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069cfbff | 1 | @c Copyright (C) 1988,89,92,93,94,96,98,99,2000 Free Software Foundation, Inc. |
c1f7febf RK |
2 | @c This is part of the GCC manual. |
3 | @c For copying conditions, see the file gcc.texi. | |
4 | ||
5 | @node C Extensions | |
6 | @chapter Extensions to the C Language Family | |
7 | @cindex extensions, C language | |
8 | @cindex C language extensions | |
9 | ||
10 | GNU C provides several language features not found in ANSI standard C. | |
11 | (The @samp{-pedantic} option directs GNU CC to print a warning message if | |
12 | any of these features is used.) To test for the availability of these | |
13 | features in conditional compilation, check for a predefined macro | |
14 | @code{__GNUC__}, which is always defined under GNU CC. | |
15 | ||
16 | These extensions are available in C and Objective C. Most of them are | |
17 | also available in C++. @xref{C++ Extensions,,Extensions to the | |
18 | C++ Language}, for extensions that apply @emph{only} to C++. | |
19 | ||
20 | @c The only difference between the two versions of this menu is that the | |
21 | @c version for clear INTERNALS has an extra node, "Constraints" (which | |
22 | @c appears in a separate chapter in the other version of the manual). | |
23 | @ifset INTERNALS | |
24 | @menu | |
25 | * Statement Exprs:: Putting statements and declarations inside expressions. | |
26 | * Local Labels:: Labels local to a statement-expression. | |
27 | * Labels as Values:: Getting pointers to labels, and computed gotos. | |
28 | * Nested Functions:: As in Algol and Pascal, lexical scoping of functions. | |
29 | * Constructing Calls:: Dispatching a call to another function. | |
30 | * Naming Types:: Giving a name to the type of some expression. | |
31 | * Typeof:: @code{typeof}: referring to the type of an expression. | |
32 | * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues. | |
33 | * Conditionals:: Omitting the middle operand of a @samp{?:} expression. | |
34 | * Long Long:: Double-word integers---@code{long long int}. | |
35 | * Complex:: Data types for complex numbers. | |
6f4d7222 | 36 | * Hex Floats:: Hexadecimal floating-point constants. |
c1f7febf RK |
37 | * Zero Length:: Zero-length arrays. |
38 | * Variable Length:: Arrays whose length is computed at run time. | |
39 | * Macro Varargs:: Macros with variable number of arguments. | |
40 | * Subscripting:: Any array can be subscripted, even if not an lvalue. | |
41 | * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers. | |
42 | * Initializers:: Non-constant initializers. | |
43 | * Constructors:: Constructor expressions give structures, unions | |
44 | or arrays as values. | |
45 | * Labeled Elements:: Labeling elements of initializers. | |
46 | * Cast to Union:: Casting to union type from any member of the union. | |
47 | * Case Ranges:: `case 1 ... 9' and such. | |
48 | * Function Attributes:: Declaring that functions have no side effects, | |
49 | or that they can never return. | |
50 | * Function Prototypes:: Prototype declarations and old-style definitions. | |
51 | * C++ Comments:: C++ comments are recognized. | |
52 | * Dollar Signs:: Dollar sign is allowed in identifiers. | |
53 | * Character Escapes:: @samp{\e} stands for the character @key{ESC}. | |
54 | * Variable Attributes:: Specifying attributes of variables. | |
55 | * Type Attributes:: Specifying attributes of types. | |
56 | * Alignment:: Inquiring about the alignment of a type or variable. | |
57 | * Inline:: Defining inline functions (as fast as macros). | |
58 | * Extended Asm:: Assembler instructions with C expressions as operands. | |
59 | (With them you can define ``built-in'' functions.) | |
60 | * Asm Labels:: Specifying the assembler name to use for a C symbol. | |
61 | * Explicit Reg Vars:: Defining variables residing in specified registers. | |
62 | * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files. | |
63 | * Incomplete Enums:: @code{enum foo;}, with details to follow. | |
64 | * Function Names:: Printable strings which are the name of the current | |
65 | function. | |
66 | * Return Address:: Getting the return or frame address of a function. | |
185ebd6c | 67 | * Other Builtins:: Other built-in functions. |
2de45c06 | 68 | * Deprecated Features:: Things might disappear from g++. |
c1f7febf RK |
69 | @end menu |
70 | @end ifset | |
71 | @ifclear INTERNALS | |
72 | @menu | |
73 | * Statement Exprs:: Putting statements and declarations inside expressions. | |
74 | * Local Labels:: Labels local to a statement-expression. | |
75 | * Labels as Values:: Getting pointers to labels, and computed gotos. | |
76 | * Nested Functions:: As in Algol and Pascal, lexical scoping of functions. | |
77 | * Constructing Calls:: Dispatching a call to another function. | |
78 | * Naming Types:: Giving a name to the type of some expression. | |
79 | * Typeof:: @code{typeof}: referring to the type of an expression. | |
80 | * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues. | |
81 | * Conditionals:: Omitting the middle operand of a @samp{?:} expression. | |
82 | * Long Long:: Double-word integers---@code{long long int}. | |
83 | * Complex:: Data types for complex numbers. | |
6f4d7222 | 84 | * Hex Floats:: Hexadecimal floating-point constants. |
c1f7febf RK |
85 | * Zero Length:: Zero-length arrays. |
86 | * Variable Length:: Arrays whose length is computed at run time. | |
87 | * Macro Varargs:: Macros with variable number of arguments. | |
88 | * Subscripting:: Any array can be subscripted, even if not an lvalue. | |
89 | * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers. | |
90 | * Initializers:: Non-constant initializers. | |
91 | * Constructors:: Constructor expressions give structures, unions | |
92 | or arrays as values. | |
93 | * Labeled Elements:: Labeling elements of initializers. | |
94 | * Cast to Union:: Casting to union type from any member of the union. | |
95 | * Case Ranges:: `case 1 ... 9' and such. | |
96 | * Function Attributes:: Declaring that functions have no side effects, | |
97 | or that they can never return. | |
98 | * Function Prototypes:: Prototype declarations and old-style definitions. | |
99 | * C++ Comments:: C++ comments are recognized. | |
100 | * Dollar Signs:: Dollar sign is allowed in identifiers. | |
101 | * Character Escapes:: @samp{\e} stands for the character @key{ESC}. | |
102 | * Variable Attributes:: Specifying attributes of variables. | |
103 | * Type Attributes:: Specifying attributes of types. | |
104 | * Alignment:: Inquiring about the alignment of a type or variable. | |
105 | * Inline:: Defining inline functions (as fast as macros). | |
106 | * Extended Asm:: Assembler instructions with C expressions as operands. | |
107 | (With them you can define ``built-in'' functions.) | |
108 | * Constraints:: Constraints for asm operands | |
109 | * Asm Labels:: Specifying the assembler name to use for a C symbol. | |
110 | * Explicit Reg Vars:: Defining variables residing in specified registers. | |
111 | * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files. | |
112 | * Incomplete Enums:: @code{enum foo;}, with details to follow. | |
113 | * Function Names:: Printable strings which are the name of the current | |
114 | function. | |
115 | * Return Address:: Getting the return or frame address of a function. | |
2de45c06 | 116 | * Deprecated Features:: Things might disappear from g++. |
c5c76735 | 117 | * Other Builtins:: Other built-in functions. |
c1f7febf RK |
118 | @end menu |
119 | @end ifclear | |
120 | ||
121 | @node Statement Exprs | |
122 | @section Statements and Declarations in Expressions | |
123 | @cindex statements inside expressions | |
124 | @cindex declarations inside expressions | |
125 | @cindex expressions containing statements | |
126 | @cindex macros, statements in expressions | |
127 | ||
128 | @c the above section title wrapped and causes an underfull hbox.. i | |
129 | @c changed it from "within" to "in". --mew 4feb93 | |
130 | ||
131 | A compound statement enclosed in parentheses may appear as an expression | |
132 | in GNU C. This allows you to use loops, switches, and local variables | |
133 | within an expression. | |
134 | ||
135 | Recall that a compound statement is a sequence of statements surrounded | |
136 | by braces; in this construct, parentheses go around the braces. For | |
137 | example: | |
138 | ||
139 | @example | |
140 | (@{ int y = foo (); int z; | |
141 | if (y > 0) z = y; | |
142 | else z = - y; | |
143 | z; @}) | |
144 | @end example | |
145 | ||
146 | @noindent | |
147 | is a valid (though slightly more complex than necessary) expression | |
148 | for the absolute value of @code{foo ()}. | |
149 | ||
150 | The last thing in the compound statement should be an expression | |
151 | followed by a semicolon; the value of this subexpression serves as the | |
152 | value of the entire construct. (If you use some other kind of statement | |
153 | last within the braces, the construct has type @code{void}, and thus | |
154 | effectively no value.) | |
155 | ||
156 | This feature is especially useful in making macro definitions ``safe'' (so | |
157 | that they evaluate each operand exactly once). For example, the | |
158 | ``maximum'' function is commonly defined as a macro in standard C as | |
159 | follows: | |
160 | ||
161 | @example | |
162 | #define max(a,b) ((a) > (b) ? (a) : (b)) | |
163 | @end example | |
164 | ||
165 | @noindent | |
166 | @cindex side effects, macro argument | |
167 | But this definition computes either @var{a} or @var{b} twice, with bad | |
168 | results if the operand has side effects. In GNU C, if you know the | |
169 | type of the operands (here let's assume @code{int}), you can define | |
170 | the macro safely as follows: | |
171 | ||
172 | @example | |
173 | #define maxint(a,b) \ | |
174 | (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @}) | |
175 | @end example | |
176 | ||
177 | Embedded statements are not allowed in constant expressions, such as | |
178 | the value of an enumeration constant, the width of a bit field, or | |
179 | the initial value of a static variable. | |
180 | ||
181 | If you don't know the type of the operand, you can still do this, but you | |
182 | must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming | |
183 | Types}). | |
184 | ||
185 | @node Local Labels | |
186 | @section Locally Declared Labels | |
187 | @cindex local labels | |
188 | @cindex macros, local labels | |
189 | ||
190 | Each statement expression is a scope in which @dfn{local labels} can be | |
191 | declared. A local label is simply an identifier; you can jump to it | |
192 | with an ordinary @code{goto} statement, but only from within the | |
193 | statement expression it belongs to. | |
194 | ||
195 | A local label declaration looks like this: | |
196 | ||
197 | @example | |
198 | __label__ @var{label}; | |
199 | @end example | |
200 | ||
201 | @noindent | |
202 | or | |
203 | ||
204 | @example | |
205 | __label__ @var{label1}, @var{label2}, @dots{}; | |
206 | @end example | |
207 | ||
208 | Local label declarations must come at the beginning of the statement | |
209 | expression, right after the @samp{(@{}, before any ordinary | |
210 | declarations. | |
211 | ||
212 | The label declaration defines the label @emph{name}, but does not define | |
213 | the label itself. You must do this in the usual way, with | |
214 | @code{@var{label}:}, within the statements of the statement expression. | |
215 | ||
216 | The local label feature is useful because statement expressions are | |
217 | often used in macros. If the macro contains nested loops, a @code{goto} | |
218 | can be useful for breaking out of them. However, an ordinary label | |
219 | whose scope is the whole function cannot be used: if the macro can be | |
220 | expanded several times in one function, the label will be multiply | |
221 | defined in that function. A local label avoids this problem. For | |
222 | example: | |
223 | ||
224 | @example | |
225 | #define SEARCH(array, target) \ | |
226 | (@{ \ | |
227 | __label__ found; \ | |
228 | typeof (target) _SEARCH_target = (target); \ | |
229 | typeof (*(array)) *_SEARCH_array = (array); \ | |
230 | int i, j; \ | |
231 | int value; \ | |
232 | for (i = 0; i < max; i++) \ | |
233 | for (j = 0; j < max; j++) \ | |
234 | if (_SEARCH_array[i][j] == _SEARCH_target) \ | |
235 | @{ value = i; goto found; @} \ | |
236 | value = -1; \ | |
237 | found: \ | |
238 | value; \ | |
239 | @}) | |
240 | @end example | |
241 | ||
242 | @node Labels as Values | |
243 | @section Labels as Values | |
244 | @cindex labels as values | |
245 | @cindex computed gotos | |
246 | @cindex goto with computed label | |
247 | @cindex address of a label | |
248 | ||
249 | You can get the address of a label defined in the current function | |
250 | (or a containing function) with the unary operator @samp{&&}. The | |
251 | value has type @code{void *}. This value is a constant and can be used | |
252 | wherever a constant of that type is valid. For example: | |
253 | ||
254 | @example | |
255 | void *ptr; | |
256 | @dots{} | |
257 | ptr = &&foo; | |
258 | @end example | |
259 | ||
260 | To use these values, you need to be able to jump to one. This is done | |
261 | with the computed goto statement@footnote{The analogous feature in | |
262 | Fortran is called an assigned goto, but that name seems inappropriate in | |
263 | C, where one can do more than simply store label addresses in label | |
264 | variables.}, @code{goto *@var{exp};}. For example, | |
265 | ||
266 | @example | |
267 | goto *ptr; | |
268 | @end example | |
269 | ||
270 | @noindent | |
271 | Any expression of type @code{void *} is allowed. | |
272 | ||
273 | One way of using these constants is in initializing a static array that | |
274 | will serve as a jump table: | |
275 | ||
276 | @example | |
277 | static void *array[] = @{ &&foo, &&bar, &&hack @}; | |
278 | @end example | |
279 | ||
280 | Then you can select a label with indexing, like this: | |
281 | ||
282 | @example | |
283 | goto *array[i]; | |
284 | @end example | |
285 | ||
286 | @noindent | |
287 | Note that this does not check whether the subscript is in bounds---array | |
288 | indexing in C never does that. | |
289 | ||
290 | Such an array of label values serves a purpose much like that of the | |
291 | @code{switch} statement. The @code{switch} statement is cleaner, so | |
292 | use that rather than an array unless the problem does not fit a | |
293 | @code{switch} statement very well. | |
294 | ||
295 | Another use of label values is in an interpreter for threaded code. | |
296 | The labels within the interpreter function can be stored in the | |
297 | threaded code for super-fast dispatching. | |
298 | ||
47620e09 RH |
299 | You may not use this mechanism to jump to code in a different function. |
300 | If you do that, totally unpredictable things will happen. The best way to | |
c1f7febf RK |
301 | avoid this is to store the label address only in automatic variables and |
302 | never pass it as an argument. | |
303 | ||
47620e09 RH |
304 | An alternate way to write the above example is |
305 | ||
306 | @example | |
307 | static const int array[] = @{ &&foo - &&foo, &&bar - &&foo, &&hack - &&foo @}; | |
308 | goto *(&&foo + array[i]); | |
309 | @end example | |
310 | ||
311 | @noindent | |
312 | This is more friendly to code living in shared libraries, as it reduces | |
313 | the number of dynamic relocations that are needed, and by consequence, | |
314 | allows the data to be read-only. | |
315 | ||
c1f7febf RK |
316 | @node Nested Functions |
317 | @section Nested Functions | |
318 | @cindex nested functions | |
319 | @cindex downward funargs | |
320 | @cindex thunks | |
321 | ||
322 | A @dfn{nested function} is a function defined inside another function. | |
323 | (Nested functions are not supported for GNU C++.) The nested function's | |
324 | name is local to the block where it is defined. For example, here we | |
325 | define a nested function named @code{square}, and call it twice: | |
326 | ||
327 | @example | |
328 | @group | |
329 | foo (double a, double b) | |
330 | @{ | |
331 | double square (double z) @{ return z * z; @} | |
332 | ||
333 | return square (a) + square (b); | |
334 | @} | |
335 | @end group | |
336 | @end example | |
337 | ||
338 | The nested function can access all the variables of the containing | |
339 | function that are visible at the point of its definition. This is | |
340 | called @dfn{lexical scoping}. For example, here we show a nested | |
341 | function which uses an inherited variable named @code{offset}: | |
342 | ||
343 | @example | |
344 | bar (int *array, int offset, int size) | |
345 | @{ | |
346 | int access (int *array, int index) | |
347 | @{ return array[index + offset]; @} | |
348 | int i; | |
349 | @dots{} | |
350 | for (i = 0; i < size; i++) | |
351 | @dots{} access (array, i) @dots{} | |
352 | @} | |
353 | @end example | |
354 | ||
355 | Nested function definitions are permitted within functions in the places | |
356 | where variable definitions are allowed; that is, in any block, before | |
357 | the first statement in the block. | |
358 | ||
359 | It is possible to call the nested function from outside the scope of its | |
360 | name by storing its address or passing the address to another function: | |
361 | ||
362 | @example | |
363 | hack (int *array, int size) | |
364 | @{ | |
365 | void store (int index, int value) | |
366 | @{ array[index] = value; @} | |
367 | ||
368 | intermediate (store, size); | |
369 | @} | |
370 | @end example | |
371 | ||
372 | Here, the function @code{intermediate} receives the address of | |
373 | @code{store} as an argument. If @code{intermediate} calls @code{store}, | |
374 | the arguments given to @code{store} are used to store into @code{array}. | |
375 | But this technique works only so long as the containing function | |
376 | (@code{hack}, in this example) does not exit. | |
377 | ||
378 | If you try to call the nested function through its address after the | |
379 | containing function has exited, all hell will break loose. If you try | |
380 | to call it after a containing scope level has exited, and if it refers | |
381 | to some of the variables that are no longer in scope, you may be lucky, | |
382 | but it's not wise to take the risk. If, however, the nested function | |
383 | does not refer to anything that has gone out of scope, you should be | |
384 | safe. | |
385 | ||
386 | GNU CC implements taking the address of a nested function using a | |
674032e2 JL |
387 | technique called @dfn{trampolines}. A paper describing them is |
388 | available as @samp{http://master.debian.org/~karlheg/Usenix88-lexic.pdf}. | |
c1f7febf RK |
389 | |
390 | A nested function can jump to a label inherited from a containing | |
391 | function, provided the label was explicitly declared in the containing | |
392 | function (@pxref{Local Labels}). Such a jump returns instantly to the | |
393 | containing function, exiting the nested function which did the | |
394 | @code{goto} and any intermediate functions as well. Here is an example: | |
395 | ||
396 | @example | |
397 | @group | |
398 | bar (int *array, int offset, int size) | |
399 | @{ | |
400 | __label__ failure; | |
401 | int access (int *array, int index) | |
402 | @{ | |
403 | if (index > size) | |
404 | goto failure; | |
405 | return array[index + offset]; | |
406 | @} | |
407 | int i; | |
408 | @dots{} | |
409 | for (i = 0; i < size; i++) | |
410 | @dots{} access (array, i) @dots{} | |
411 | @dots{} | |
412 | return 0; | |
413 | ||
414 | /* @r{Control comes here from @code{access} | |
415 | if it detects an error.} */ | |
416 | failure: | |
417 | return -1; | |
418 | @} | |
419 | @end group | |
420 | @end example | |
421 | ||
422 | A nested function always has internal linkage. Declaring one with | |
423 | @code{extern} is erroneous. If you need to declare the nested function | |
424 | before its definition, use @code{auto} (which is otherwise meaningless | |
425 | for function declarations). | |
426 | ||
427 | @example | |
428 | bar (int *array, int offset, int size) | |
429 | @{ | |
430 | __label__ failure; | |
431 | auto int access (int *, int); | |
432 | @dots{} | |
433 | int access (int *array, int index) | |
434 | @{ | |
435 | if (index > size) | |
436 | goto failure; | |
437 | return array[index + offset]; | |
438 | @} | |
439 | @dots{} | |
440 | @} | |
441 | @end example | |
442 | ||
443 | @node Constructing Calls | |
444 | @section Constructing Function Calls | |
445 | @cindex constructing calls | |
446 | @cindex forwarding calls | |
447 | ||
448 | Using the built-in functions described below, you can record | |
449 | the arguments a function received, and call another function | |
450 | with the same arguments, without knowing the number or types | |
451 | of the arguments. | |
452 | ||
453 | You can also record the return value of that function call, | |
454 | and later return that value, without knowing what data type | |
455 | the function tried to return (as long as your caller expects | |
456 | that data type). | |
457 | ||
458 | @table @code | |
459 | @findex __builtin_apply_args | |
460 | @item __builtin_apply_args () | |
461 | This built-in function returns a pointer of type @code{void *} to data | |
462 | describing how to perform a call with the same arguments as were passed | |
463 | to the current function. | |
464 | ||
465 | The function saves the arg pointer register, structure value address, | |
466 | and all registers that might be used to pass arguments to a function | |
467 | into a block of memory allocated on the stack. Then it returns the | |
468 | address of that block. | |
469 | ||
470 | @findex __builtin_apply | |
471 | @item __builtin_apply (@var{function}, @var{arguments}, @var{size}) | |
472 | This built-in function invokes @var{function} (type @code{void (*)()}) | |
473 | with a copy of the parameters described by @var{arguments} (type | |
474 | @code{void *}) and @var{size} (type @code{int}). | |
475 | ||
476 | The value of @var{arguments} should be the value returned by | |
477 | @code{__builtin_apply_args}. The argument @var{size} specifies the size | |
478 | of the stack argument data, in bytes. | |
479 | ||
480 | This function returns a pointer of type @code{void *} to data describing | |
481 | how to return whatever value was returned by @var{function}. The data | |
482 | is saved in a block of memory allocated on the stack. | |
483 | ||
484 | It is not always simple to compute the proper value for @var{size}. The | |
485 | value is used by @code{__builtin_apply} to compute the amount of data | |
486 | that should be pushed on the stack and copied from the incoming argument | |
487 | area. | |
488 | ||
489 | @findex __builtin_return | |
490 | @item __builtin_return (@var{result}) | |
491 | This built-in function returns the value described by @var{result} from | |
492 | the containing function. You should specify, for @var{result}, a value | |
493 | returned by @code{__builtin_apply}. | |
494 | @end table | |
495 | ||
496 | @node Naming Types | |
497 | @section Naming an Expression's Type | |
498 | @cindex naming types | |
499 | ||
500 | You can give a name to the type of an expression using a @code{typedef} | |
501 | declaration with an initializer. Here is how to define @var{name} as a | |
502 | type name for the type of @var{exp}: | |
503 | ||
504 | @example | |
505 | typedef @var{name} = @var{exp}; | |
506 | @end example | |
507 | ||
508 | This is useful in conjunction with the statements-within-expressions | |
509 | feature. Here is how the two together can be used to define a safe | |
510 | ``maximum'' macro that operates on any arithmetic type: | |
511 | ||
512 | @example | |
513 | #define max(a,b) \ | |
514 | (@{typedef _ta = (a), _tb = (b); \ | |
515 | _ta _a = (a); _tb _b = (b); \ | |
516 | _a > _b ? _a : _b; @}) | |
517 | @end example | |
518 | ||
519 | @cindex underscores in variables in macros | |
520 | @cindex @samp{_} in variables in macros | |
521 | @cindex local variables in macros | |
522 | @cindex variables, local, in macros | |
523 | @cindex macros, local variables in | |
524 | ||
525 | The reason for using names that start with underscores for the local | |
526 | variables is to avoid conflicts with variable names that occur within the | |
527 | expressions that are substituted for @code{a} and @code{b}. Eventually we | |
528 | hope to design a new form of declaration syntax that allows you to declare | |
529 | variables whose scopes start only after their initializers; this will be a | |
530 | more reliable way to prevent such conflicts. | |
531 | ||
532 | @node Typeof | |
533 | @section Referring to a Type with @code{typeof} | |
534 | @findex typeof | |
535 | @findex sizeof | |
536 | @cindex macros, types of arguments | |
537 | ||
538 | Another way to refer to the type of an expression is with @code{typeof}. | |
539 | The syntax of using of this keyword looks like @code{sizeof}, but the | |
540 | construct acts semantically like a type name defined with @code{typedef}. | |
541 | ||
542 | There are two ways of writing the argument to @code{typeof}: with an | |
543 | expression or with a type. Here is an example with an expression: | |
544 | ||
545 | @example | |
546 | typeof (x[0](1)) | |
547 | @end example | |
548 | ||
549 | @noindent | |
550 | This assumes that @code{x} is an array of functions; the type described | |
551 | is that of the values of the functions. | |
552 | ||
553 | Here is an example with a typename as the argument: | |
554 | ||
555 | @example | |
556 | typeof (int *) | |
557 | @end example | |
558 | ||
559 | @noindent | |
560 | Here the type described is that of pointers to @code{int}. | |
561 | ||
562 | If you are writing a header file that must work when included in ANSI C | |
563 | programs, write @code{__typeof__} instead of @code{typeof}. | |
564 | @xref{Alternate Keywords}. | |
565 | ||
566 | A @code{typeof}-construct can be used anywhere a typedef name could be | |
567 | used. For example, you can use it in a declaration, in a cast, or inside | |
568 | of @code{sizeof} or @code{typeof}. | |
569 | ||
570 | @itemize @bullet | |
571 | @item | |
572 | This declares @code{y} with the type of what @code{x} points to. | |
573 | ||
574 | @example | |
575 | typeof (*x) y; | |
576 | @end example | |
577 | ||
578 | @item | |
579 | This declares @code{y} as an array of such values. | |
580 | ||
581 | @example | |
582 | typeof (*x) y[4]; | |
583 | @end example | |
584 | ||
585 | @item | |
586 | This declares @code{y} as an array of pointers to characters: | |
587 | ||
588 | @example | |
589 | typeof (typeof (char *)[4]) y; | |
590 | @end example | |
591 | ||
592 | @noindent | |
593 | It is equivalent to the following traditional C declaration: | |
594 | ||
595 | @example | |
596 | char *y[4]; | |
597 | @end example | |
598 | ||
599 | To see the meaning of the declaration using @code{typeof}, and why it | |
600 | might be a useful way to write, let's rewrite it with these macros: | |
601 | ||
602 | @example | |
603 | #define pointer(T) typeof(T *) | |
604 | #define array(T, N) typeof(T [N]) | |
605 | @end example | |
606 | ||
607 | @noindent | |
608 | Now the declaration can be rewritten this way: | |
609 | ||
610 | @example | |
611 | array (pointer (char), 4) y; | |
612 | @end example | |
613 | ||
614 | @noindent | |
615 | Thus, @code{array (pointer (char), 4)} is the type of arrays of 4 | |
616 | pointers to @code{char}. | |
617 | @end itemize | |
618 | ||
619 | @node Lvalues | |
620 | @section Generalized Lvalues | |
621 | @cindex compound expressions as lvalues | |
622 | @cindex expressions, compound, as lvalues | |
623 | @cindex conditional expressions as lvalues | |
624 | @cindex expressions, conditional, as lvalues | |
625 | @cindex casts as lvalues | |
626 | @cindex generalized lvalues | |
627 | @cindex lvalues, generalized | |
628 | @cindex extensions, @code{?:} | |
629 | @cindex @code{?:} extensions | |
630 | Compound expressions, conditional expressions and casts are allowed as | |
631 | lvalues provided their operands are lvalues. This means that you can take | |
632 | their addresses or store values into them. | |
633 | ||
634 | Standard C++ allows compound expressions and conditional expressions as | |
635 | lvalues, and permits casts to reference type, so use of this extension | |
636 | is deprecated for C++ code. | |
637 | ||
638 | For example, a compound expression can be assigned, provided the last | |
639 | expression in the sequence is an lvalue. These two expressions are | |
640 | equivalent: | |
641 | ||
642 | @example | |
643 | (a, b) += 5 | |
644 | a, (b += 5) | |
645 | @end example | |
646 | ||
647 | Similarly, the address of the compound expression can be taken. These two | |
648 | expressions are equivalent: | |
649 | ||
650 | @example | |
651 | &(a, b) | |
652 | a, &b | |
653 | @end example | |
654 | ||
655 | A conditional expression is a valid lvalue if its type is not void and the | |
656 | true and false branches are both valid lvalues. For example, these two | |
657 | expressions are equivalent: | |
658 | ||
659 | @example | |
660 | (a ? b : c) = 5 | |
661 | (a ? b = 5 : (c = 5)) | |
662 | @end example | |
663 | ||
664 | A cast is a valid lvalue if its operand is an lvalue. A simple | |
665 | assignment whose left-hand side is a cast works by converting the | |
666 | right-hand side first to the specified type, then to the type of the | |
667 | inner left-hand side expression. After this is stored, the value is | |
668 | converted back to the specified type to become the value of the | |
669 | assignment. Thus, if @code{a} has type @code{char *}, the following two | |
670 | expressions are equivalent: | |
671 | ||
672 | @example | |
673 | (int)a = 5 | |
674 | (int)(a = (char *)(int)5) | |
675 | @end example | |
676 | ||
677 | An assignment-with-arithmetic operation such as @samp{+=} applied to a cast | |
678 | performs the arithmetic using the type resulting from the cast, and then | |
679 | continues as in the previous case. Therefore, these two expressions are | |
680 | equivalent: | |
681 | ||
682 | @example | |
683 | (int)a += 5 | |
684 | (int)(a = (char *)(int) ((int)a + 5)) | |
685 | @end example | |
686 | ||
687 | You cannot take the address of an lvalue cast, because the use of its | |
688 | address would not work out coherently. Suppose that @code{&(int)f} were | |
689 | permitted, where @code{f} has type @code{float}. Then the following | |
690 | statement would try to store an integer bit-pattern where a floating | |
691 | point number belongs: | |
692 | ||
693 | @example | |
694 | *&(int)f = 1; | |
695 | @end example | |
696 | ||
697 | This is quite different from what @code{(int)f = 1} would do---that | |
698 | would convert 1 to floating point and store it. Rather than cause this | |
699 | inconsistency, we think it is better to prohibit use of @samp{&} on a cast. | |
700 | ||
701 | If you really do want an @code{int *} pointer with the address of | |
702 | @code{f}, you can simply write @code{(int *)&f}. | |
703 | ||
704 | @node Conditionals | |
705 | @section Conditionals with Omitted Operands | |
706 | @cindex conditional expressions, extensions | |
707 | @cindex omitted middle-operands | |
708 | @cindex middle-operands, omitted | |
709 | @cindex extensions, @code{?:} | |
710 | @cindex @code{?:} extensions | |
711 | ||
712 | The middle operand in a conditional expression may be omitted. Then | |
713 | if the first operand is nonzero, its value is the value of the conditional | |
714 | expression. | |
715 | ||
716 | Therefore, the expression | |
717 | ||
718 | @example | |
719 | x ? : y | |
720 | @end example | |
721 | ||
722 | @noindent | |
723 | has the value of @code{x} if that is nonzero; otherwise, the value of | |
724 | @code{y}. | |
725 | ||
726 | This example is perfectly equivalent to | |
727 | ||
728 | @example | |
729 | x ? x : y | |
730 | @end example | |
731 | ||
732 | @cindex side effect in ?: | |
733 | @cindex ?: side effect | |
734 | @noindent | |
735 | In this simple case, the ability to omit the middle operand is not | |
736 | especially useful. When it becomes useful is when the first operand does, | |
737 | or may (if it is a macro argument), contain a side effect. Then repeating | |
738 | the operand in the middle would perform the side effect twice. Omitting | |
739 | the middle operand uses the value already computed without the undesirable | |
740 | effects of recomputing it. | |
741 | ||
742 | @node Long Long | |
743 | @section Double-Word Integers | |
744 | @cindex @code{long long} data types | |
745 | @cindex double-word arithmetic | |
746 | @cindex multiprecision arithmetic | |
747 | ||
748 | GNU C supports data types for integers that are twice as long as | |
749 | @code{int}. Simply write @code{long long int} for a signed integer, or | |
750 | @code{unsigned long long int} for an unsigned integer. To make an | |
751 | integer constant of type @code{long long int}, add the suffix @code{LL} | |
752 | to the integer. To make an integer constant of type @code{unsigned long | |
753 | long int}, add the suffix @code{ULL} to the integer. | |
754 | ||
755 | You can use these types in arithmetic like any other integer types. | |
756 | Addition, subtraction, and bitwise boolean operations on these types | |
757 | are open-coded on all types of machines. Multiplication is open-coded | |
758 | if the machine supports fullword-to-doubleword a widening multiply | |
759 | instruction. Division and shifts are open-coded only on machines that | |
760 | provide special support. The operations that are not open-coded use | |
761 | special library routines that come with GNU CC. | |
762 | ||
763 | There may be pitfalls when you use @code{long long} types for function | |
764 | arguments, unless you declare function prototypes. If a function | |
765 | expects type @code{int} for its argument, and you pass a value of type | |
766 | @code{long long int}, confusion will result because the caller and the | |
767 | subroutine will disagree about the number of bytes for the argument. | |
768 | Likewise, if the function expects @code{long long int} and you pass | |
769 | @code{int}. The best way to avoid such problems is to use prototypes. | |
770 | ||
771 | @node Complex | |
772 | @section Complex Numbers | |
773 | @cindex complex numbers | |
774 | ||
775 | GNU C supports complex data types. You can declare both complex integer | |
776 | types and complex floating types, using the keyword @code{__complex__}. | |
777 | ||
778 | For example, @samp{__complex__ double x;} declares @code{x} as a | |
779 | variable whose real part and imaginary part are both of type | |
780 | @code{double}. @samp{__complex__ short int y;} declares @code{y} to | |
781 | have real and imaginary parts of type @code{short int}; this is not | |
782 | likely to be useful, but it shows that the set of complex types is | |
783 | complete. | |
784 | ||
785 | To write a constant with a complex data type, use the suffix @samp{i} or | |
786 | @samp{j} (either one; they are equivalent). For example, @code{2.5fi} | |
787 | has type @code{__complex__ float} and @code{3i} has type | |
788 | @code{__complex__ int}. Such a constant always has a pure imaginary | |
789 | value, but you can form any complex value you like by adding one to a | |
790 | real constant. | |
791 | ||
792 | To extract the real part of a complex-valued expression @var{exp}, write | |
793 | @code{__real__ @var{exp}}. Likewise, use @code{__imag__} to | |
794 | extract the imaginary part. | |
795 | ||
796 | The operator @samp{~} performs complex conjugation when used on a value | |
797 | with a complex type. | |
798 | ||
799 | GNU CC can allocate complex automatic variables in a noncontiguous | |
800 | fashion; it's even possible for the real part to be in a register while | |
801 | the imaginary part is on the stack (or vice-versa). None of the | |
802 | supported debugging info formats has a way to represent noncontiguous | |
803 | allocation like this, so GNU CC describes a noncontiguous complex | |
804 | variable as if it were two separate variables of noncomplex type. | |
805 | If the variable's actual name is @code{foo}, the two fictitious | |
806 | variables are named @code{foo$real} and @code{foo$imag}. You can | |
807 | examine and set these two fictitious variables with your debugger. | |
808 | ||
809 | A future version of GDB will know how to recognize such pairs and treat | |
810 | them as a single variable with a complex type. | |
811 | ||
6f4d7222 | 812 | @node Hex Floats |
6b42b9ea UD |
813 | @section Hex Floats |
814 | @cindex hex floats | |
c5c76735 JL |
815 | |
816 | GNU CC recognizes floating-point numbers writen not only in the usual | |
6f4d7222 UD |
817 | decimal notation, such as @code{1.55e1}, but also numbers such as |
818 | @code{0x1.fp3} written in hexadecimal format. In that format the | |
819 | @code{0x} hex introducer and the @code{p} or @code{P} exponent field are | |
820 | mandatory. The exponent is a decimal number that indicates the power of | |
821 | 2 by which the significand part will be multiplied. Thus @code{0x1.f} is | |
822 | 1 15/16, @code{p3} multiplies it by 8, and the value of @code{0x1.fp3} | |
823 | is the same as @code{1.55e1}. | |
824 | ||
825 | Unlike for floating-point numbers in the decimal notation the exponent | |
826 | is always required in the hexadecimal notation. Otherwise the compiler | |
827 | would not be able to resolve the ambiguity of, e.g., @code{0x1.f}. This | |
828 | could mean @code{1.0f} or @code{1.9375} since @code{f} is also the | |
829 | extension for floating-point constants of type @code{float}. | |
830 | ||
c1f7febf RK |
831 | @node Zero Length |
832 | @section Arrays of Length Zero | |
833 | @cindex arrays of length zero | |
834 | @cindex zero-length arrays | |
835 | @cindex length-zero arrays | |
836 | ||
837 | Zero-length arrays are allowed in GNU C. They are very useful as the last | |
838 | element of a structure which is really a header for a variable-length | |
839 | object: | |
840 | ||
841 | @example | |
842 | struct line @{ | |
843 | int length; | |
844 | char contents[0]; | |
845 | @}; | |
846 | ||
847 | @{ | |
848 | struct line *thisline = (struct line *) | |
849 | malloc (sizeof (struct line) + this_length); | |
850 | thisline->length = this_length; | |
851 | @} | |
852 | @end example | |
853 | ||
854 | In standard C, you would have to give @code{contents} a length of 1, which | |
855 | means either you waste space or complicate the argument to @code{malloc}. | |
856 | ||
857 | @node Variable Length | |
858 | @section Arrays of Variable Length | |
859 | @cindex variable-length arrays | |
860 | @cindex arrays of variable length | |
861 | ||
862 | Variable-length automatic arrays are allowed in GNU C. These arrays are | |
863 | declared like any other automatic arrays, but with a length that is not | |
864 | a constant expression. The storage is allocated at the point of | |
865 | declaration and deallocated when the brace-level is exited. For | |
866 | example: | |
867 | ||
868 | @example | |
869 | FILE * | |
870 | concat_fopen (char *s1, char *s2, char *mode) | |
871 | @{ | |
872 | char str[strlen (s1) + strlen (s2) + 1]; | |
873 | strcpy (str, s1); | |
874 | strcat (str, s2); | |
875 | return fopen (str, mode); | |
876 | @} | |
877 | @end example | |
878 | ||
879 | @cindex scope of a variable length array | |
880 | @cindex variable-length array scope | |
881 | @cindex deallocating variable length arrays | |
882 | Jumping or breaking out of the scope of the array name deallocates the | |
883 | storage. Jumping into the scope is not allowed; you get an error | |
884 | message for it. | |
885 | ||
886 | @cindex @code{alloca} vs variable-length arrays | |
887 | You can use the function @code{alloca} to get an effect much like | |
888 | variable-length arrays. The function @code{alloca} is available in | |
889 | many other C implementations (but not in all). On the other hand, | |
890 | variable-length arrays are more elegant. | |
891 | ||
892 | There are other differences between these two methods. Space allocated | |
893 | with @code{alloca} exists until the containing @emph{function} returns. | |
894 | The space for a variable-length array is deallocated as soon as the array | |
895 | name's scope ends. (If you use both variable-length arrays and | |
896 | @code{alloca} in the same function, deallocation of a variable-length array | |
897 | will also deallocate anything more recently allocated with @code{alloca}.) | |
898 | ||
899 | You can also use variable-length arrays as arguments to functions: | |
900 | ||
901 | @example | |
902 | struct entry | |
903 | tester (int len, char data[len][len]) | |
904 | @{ | |
905 | @dots{} | |
906 | @} | |
907 | @end example | |
908 | ||
909 | The length of an array is computed once when the storage is allocated | |
910 | and is remembered for the scope of the array in case you access it with | |
911 | @code{sizeof}. | |
912 | ||
913 | If you want to pass the array first and the length afterward, you can | |
914 | use a forward declaration in the parameter list---another GNU extension. | |
915 | ||
916 | @example | |
917 | struct entry | |
918 | tester (int len; char data[len][len], int len) | |
919 | @{ | |
920 | @dots{} | |
921 | @} | |
922 | @end example | |
923 | ||
924 | @cindex parameter forward declaration | |
925 | The @samp{int len} before the semicolon is a @dfn{parameter forward | |
926 | declaration}, and it serves the purpose of making the name @code{len} | |
927 | known when the declaration of @code{data} is parsed. | |
928 | ||
929 | You can write any number of such parameter forward declarations in the | |
930 | parameter list. They can be separated by commas or semicolons, but the | |
931 | last one must end with a semicolon, which is followed by the ``real'' | |
932 | parameter declarations. Each forward declaration must match a ``real'' | |
933 | declaration in parameter name and data type. | |
934 | ||
935 | @node Macro Varargs | |
936 | @section Macros with Variable Numbers of Arguments | |
937 | @cindex variable number of arguments | |
938 | @cindex macro with variable arguments | |
939 | @cindex rest argument (in macro) | |
940 | ||
941 | In GNU C, a macro can accept a variable number of arguments, much as a | |
942 | function can. The syntax for defining the macro looks much like that | |
943 | used for a function. Here is an example: | |
944 | ||
945 | @example | |
946 | #define eprintf(format, args...) \ | |
947 | fprintf (stderr, format , ## args) | |
948 | @end example | |
949 | ||
950 | Here @code{args} is a @dfn{rest argument}: it takes in zero or more | |
951 | arguments, as many as the call contains. All of them plus the commas | |
952 | between them form the value of @code{args}, which is substituted into | |
953 | the macro body where @code{args} is used. Thus, we have this expansion: | |
954 | ||
955 | @example | |
956 | eprintf ("%s:%d: ", input_file_name, line_number) | |
957 | @expansion{} | |
958 | fprintf (stderr, "%s:%d: " , input_file_name, line_number) | |
959 | @end example | |
960 | ||
961 | @noindent | |
962 | Note that the comma after the string constant comes from the definition | |
963 | of @code{eprintf}, whereas the last comma comes from the value of | |
964 | @code{args}. | |
965 | ||
966 | The reason for using @samp{##} is to handle the case when @code{args} | |
967 | matches no arguments at all. In this case, @code{args} has an empty | |
968 | value. In this case, the second comma in the definition becomes an | |
969 | embarrassment: if it got through to the expansion of the macro, we would | |
970 | get something like this: | |
971 | ||
972 | @example | |
973 | fprintf (stderr, "success!\n" , ) | |
974 | @end example | |
975 | ||
976 | @noindent | |
977 | which is invalid C syntax. @samp{##} gets rid of the comma, so we get | |
978 | the following instead: | |
979 | ||
980 | @example | |
981 | fprintf (stderr, "success!\n") | |
982 | @end example | |
983 | ||
984 | This is a special feature of the GNU C preprocessor: @samp{##} before a | |
985 | rest argument that is empty discards the preceding sequence of | |
986 | non-whitespace characters from the macro definition. (If another macro | |
987 | argument precedes, none of it is discarded.) | |
988 | ||
989 | It might be better to discard the last preprocessor token instead of the | |
990 | last preceding sequence of non-whitespace characters; in fact, we may | |
991 | someday change this feature to do so. We advise you to write the macro | |
992 | definition so that the preceding sequence of non-whitespace characters | |
993 | is just a single token, so that the meaning will not change if we change | |
994 | the definition of this feature. | |
995 | ||
996 | @node Subscripting | |
997 | @section Non-Lvalue Arrays May Have Subscripts | |
998 | @cindex subscripting | |
999 | @cindex arrays, non-lvalue | |
1000 | ||
1001 | @cindex subscripting and function values | |
1002 | Subscripting is allowed on arrays that are not lvalues, even though the | |
1003 | unary @samp{&} operator is not. For example, this is valid in GNU C though | |
1004 | not valid in other C dialects: | |
1005 | ||
1006 | @example | |
1007 | @group | |
1008 | struct foo @{int a[4];@}; | |
1009 | ||
1010 | struct foo f(); | |
1011 | ||
1012 | bar (int index) | |
1013 | @{ | |
1014 | return f().a[index]; | |
1015 | @} | |
1016 | @end group | |
1017 | @end example | |
1018 | ||
1019 | @node Pointer Arith | |
1020 | @section Arithmetic on @code{void}- and Function-Pointers | |
1021 | @cindex void pointers, arithmetic | |
1022 | @cindex void, size of pointer to | |
1023 | @cindex function pointers, arithmetic | |
1024 | @cindex function, size of pointer to | |
1025 | ||
1026 | In GNU C, addition and subtraction operations are supported on pointers to | |
1027 | @code{void} and on pointers to functions. This is done by treating the | |
1028 | size of a @code{void} or of a function as 1. | |
1029 | ||
1030 | A consequence of this is that @code{sizeof} is also allowed on @code{void} | |
1031 | and on function types, and returns 1. | |
1032 | ||
1033 | The option @samp{-Wpointer-arith} requests a warning if these extensions | |
1034 | are used. | |
1035 | ||
1036 | @node Initializers | |
1037 | @section Non-Constant Initializers | |
1038 | @cindex initializers, non-constant | |
1039 | @cindex non-constant initializers | |
1040 | ||
1041 | As in standard C++, the elements of an aggregate initializer for an | |
1042 | automatic variable are not required to be constant expressions in GNU C. | |
1043 | Here is an example of an initializer with run-time varying elements: | |
1044 | ||
1045 | @example | |
1046 | foo (float f, float g) | |
1047 | @{ | |
1048 | float beat_freqs[2] = @{ f-g, f+g @}; | |
1049 | @dots{} | |
1050 | @} | |
1051 | @end example | |
1052 | ||
1053 | @node Constructors | |
1054 | @section Constructor Expressions | |
1055 | @cindex constructor expressions | |
1056 | @cindex initializations in expressions | |
1057 | @cindex structures, constructor expression | |
1058 | @cindex expressions, constructor | |
1059 | ||
1060 | GNU C supports constructor expressions. A constructor looks like | |
1061 | a cast containing an initializer. Its value is an object of the | |
1062 | type specified in the cast, containing the elements specified in | |
1063 | the initializer. | |
1064 | ||
1065 | Usually, the specified type is a structure. Assume that | |
1066 | @code{struct foo} and @code{structure} are declared as shown: | |
1067 | ||
1068 | @example | |
1069 | struct foo @{int a; char b[2];@} structure; | |
1070 | @end example | |
1071 | ||
1072 | @noindent | |
1073 | Here is an example of constructing a @code{struct foo} with a constructor: | |
1074 | ||
1075 | @example | |
1076 | structure = ((struct foo) @{x + y, 'a', 0@}); | |
1077 | @end example | |
1078 | ||
1079 | @noindent | |
1080 | This is equivalent to writing the following: | |
1081 | ||
1082 | @example | |
1083 | @{ | |
1084 | struct foo temp = @{x + y, 'a', 0@}; | |
1085 | structure = temp; | |
1086 | @} | |
1087 | @end example | |
1088 | ||
1089 | You can also construct an array. If all the elements of the constructor | |
1090 | are (made up of) simple constant expressions, suitable for use in | |
1091 | initializers, then the constructor is an lvalue and can be coerced to a | |
1092 | pointer to its first element, as shown here: | |
1093 | ||
1094 | @example | |
1095 | char **foo = (char *[]) @{ "x", "y", "z" @}; | |
1096 | @end example | |
1097 | ||
1098 | Array constructors whose elements are not simple constants are | |
1099 | not very useful, because the constructor is not an lvalue. There | |
1100 | are only two valid ways to use it: to subscript it, or initialize | |
1101 | an array variable with it. The former is probably slower than a | |
1102 | @code{switch} statement, while the latter does the same thing an | |
1103 | ordinary C initializer would do. Here is an example of | |
1104 | subscripting an array constructor: | |
1105 | ||
1106 | @example | |
1107 | output = ((int[]) @{ 2, x, 28 @}) [input]; | |
1108 | @end example | |
1109 | ||
1110 | Constructor expressions for scalar types and union types are is | |
1111 | also allowed, but then the constructor expression is equivalent | |
1112 | to a cast. | |
1113 | ||
1114 | @node Labeled Elements | |
1115 | @section Labeled Elements in Initializers | |
1116 | @cindex initializers with labeled elements | |
1117 | @cindex labeled elements in initializers | |
1118 | @cindex case labels in initializers | |
1119 | ||
1120 | Standard C requires the elements of an initializer to appear in a fixed | |
1121 | order, the same as the order of the elements in the array or structure | |
1122 | being initialized. | |
1123 | ||
1124 | In GNU C you can give the elements in any order, specifying the array | |
1125 | indices or structure field names they apply to. This extension is not | |
1126 | implemented in GNU C++. | |
1127 | ||
1128 | To specify an array index, write @samp{[@var{index}]} or | |
1129 | @samp{[@var{index}] =} before the element value. For example, | |
1130 | ||
1131 | @example | |
1132 | int a[6] = @{ [4] 29, [2] = 15 @}; | |
1133 | @end example | |
1134 | ||
1135 | @noindent | |
1136 | is equivalent to | |
1137 | ||
1138 | @example | |
1139 | int a[6] = @{ 0, 0, 15, 0, 29, 0 @}; | |
1140 | @end example | |
1141 | ||
1142 | @noindent | |
1143 | The index values must be constant expressions, even if the array being | |
1144 | initialized is automatic. | |
1145 | ||
1146 | To initialize a range of elements to the same value, write | |
1147 | @samp{[@var{first} ... @var{last}] = @var{value}}. For example, | |
1148 | ||
1149 | @example | |
1150 | int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @}; | |
1151 | @end example | |
1152 | ||
1153 | @noindent | |
1154 | Note that the length of the array is the highest value specified | |
1155 | plus one. | |
1156 | ||
1157 | In a structure initializer, specify the name of a field to initialize | |
1158 | with @samp{@var{fieldname}:} before the element value. For example, | |
1159 | given the following structure, | |
1160 | ||
1161 | @example | |
1162 | struct point @{ int x, y; @}; | |
1163 | @end example | |
1164 | ||
1165 | @noindent | |
1166 | the following initialization | |
1167 | ||
1168 | @example | |
1169 | struct point p = @{ y: yvalue, x: xvalue @}; | |
1170 | @end example | |
1171 | ||
1172 | @noindent | |
1173 | is equivalent to | |
1174 | ||
1175 | @example | |
1176 | struct point p = @{ xvalue, yvalue @}; | |
1177 | @end example | |
1178 | ||
1179 | Another syntax which has the same meaning is @samp{.@var{fieldname} =}., | |
1180 | as shown here: | |
1181 | ||
1182 | @example | |
1183 | struct point p = @{ .y = yvalue, .x = xvalue @}; | |
1184 | @end example | |
1185 | ||
1186 | You can also use an element label (with either the colon syntax or the | |
1187 | period-equal syntax) when initializing a union, to specify which element | |
1188 | of the union should be used. For example, | |
1189 | ||
1190 | @example | |
1191 | union foo @{ int i; double d; @}; | |
1192 | ||
1193 | union foo f = @{ d: 4 @}; | |
1194 | @end example | |
1195 | ||
1196 | @noindent | |
1197 | will convert 4 to a @code{double} to store it in the union using | |
1198 | the second element. By contrast, casting 4 to type @code{union foo} | |
1199 | would store it into the union as the integer @code{i}, since it is | |
1200 | an integer. (@xref{Cast to Union}.) | |
1201 | ||
1202 | You can combine this technique of naming elements with ordinary C | |
1203 | initialization of successive elements. Each initializer element that | |
1204 | does not have a label applies to the next consecutive element of the | |
1205 | array or structure. For example, | |
1206 | ||
1207 | @example | |
1208 | int a[6] = @{ [1] = v1, v2, [4] = v4 @}; | |
1209 | @end example | |
1210 | ||
1211 | @noindent | |
1212 | is equivalent to | |
1213 | ||
1214 | @example | |
1215 | int a[6] = @{ 0, v1, v2, 0, v4, 0 @}; | |
1216 | @end example | |
1217 | ||
1218 | Labeling the elements of an array initializer is especially useful | |
1219 | when the indices are characters or belong to an @code{enum} type. | |
1220 | For example: | |
1221 | ||
1222 | @example | |
1223 | int whitespace[256] | |
1224 | = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1, | |
1225 | ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @}; | |
1226 | @end example | |
1227 | ||
1228 | @node Case Ranges | |
1229 | @section Case Ranges | |
1230 | @cindex case ranges | |
1231 | @cindex ranges in case statements | |
1232 | ||
1233 | You can specify a range of consecutive values in a single @code{case} label, | |
1234 | like this: | |
1235 | ||
1236 | @example | |
1237 | case @var{low} ... @var{high}: | |
1238 | @end example | |
1239 | ||
1240 | @noindent | |
1241 | This has the same effect as the proper number of individual @code{case} | |
1242 | labels, one for each integer value from @var{low} to @var{high}, inclusive. | |
1243 | ||
1244 | This feature is especially useful for ranges of ASCII character codes: | |
1245 | ||
1246 | @example | |
1247 | case 'A' ... 'Z': | |
1248 | @end example | |
1249 | ||
1250 | @strong{Be careful:} Write spaces around the @code{...}, for otherwise | |
1251 | it may be parsed wrong when you use it with integer values. For example, | |
1252 | write this: | |
1253 | ||
1254 | @example | |
1255 | case 1 ... 5: | |
1256 | @end example | |
1257 | ||
1258 | @noindent | |
1259 | rather than this: | |
1260 | ||
1261 | @example | |
1262 | case 1...5: | |
1263 | @end example | |
1264 | ||
1265 | @node Cast to Union | |
1266 | @section Cast to a Union Type | |
1267 | @cindex cast to a union | |
1268 | @cindex union, casting to a | |
1269 | ||
1270 | A cast to union type is similar to other casts, except that the type | |
1271 | specified is a union type. You can specify the type either with | |
1272 | @code{union @var{tag}} or with a typedef name. A cast to union is actually | |
1273 | a constructor though, not a cast, and hence does not yield an lvalue like | |
1274 | normal casts. (@xref{Constructors}.) | |
1275 | ||
1276 | The types that may be cast to the union type are those of the members | |
1277 | of the union. Thus, given the following union and variables: | |
1278 | ||
1279 | @example | |
1280 | union foo @{ int i; double d; @}; | |
1281 | int x; | |
1282 | double y; | |
1283 | @end example | |
1284 | ||
1285 | @noindent | |
1286 | both @code{x} and @code{y} can be cast to type @code{union} foo. | |
1287 | ||
1288 | Using the cast as the right-hand side of an assignment to a variable of | |
1289 | union type is equivalent to storing in a member of the union: | |
1290 | ||
1291 | @example | |
1292 | union foo u; | |
1293 | @dots{} | |
1294 | u = (union foo) x @equiv{} u.i = x | |
1295 | u = (union foo) y @equiv{} u.d = y | |
1296 | @end example | |
1297 | ||
1298 | You can also use the union cast as a function argument: | |
1299 | ||
1300 | @example | |
1301 | void hack (union foo); | |
1302 | @dots{} | |
1303 | hack ((union foo) x); | |
1304 | @end example | |
1305 | ||
1306 | @node Function Attributes | |
1307 | @section Declaring Attributes of Functions | |
1308 | @cindex function attributes | |
1309 | @cindex declaring attributes of functions | |
1310 | @cindex functions that never return | |
1311 | @cindex functions that have no side effects | |
1312 | @cindex functions in arbitrary sections | |
140592a0 | 1313 | @cindex functions that bahave like malloc |
c1f7febf RK |
1314 | @cindex @code{volatile} applied to function |
1315 | @cindex @code{const} applied to function | |
bb72a084 | 1316 | @cindex functions with @code{printf}, @code{scanf} or @code{strftime} style arguments |
c1f7febf RK |
1317 | @cindex functions that are passed arguments in registers on the 386 |
1318 | @cindex functions that pop the argument stack on the 386 | |
1319 | @cindex functions that do not pop the argument stack on the 386 | |
1320 | ||
1321 | In GNU C, you declare certain things about functions called in your program | |
1322 | which help the compiler optimize function calls and check your code more | |
1323 | carefully. | |
1324 | ||
1325 | The keyword @code{__attribute__} allows you to specify special | |
1326 | attributes when making a declaration. This keyword is followed by an | |
140592a0 | 1327 | attribute specification inside double parentheses. Ten attributes, |
07417085 | 1328 | @code{noreturn}, @code{const}, @code{format}, |
140592a0 AG |
1329 | @code{no_instrument_function}, @code{section}, @code{constructor}, |
1330 | @code{destructor}, @code{unused}, @code{weak} and @code{malloc} are | |
c1f7febf RK |
1331 | currently defined for functions. Other attributes, including |
1332 | @code{section} are supported for variables declarations (@pxref{Variable | |
1333 | Attributes}) and for types (@pxref{Type Attributes}). | |
1334 | ||
1335 | You may also specify attributes with @samp{__} preceding and following | |
1336 | each keyword. This allows you to use them in header files without | |
1337 | being concerned about a possible macro of the same name. For example, | |
1338 | you may use @code{__noreturn__} instead of @code{noreturn}. | |
1339 | ||
1340 | @table @code | |
1341 | @cindex @code{noreturn} function attribute | |
1342 | @item noreturn | |
1343 | A few standard library functions, such as @code{abort} and @code{exit}, | |
1344 | cannot return. GNU CC knows this automatically. Some programs define | |
1345 | their own functions that never return. You can declare them | |
1346 | @code{noreturn} to tell the compiler this fact. For example, | |
1347 | ||
1348 | @smallexample | |
1349 | void fatal () __attribute__ ((noreturn)); | |
1350 | ||
1351 | void | |
1352 | fatal (@dots{}) | |
1353 | @{ | |
1354 | @dots{} /* @r{Print error message.} */ @dots{} | |
1355 | exit (1); | |
1356 | @} | |
1357 | @end smallexample | |
1358 | ||
1359 | The @code{noreturn} keyword tells the compiler to assume that | |
1360 | @code{fatal} cannot return. It can then optimize without regard to what | |
1361 | would happen if @code{fatal} ever did return. This makes slightly | |
1362 | better code. More importantly, it helps avoid spurious warnings of | |
1363 | uninitialized variables. | |
1364 | ||
1365 | Do not assume that registers saved by the calling function are | |
1366 | restored before calling the @code{noreturn} function. | |
1367 | ||
1368 | It does not make sense for a @code{noreturn} function to have a return | |
1369 | type other than @code{void}. | |
1370 | ||
1371 | The attribute @code{noreturn} is not implemented in GNU C versions | |
1372 | earlier than 2.5. An alternative way to declare that a function does | |
1373 | not return, which works in the current version and in some older | |
1374 | versions, is as follows: | |
1375 | ||
1376 | @smallexample | |
1377 | typedef void voidfn (); | |
1378 | ||
1379 | volatile voidfn fatal; | |
1380 | @end smallexample | |
1381 | ||
2a8f6b90 JH |
1382 | @cindex @code{pure} function attribute |
1383 | @item pure | |
1384 | Many functions have no effects except the return value and their | |
1385 | return value and depends only on the parameters and/or global variables. | |
1386 | Such a function can be subject | |
c1f7febf RK |
1387 | to common subexpression elimination and loop optimization just as an |
1388 | arithmetic operator would be. These functions should be declared | |
2a8f6b90 | 1389 | with the attribute @code{pure}. For example, |
c1f7febf RK |
1390 | |
1391 | @smallexample | |
2a8f6b90 | 1392 | int square (int) __attribute__ ((pure)); |
c1f7febf RK |
1393 | @end smallexample |
1394 | ||
1395 | @noindent | |
1396 | says that the hypothetical function @code{square} is safe to call | |
1397 | fewer times than the program says. | |
1398 | ||
2a8f6b90 JH |
1399 | Some of common examples of pure functions are @code{strlen} or @code{memcmp}. |
1400 | Interesting non-pure functions are functions with infinite loops or those | |
1401 | depending on volatile memory or other system resource, that may change between | |
1402 | two consetuctive calls (such as @code{feof} in multithreding environment). | |
1403 | ||
1404 | The attribute @code{pure} is not implemented in GNU C versions earlier | |
1405 | than 2.96. | |
1406 | @cindex @code{const} function attribute | |
1407 | @item const | |
1408 | Many functions do not examine any values except their arguments, and | |
1409 | have no effects except the return value. Basically this is just slightly | |
1410 | more strict class than the "pure" attribute above, since function is not | |
1411 | alloved to read global memory. | |
1412 | ||
1413 | @cindex pointer arguments | |
1414 | Note that a function that has pointer arguments and examines the data | |
1415 | pointed to must @emph{not} be declared @code{const}. Likewise, a | |
1416 | function that calls a non-@code{const} function usually must not be | |
1417 | @code{const}. It does not make sense for a @code{const} function to | |
1418 | return @code{void}. | |
1419 | ||
c1f7febf RK |
1420 | The attribute @code{const} is not implemented in GNU C versions earlier |
1421 | than 2.5. An alternative way to declare that a function has no side | |
1422 | effects, which works in the current version and in some older versions, | |
1423 | is as follows: | |
1424 | ||
1425 | @smallexample | |
1426 | typedef int intfn (); | |
1427 | ||
1428 | extern const intfn square; | |
1429 | @end smallexample | |
1430 | ||
1431 | This approach does not work in GNU C++ from 2.6.0 on, since the language | |
1432 | specifies that the @samp{const} must be attached to the return value. | |
1433 | ||
c1f7febf RK |
1434 | |
1435 | @item format (@var{archetype}, @var{string-index}, @var{first-to-check}) | |
1436 | @cindex @code{format} function attribute | |
bb72a084 PE |
1437 | The @code{format} attribute specifies that a function takes @code{printf}, |
1438 | @code{scanf}, or @code{strftime} style arguments which should be type-checked | |
1439 | against a format string. For example, the declaration: | |
c1f7febf RK |
1440 | |
1441 | @smallexample | |
1442 | extern int | |
1443 | my_printf (void *my_object, const char *my_format, ...) | |
1444 | __attribute__ ((format (printf, 2, 3))); | |
1445 | @end smallexample | |
1446 | ||
1447 | @noindent | |
1448 | causes the compiler to check the arguments in calls to @code{my_printf} | |
1449 | for consistency with the @code{printf} style format string argument | |
1450 | @code{my_format}. | |
1451 | ||
1452 | The parameter @var{archetype} determines how the format string is | |
bb72a084 PE |
1453 | interpreted, and should be either @code{printf}, @code{scanf}, or |
1454 | @code{strftime}. The | |
c1f7febf RK |
1455 | parameter @var{string-index} specifies which argument is the format |
1456 | string argument (starting from 1), while @var{first-to-check} is the | |
1457 | number of the first argument to check against the format string. For | |
1458 | functions where the arguments are not available to be checked (such as | |
1459 | @code{vprintf}), specify the third parameter as zero. In this case the | |
1460 | compiler only checks the format string for consistency. | |
1461 | ||
1462 | In the example above, the format string (@code{my_format}) is the second | |
1463 | argument of the function @code{my_print}, and the arguments to check | |
1464 | start with the third argument, so the correct parameters for the format | |
1465 | attribute are 2 and 3. | |
1466 | ||
1467 | The @code{format} attribute allows you to identify your own functions | |
1468 | which take format strings as arguments, so that GNU CC can check the | |
1469 | calls to these functions for errors. The compiler always checks formats | |
1470 | for the ANSI library functions @code{printf}, @code{fprintf}, | |
bb72a084 | 1471 | @code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime}, |
c1f7febf RK |
1472 | @code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such |
1473 | warnings are requested (using @samp{-Wformat}), so there is no need to | |
1474 | modify the header file @file{stdio.h}. | |
1475 | ||
1476 | @item format_arg (@var{string-index}) | |
1477 | @cindex @code{format_arg} function attribute | |
1478 | The @code{format_arg} attribute specifies that a function takes | |
1479 | @code{printf} or @code{scanf} style arguments, modifies it (for example, | |
1480 | to translate it into another language), and passes it to a @code{printf} | |
1481 | or @code{scanf} style function. For example, the declaration: | |
1482 | ||
1483 | @smallexample | |
1484 | extern char * | |
1485 | my_dgettext (char *my_domain, const char *my_format) | |
1486 | __attribute__ ((format_arg (2))); | |
1487 | @end smallexample | |
1488 | ||
1489 | @noindent | |
1490 | causes the compiler to check the arguments in calls to | |
bb72a084 PE |
1491 | @code{my_dgettext} whose result is passed to a @code{printf}, |
1492 | @code{scanf}, or @code{strftime} type function for consistency with the | |
1493 | @code{printf} style format string argument @code{my_format}. | |
c1f7febf RK |
1494 | |
1495 | The parameter @var{string-index} specifies which argument is the format | |
1496 | string argument (starting from 1). | |
1497 | ||
1498 | The @code{format-arg} attribute allows you to identify your own | |
1499 | functions which modify format strings, so that GNU CC can check the | |
bb72a084 PE |
1500 | calls to @code{printf}, @code{scanf}, or @code{strftime} function whose |
1501 | operands are a call to one of your own function. The compiler always | |
1502 | treats @code{gettext}, @code{dgettext}, and @code{dcgettext} in this | |
1503 | manner. | |
c1f7febf | 1504 | |
07417085 KR |
1505 | @item no_instrument_function |
1506 | @cindex @code{no_instrument_function} function attribute | |
1507 | If @samp{-finstrument-functions} is given, profiling function calls will | |
1508 | be generated at entry and exit of most user-compiled functions. | |
1509 | Functions with this attribute will not be so instrumented. | |
1510 | ||
c1f7febf RK |
1511 | @item section ("section-name") |
1512 | @cindex @code{section} function attribute | |
1513 | Normally, the compiler places the code it generates in the @code{text} section. | |
1514 | Sometimes, however, you need additional sections, or you need certain | |
1515 | particular functions to appear in special sections. The @code{section} | |
1516 | attribute specifies that a function lives in a particular section. | |
1517 | For example, the declaration: | |
1518 | ||
1519 | @smallexample | |
1520 | extern void foobar (void) __attribute__ ((section ("bar"))); | |
1521 | @end smallexample | |
1522 | ||
1523 | @noindent | |
1524 | puts the function @code{foobar} in the @code{bar} section. | |
1525 | ||
1526 | Some file formats do not support arbitrary sections so the @code{section} | |
1527 | attribute is not available on all platforms. | |
1528 | If you need to map the entire contents of a module to a particular | |
1529 | section, consider using the facilities of the linker instead. | |
1530 | ||
1531 | @item constructor | |
1532 | @itemx destructor | |
1533 | @cindex @code{constructor} function attribute | |
1534 | @cindex @code{destructor} function attribute | |
1535 | The @code{constructor} attribute causes the function to be called | |
1536 | automatically before execution enters @code{main ()}. Similarly, the | |
1537 | @code{destructor} attribute causes the function to be called | |
1538 | automatically after @code{main ()} has completed or @code{exit ()} has | |
1539 | been called. Functions with these attributes are useful for | |
1540 | initializing data that will be used implicitly during the execution of | |
1541 | the program. | |
1542 | ||
1543 | These attributes are not currently implemented for Objective C. | |
1544 | ||
1545 | @item unused | |
1546 | This attribute, attached to a function, means that the function is meant | |
1547 | to be possibly unused. GNU CC will not produce a warning for this | |
1548 | function. GNU C++ does not currently support this attribute as | |
1549 | definitions without parameters are valid in C++. | |
1550 | ||
1551 | @item weak | |
1552 | @cindex @code{weak} attribute | |
1553 | The @code{weak} attribute causes the declaration to be emitted as a weak | |
1554 | symbol rather than a global. This is primarily useful in defining | |
1555 | library functions which can be overridden in user code, though it can | |
1556 | also be used with non-function declarations. Weak symbols are supported | |
1557 | for ELF targets, and also for a.out targets when using the GNU assembler | |
1558 | and linker. | |
1559 | ||
140592a0 AG |
1560 | @item malloc |
1561 | @cindex @code{malloc} attribute | |
1562 | The @code{malloc} attribute is used to tell the compiler that a function | |
1563 | may be treated as if it were the malloc function. The compiler assumes | |
1564 | that calls to malloc result in a pointers that cannot alias anything. | |
1565 | This will often improve optimization. | |
1566 | ||
c1f7febf RK |
1567 | @item alias ("target") |
1568 | @cindex @code{alias} attribute | |
1569 | The @code{alias} attribute causes the declaration to be emitted as an | |
1570 | alias for another symbol, which must be specified. For instance, | |
1571 | ||
1572 | @smallexample | |
1573 | void __f () @{ /* do something */; @} | |
1574 | void f () __attribute__ ((weak, alias ("__f"))); | |
1575 | @end smallexample | |
1576 | ||
1577 | declares @samp{f} to be a weak alias for @samp{__f}. In C++, the | |
1578 | mangled name for the target must be used. | |
1579 | ||
af3e86c2 RK |
1580 | Not all target machines support this attribute. |
1581 | ||
7d384cc0 KR |
1582 | @item no_check_memory_usage |
1583 | @cindex @code{no_check_memory_usage} function attribute | |
c5c76735 JL |
1584 | The @code{no_check_memory_usage} attribute causes GNU CC to omit checks |
1585 | of memory references when it generates code for that function. Normally | |
1586 | if you specify @samp{-fcheck-memory-usage} (see @pxref{Code Gen | |
1587 | Options}), GNU CC generates calls to support routines before most memory | |
1588 | accesses to permit support code to record usage and detect uses of | |
1589 | uninitialized or unallocated storage. Since GNU CC cannot handle | |
1590 | @code{asm} statements properly they are not allowed in such functions. | |
1591 | If you declare a function with this attribute, GNU CC will not generate | |
7d384cc0 | 1592 | memory checking code for that function, permitting the use of @code{asm} |
c5c76735 JL |
1593 | statements without having to compile that function with different |
1594 | options. This also allows you to write support routines of your own if | |
1595 | you wish, without getting infinite recursion if they get compiled with | |
1596 | @code{-fcheck-memory-usage}. | |
7d384cc0 | 1597 | |
c1f7febf RK |
1598 | @item regparm (@var{number}) |
1599 | @cindex functions that are passed arguments in registers on the 386 | |
1600 | On the Intel 386, the @code{regparm} attribute causes the compiler to | |
1601 | pass up to @var{number} integer arguments in registers @var{EAX}, | |
1602 | @var{EDX}, and @var{ECX} instead of on the stack. Functions that take a | |
1603 | variable number of arguments will continue to be passed all of their | |
1604 | arguments on the stack. | |
1605 | ||
1606 | @item stdcall | |
1607 | @cindex functions that pop the argument stack on the 386 | |
1608 | On the Intel 386, the @code{stdcall} attribute causes the compiler to | |
1609 | assume that the called function will pop off the stack space used to | |
1610 | pass arguments, unless it takes a variable number of arguments. | |
1611 | ||
1612 | The PowerPC compiler for Windows NT currently ignores the @code{stdcall} | |
1613 | attribute. | |
1614 | ||
1615 | @item cdecl | |
1616 | @cindex functions that do pop the argument stack on the 386 | |
1617 | On the Intel 386, the @code{cdecl} attribute causes the compiler to | |
1618 | assume that the calling function will pop off the stack space used to | |
1619 | pass arguments. This is | |
1620 | useful to override the effects of the @samp{-mrtd} switch. | |
1621 | ||
1622 | The PowerPC compiler for Windows NT currently ignores the @code{cdecl} | |
1623 | attribute. | |
1624 | ||
1625 | @item longcall | |
1626 | @cindex functions called via pointer on the RS/6000 and PowerPC | |
1627 | On the RS/6000 and PowerPC, the @code{longcall} attribute causes the | |
1628 | compiler to always call the function via a pointer, so that functions | |
1629 | which reside further than 64 megabytes (67,108,864 bytes) from the | |
1630 | current location can be called. | |
1631 | ||
c27ba912 DM |
1632 | @item long_call/short_call |
1633 | @cindex indirect calls on ARM | |
1634 | This attribute allows to specify how to call a particular function on | |
1635 | ARM. Both attributes override the @code{-mlong-calls} (@pxref{ARM Options}) | |
1636 | command line switch and @code{#pragma long_calls} settings. The | |
1637 | @code{long_call} attribute causes the compiler to always call the | |
1638 | function by first loading its address into a register and then using the | |
1639 | contents of that register. The @code{short_call} attribute always places | |
1640 | the offset to the function from the call site into the @samp{BL} | |
1641 | instruction directly. | |
1642 | ||
c1f7febf RK |
1643 | @item dllimport |
1644 | @cindex functions which are imported from a dll on PowerPC Windows NT | |
1645 | On the PowerPC running Windows NT, the @code{dllimport} attribute causes | |
1646 | the compiler to call the function via a global pointer to the function | |
1647 | pointer that is set up by the Windows NT dll library. The pointer name | |
1648 | is formed by combining @code{__imp_} and the function name. | |
1649 | ||
1650 | @item dllexport | |
1651 | @cindex functions which are exported from a dll on PowerPC Windows NT | |
1652 | On the PowerPC running Windows NT, the @code{dllexport} attribute causes | |
1653 | the compiler to provide a global pointer to the function pointer, so | |
1654 | that it can be called with the @code{dllimport} attribute. The pointer | |
1655 | name is formed by combining @code{__imp_} and the function name. | |
1656 | ||
1657 | @item exception (@var{except-func} [, @var{except-arg}]) | |
1658 | @cindex functions which specify exception handling on PowerPC Windows NT | |
1659 | On the PowerPC running Windows NT, the @code{exception} attribute causes | |
1660 | the compiler to modify the structured exception table entry it emits for | |
1661 | the declared function. The string or identifier @var{except-func} is | |
1662 | placed in the third entry of the structured exception table. It | |
1663 | represents a function, which is called by the exception handling | |
1664 | mechanism if an exception occurs. If it was specified, the string or | |
1665 | identifier @var{except-arg} is placed in the fourth entry of the | |
1666 | structured exception table. | |
1667 | ||
1668 | @item function_vector | |
1669 | @cindex calling functions through the function vector on the H8/300 processors | |
1670 | Use this option on the H8/300 and H8/300H to indicate that the specified | |
1671 | function should be called through the function vector. Calling a | |
1672 | function through the function vector will reduce code size, however; | |
1673 | the function vector has a limited size (maximum 128 entries on the H8/300 | |
1674 | and 64 entries on the H8/300H) and shares space with the interrupt vector. | |
1675 | ||
1676 | You must use GAS and GLD from GNU binutils version 2.7 or later for | |
1677 | this option to work correctly. | |
1678 | ||
1679 | @item interrupt_handler | |
1680 | @cindex interrupt handler functions on the H8/300 processors | |
1681 | Use this option on the H8/300 and H8/300H to indicate that the specified | |
1682 | function is an interrupt handler. The compiler will generate function | |
1683 | entry and exit sequences suitable for use in an interrupt handler when this | |
1684 | attribute is present. | |
1685 | ||
1686 | @item eightbit_data | |
1687 | @cindex eight bit data on the H8/300 and H8/300H | |
1688 | Use this option on the H8/300 and H8/300H to indicate that the specified | |
1689 | variable should be placed into the eight bit data section. | |
1690 | The compiler will generate more efficient code for certain operations | |
1691 | on data in the eight bit data area. Note the eight bit data area is limited to | |
1692 | 256 bytes of data. | |
1693 | ||
1694 | You must use GAS and GLD from GNU binutils version 2.7 or later for | |
1695 | this option to work correctly. | |
1696 | ||
1697 | @item tiny_data | |
1698 | @cindex tiny data section on the H8/300H | |
1699 | Use this option on the H8/300H to indicate that the specified | |
1700 | variable should be placed into the tiny data section. | |
1701 | The compiler will generate more efficient code for loads and stores | |
1702 | on data in the tiny data section. Note the tiny data area is limited to | |
1703 | slightly under 32kbytes of data. | |
845da534 DE |
1704 | |
1705 | @item interrupt | |
1706 | @cindex interrupt handlers on the M32R/D | |
1707 | Use this option on the M32R/D to indicate that the specified | |
1708 | function is an interrupt handler. The compiler will generate function | |
1709 | entry and exit sequences suitable for use in an interrupt handler when this | |
1710 | attribute is present. | |
1711 | ||
052a4b28 DC |
1712 | Interrupt handler functions on the AVR processors |
1713 | Use this option on the AVR to indicate that the specified | |
1714 | function is an interrupt handler. The compiler will generate function | |
1715 | entry and exit sequences suitable for use in an interrupt handler when this | |
1716 | attribute is present. Interrupts will be enabled inside function. | |
1717 | ||
1718 | @item signal | |
1719 | @cindex signal handler functions on the AVR processors | |
1720 | Use this option on the AVR to indicate that the specified | |
1721 | function is an signal handler. The compiler will generate function | |
1722 | entry and exit sequences suitable for use in an signal handler when this | |
1723 | attribute is present. Interrupts will be disabled inside function. | |
1724 | ||
1725 | @item naked | |
1726 | @cindex function without a prologue/epilogue code on the AVR processors | |
1727 | Use this option on the AVR to indicate that the specified | |
1728 | function don't have a prologue/epilogue. The compiler don't generate | |
1729 | function entry and exit sequences. | |
1730 | ||
845da534 DE |
1731 | @item model (@var{model-name}) |
1732 | @cindex function addressability on the M32R/D | |
1733 | Use this attribute on the M32R/D to set the addressability of an object, | |
1734 | and the code generated for a function. | |
1735 | The identifier @var{model-name} is one of @code{small}, @code{medium}, | |
1736 | or @code{large}, representing each of the code models. | |
1737 | ||
1738 | Small model objects live in the lower 16MB of memory (so that their | |
1739 | addresses can be loaded with the @code{ld24} instruction), and are | |
1740 | callable with the @code{bl} instruction. | |
1741 | ||
1742 | Medium model objects may live anywhere in the 32 bit address space (the | |
1743 | compiler will generate @code{seth/add3} instructions to load their addresses), | |
1744 | and are callable with the @code{bl} instruction. | |
1745 | ||
1746 | Large model objects may live anywhere in the 32 bit address space (the | |
1747 | compiler will generate @code{seth/add3} instructions to load their addresses), | |
1748 | and may not be reachable with the @code{bl} instruction (the compiler will | |
1749 | generate the much slower @code{seth/add3/jl} instruction sequence). | |
1750 | ||
c1f7febf RK |
1751 | @end table |
1752 | ||
1753 | You can specify multiple attributes in a declaration by separating them | |
1754 | by commas within the double parentheses or by immediately following an | |
1755 | attribute declaration with another attribute declaration. | |
1756 | ||
1757 | @cindex @code{#pragma}, reason for not using | |
1758 | @cindex pragma, reason for not using | |
1759 | Some people object to the @code{__attribute__} feature, suggesting that ANSI C's | |
1760 | @code{#pragma} should be used instead. There are two reasons for not | |
1761 | doing this. | |
1762 | ||
1763 | @enumerate | |
1764 | @item | |
1765 | It is impossible to generate @code{#pragma} commands from a macro. | |
1766 | ||
1767 | @item | |
1768 | There is no telling what the same @code{#pragma} might mean in another | |
1769 | compiler. | |
1770 | @end enumerate | |
1771 | ||
1772 | These two reasons apply to almost any application that might be proposed | |
1773 | for @code{#pragma}. It is basically a mistake to use @code{#pragma} for | |
1774 | @emph{anything}. | |
1775 | ||
1776 | @node Function Prototypes | |
1777 | @section Prototypes and Old-Style Function Definitions | |
1778 | @cindex function prototype declarations | |
1779 | @cindex old-style function definitions | |
1780 | @cindex promotion of formal parameters | |
1781 | ||
1782 | GNU C extends ANSI C to allow a function prototype to override a later | |
1783 | old-style non-prototype definition. Consider the following example: | |
1784 | ||
1785 | @example | |
1786 | /* @r{Use prototypes unless the compiler is old-fashioned.} */ | |
d863830b | 1787 | #ifdef __STDC__ |
c1f7febf RK |
1788 | #define P(x) x |
1789 | #else | |
1790 | #define P(x) () | |
1791 | #endif | |
1792 | ||
1793 | /* @r{Prototype function declaration.} */ | |
1794 | int isroot P((uid_t)); | |
1795 | ||
1796 | /* @r{Old-style function definition.} */ | |
1797 | int | |
1798 | isroot (x) /* ??? lossage here ??? */ | |
1799 | uid_t x; | |
1800 | @{ | |
1801 | return x == 0; | |
1802 | @} | |
1803 | @end example | |
1804 | ||
1805 | Suppose the type @code{uid_t} happens to be @code{short}. ANSI C does | |
1806 | not allow this example, because subword arguments in old-style | |
1807 | non-prototype definitions are promoted. Therefore in this example the | |
1808 | function definition's argument is really an @code{int}, which does not | |
1809 | match the prototype argument type of @code{short}. | |
1810 | ||
1811 | This restriction of ANSI C makes it hard to write code that is portable | |
1812 | to traditional C compilers, because the programmer does not know | |
1813 | whether the @code{uid_t} type is @code{short}, @code{int}, or | |
1814 | @code{long}. Therefore, in cases like these GNU C allows a prototype | |
1815 | to override a later old-style definition. More precisely, in GNU C, a | |
1816 | function prototype argument type overrides the argument type specified | |
1817 | by a later old-style definition if the former type is the same as the | |
1818 | latter type before promotion. Thus in GNU C the above example is | |
1819 | equivalent to the following: | |
1820 | ||
1821 | @example | |
1822 | int isroot (uid_t); | |
1823 | ||
1824 | int | |
1825 | isroot (uid_t x) | |
1826 | @{ | |
1827 | return x == 0; | |
1828 | @} | |
1829 | @end example | |
1830 | ||
1831 | GNU C++ does not support old-style function definitions, so this | |
1832 | extension is irrelevant. | |
1833 | ||
1834 | @node C++ Comments | |
1835 | @section C++ Style Comments | |
1836 | @cindex // | |
1837 | @cindex C++ comments | |
1838 | @cindex comments, C++ style | |
1839 | ||
1840 | In GNU C, you may use C++ style comments, which start with @samp{//} and | |
1841 | continue until the end of the line. Many other C implementations allow | |
1842 | such comments, and they are likely to be in a future C standard. | |
1843 | However, C++ style comments are not recognized if you specify | |
1844 | @w{@samp{-ansi}} or @w{@samp{-traditional}}, since they are incompatible | |
1845 | with traditional constructs like @code{dividend//*comment*/divisor}. | |
1846 | ||
1847 | @node Dollar Signs | |
1848 | @section Dollar Signs in Identifier Names | |
1849 | @cindex $ | |
1850 | @cindex dollar signs in identifier names | |
1851 | @cindex identifier names, dollar signs in | |
1852 | ||
79188db9 RK |
1853 | In GNU C, you may normally use dollar signs in identifier names. |
1854 | This is because many traditional C implementations allow such identifiers. | |
1855 | However, dollar signs in identifiers are not supported on a few target | |
1856 | machines, typically because the target assembler does not allow them. | |
c1f7febf RK |
1857 | |
1858 | @node Character Escapes | |
1859 | @section The Character @key{ESC} in Constants | |
1860 | ||
1861 | You can use the sequence @samp{\e} in a string or character constant to | |
1862 | stand for the ASCII character @key{ESC}. | |
1863 | ||
1864 | @node Alignment | |
1865 | @section Inquiring on Alignment of Types or Variables | |
1866 | @cindex alignment | |
1867 | @cindex type alignment | |
1868 | @cindex variable alignment | |
1869 | ||
1870 | The keyword @code{__alignof__} allows you to inquire about how an object | |
1871 | is aligned, or the minimum alignment usually required by a type. Its | |
1872 | syntax is just like @code{sizeof}. | |
1873 | ||
1874 | For example, if the target machine requires a @code{double} value to be | |
1875 | aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8. | |
1876 | This is true on many RISC machines. On more traditional machine | |
1877 | designs, @code{__alignof__ (double)} is 4 or even 2. | |
1878 | ||
1879 | Some machines never actually require alignment; they allow reference to any | |
1880 | data type even at an odd addresses. For these machines, @code{__alignof__} | |
1881 | reports the @emph{recommended} alignment of a type. | |
1882 | ||
1883 | When the operand of @code{__alignof__} is an lvalue rather than a type, the | |
1884 | value is the largest alignment that the lvalue is known to have. It may | |
1885 | have this alignment as a result of its data type, or because it is part of | |
1886 | a structure and inherits alignment from that structure. For example, after | |
1887 | this declaration: | |
1888 | ||
1889 | @example | |
1890 | struct foo @{ int x; char y; @} foo1; | |
1891 | @end example | |
1892 | ||
1893 | @noindent | |
1894 | the value of @code{__alignof__ (foo1.y)} is probably 2 or 4, the same as | |
1895 | @code{__alignof__ (int)}, even though the data type of @code{foo1.y} | |
1896 | does not itself demand any alignment.@refill | |
1897 | ||
9d27bffe SS |
1898 | It is an error to ask for the alignment of an incomplete type. |
1899 | ||
c1f7febf RK |
1900 | A related feature which lets you specify the alignment of an object is |
1901 | @code{__attribute__ ((aligned (@var{alignment})))}; see the following | |
1902 | section. | |
1903 | ||
1904 | @node Variable Attributes | |
1905 | @section Specifying Attributes of Variables | |
1906 | @cindex attribute of variables | |
1907 | @cindex variable attributes | |
1908 | ||
1909 | The keyword @code{__attribute__} allows you to specify special | |
1910 | attributes of variables or structure fields. This keyword is followed | |
1911 | by an attribute specification inside double parentheses. Eight | |
1912 | attributes are currently defined for variables: @code{aligned}, | |
1913 | @code{mode}, @code{nocommon}, @code{packed}, @code{section}, | |
1914 | @code{transparent_union}, @code{unused}, and @code{weak}. Other | |
1915 | attributes are available for functions (@pxref{Function Attributes}) and | |
1916 | for types (@pxref{Type Attributes}). | |
1917 | ||
1918 | You may also specify attributes with @samp{__} preceding and following | |
1919 | each keyword. This allows you to use them in header files without | |
1920 | being concerned about a possible macro of the same name. For example, | |
1921 | you may use @code{__aligned__} instead of @code{aligned}. | |
1922 | ||
1923 | @table @code | |
1924 | @cindex @code{aligned} attribute | |
1925 | @item aligned (@var{alignment}) | |
1926 | This attribute specifies a minimum alignment for the variable or | |
1927 | structure field, measured in bytes. For example, the declaration: | |
1928 | ||
1929 | @smallexample | |
1930 | int x __attribute__ ((aligned (16))) = 0; | |
1931 | @end smallexample | |
1932 | ||
1933 | @noindent | |
1934 | causes the compiler to allocate the global variable @code{x} on a | |
1935 | 16-byte boundary. On a 68040, this could be used in conjunction with | |
1936 | an @code{asm} expression to access the @code{move16} instruction which | |
1937 | requires 16-byte aligned operands. | |
1938 | ||
1939 | You can also specify the alignment of structure fields. For example, to | |
1940 | create a double-word aligned @code{int} pair, you could write: | |
1941 | ||
1942 | @smallexample | |
1943 | struct foo @{ int x[2] __attribute__ ((aligned (8))); @}; | |
1944 | @end smallexample | |
1945 | ||
1946 | @noindent | |
1947 | This is an alternative to creating a union with a @code{double} member | |
1948 | that forces the union to be double-word aligned. | |
1949 | ||
1950 | It is not possible to specify the alignment of functions; the alignment | |
1951 | of functions is determined by the machine's requirements and cannot be | |
1952 | changed. You cannot specify alignment for a typedef name because such a | |
1953 | name is just an alias, not a distinct type. | |
1954 | ||
1955 | As in the preceding examples, you can explicitly specify the alignment | |
1956 | (in bytes) that you wish the compiler to use for a given variable or | |
1957 | structure field. Alternatively, you can leave out the alignment factor | |
1958 | and just ask the compiler to align a variable or field to the maximum | |
1959 | useful alignment for the target machine you are compiling for. For | |
1960 | example, you could write: | |
1961 | ||
1962 | @smallexample | |
1963 | short array[3] __attribute__ ((aligned)); | |
1964 | @end smallexample | |
1965 | ||
1966 | Whenever you leave out the alignment factor in an @code{aligned} attribute | |
1967 | specification, the compiler automatically sets the alignment for the declared | |
1968 | variable or field to the largest alignment which is ever used for any data | |
1969 | type on the target machine you are compiling for. Doing this can often make | |
1970 | copy operations more efficient, because the compiler can use whatever | |
1971 | instructions copy the biggest chunks of memory when performing copies to | |
1972 | or from the variables or fields that you have aligned this way. | |
1973 | ||
1974 | The @code{aligned} attribute can only increase the alignment; but you | |
1975 | can decrease it by specifying @code{packed} as well. See below. | |
1976 | ||
1977 | Note that the effectiveness of @code{aligned} attributes may be limited | |
1978 | by inherent limitations in your linker. On many systems, the linker is | |
1979 | only able to arrange for variables to be aligned up to a certain maximum | |
1980 | alignment. (For some linkers, the maximum supported alignment may | |
1981 | be very very small.) If your linker is only able to align variables | |
1982 | up to a maximum of 8 byte alignment, then specifying @code{aligned(16)} | |
1983 | in an @code{__attribute__} will still only provide you with 8 byte | |
1984 | alignment. See your linker documentation for further information. | |
1985 | ||
1986 | @item mode (@var{mode}) | |
1987 | @cindex @code{mode} attribute | |
1988 | This attribute specifies the data type for the declaration---whichever | |
1989 | type corresponds to the mode @var{mode}. This in effect lets you | |
1990 | request an integer or floating point type according to its width. | |
1991 | ||
1992 | You may also specify a mode of @samp{byte} or @samp{__byte__} to | |
1993 | indicate the mode corresponding to a one-byte integer, @samp{word} or | |
1994 | @samp{__word__} for the mode of a one-word integer, and @samp{pointer} | |
1995 | or @samp{__pointer__} for the mode used to represent pointers. | |
1996 | ||
1997 | @item nocommon | |
1998 | @cindex @code{nocommon} attribute | |
1999 | This attribute specifies requests GNU CC not to place a variable | |
2000 | ``common'' but instead to allocate space for it directly. If you | |
2001 | specify the @samp{-fno-common} flag, GNU CC will do this for all | |
2002 | variables. | |
2003 | ||
2004 | Specifying the @code{nocommon} attribute for a variable provides an | |
2005 | initialization of zeros. A variable may only be initialized in one | |
2006 | source file. | |
2007 | ||
2008 | @item packed | |
2009 | @cindex @code{packed} attribute | |
2010 | The @code{packed} attribute specifies that a variable or structure field | |
2011 | should have the smallest possible alignment---one byte for a variable, | |
2012 | and one bit for a field, unless you specify a larger value with the | |
2013 | @code{aligned} attribute. | |
2014 | ||
2015 | Here is a structure in which the field @code{x} is packed, so that it | |
2016 | immediately follows @code{a}: | |
2017 | ||
2018 | @example | |
2019 | struct foo | |
2020 | @{ | |
2021 | char a; | |
2022 | int x[2] __attribute__ ((packed)); | |
2023 | @}; | |
2024 | @end example | |
2025 | ||
2026 | @item section ("section-name") | |
2027 | @cindex @code{section} variable attribute | |
2028 | Normally, the compiler places the objects it generates in sections like | |
2029 | @code{data} and @code{bss}. Sometimes, however, you need additional sections, | |
2030 | or you need certain particular variables to appear in special sections, | |
2031 | for example to map to special hardware. The @code{section} | |
2032 | attribute specifies that a variable (or function) lives in a particular | |
2033 | section. For example, this small program uses several specific section names: | |
2034 | ||
2035 | @smallexample | |
2036 | struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @}; | |
2037 | struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @}; | |
2038 | char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @}; | |
2039 | int init_data __attribute__ ((section ("INITDATA"))) = 0; | |
2040 | ||
2041 | main() | |
2042 | @{ | |
2043 | /* Initialize stack pointer */ | |
2044 | init_sp (stack + sizeof (stack)); | |
2045 | ||
2046 | /* Initialize initialized data */ | |
2047 | memcpy (&init_data, &data, &edata - &data); | |
2048 | ||
2049 | /* Turn on the serial ports */ | |
2050 | init_duart (&a); | |
2051 | init_duart (&b); | |
2052 | @} | |
2053 | @end smallexample | |
2054 | ||
2055 | @noindent | |
2056 | Use the @code{section} attribute with an @emph{initialized} definition | |
2057 | of a @emph{global} variable, as shown in the example. GNU CC issues | |
2058 | a warning and otherwise ignores the @code{section} attribute in | |
2059 | uninitialized variable declarations. | |
2060 | ||
2061 | You may only use the @code{section} attribute with a fully initialized | |
2062 | global definition because of the way linkers work. The linker requires | |
2063 | each object be defined once, with the exception that uninitialized | |
2064 | variables tentatively go in the @code{common} (or @code{bss}) section | |
2065 | and can be multiply "defined". You can force a variable to be | |
2066 | initialized with the @samp{-fno-common} flag or the @code{nocommon} | |
2067 | attribute. | |
2068 | ||
2069 | Some file formats do not support arbitrary sections so the @code{section} | |
2070 | attribute is not available on all platforms. | |
2071 | If you need to map the entire contents of a module to a particular | |
2072 | section, consider using the facilities of the linker instead. | |
2073 | ||
593d3a34 MK |
2074 | @item shared |
2075 | @cindex @code{shared} variable attribute | |
2076 | On Windows NT, in addition to nputting variable definitions in a named | |
2077 | section, the section can also be shared among all running copies of an | |
2078 | executable or DLL. For example, this small program defines shared data | |
2079 | by putting it in a named section "shared" and marking the section | |
2080 | shareable: | |
2081 | ||
2082 | @smallexample | |
2083 | int foo __attribute__((section ("shared"), shared)) = 0; | |
2084 | ||
2085 | int | |
2086 | main() | |
2087 | @{ | |
2088 | /* Read and write foo. All running copies see the same value. */ | |
2089 | return 0; | |
2090 | @} | |
2091 | @end smallexample | |
2092 | ||
2093 | @noindent | |
2094 | You may only use the @code{shared} attribute along with @code{section} | |
2095 | attribute with a fully initialized global definition because of the way | |
2096 | linkers work. See @code{section} attribute for more information. | |
2097 | ||
2098 | The @code{shared} attribute is only available on Windows NT. | |
2099 | ||
c1f7febf RK |
2100 | @item transparent_union |
2101 | This attribute, attached to a function parameter which is a union, means | |
2102 | that the corresponding argument may have the type of any union member, | |
2103 | but the argument is passed as if its type were that of the first union | |
2104 | member. For more details see @xref{Type Attributes}. You can also use | |
2105 | this attribute on a @code{typedef} for a union data type; then it | |
2106 | applies to all function parameters with that type. | |
2107 | ||
2108 | @item unused | |
2109 | This attribute, attached to a variable, means that the variable is meant | |
2110 | to be possibly unused. GNU CC will not produce a warning for this | |
2111 | variable. | |
2112 | ||
2113 | @item weak | |
2114 | The @code{weak} attribute is described in @xref{Function Attributes}. | |
845da534 DE |
2115 | |
2116 | @item model (@var{model-name}) | |
2117 | @cindex variable addressability on the M32R/D | |
2118 | Use this attribute on the M32R/D to set the addressability of an object. | |
2119 | The identifier @var{model-name} is one of @code{small}, @code{medium}, | |
2120 | or @code{large}, representing each of the code models. | |
2121 | ||
2122 | Small model objects live in the lower 16MB of memory (so that their | |
2123 | addresses can be loaded with the @code{ld24} instruction). | |
2124 | ||
2125 | Medium and large model objects may live anywhere in the 32 bit address space | |
2126 | (the compiler will generate @code{seth/add3} instructions to load their | |
2127 | addresses). | |
2128 | ||
c1f7febf RK |
2129 | @end table |
2130 | ||
2131 | To specify multiple attributes, separate them by commas within the | |
2132 | double parentheses: for example, @samp{__attribute__ ((aligned (16), | |
2133 | packed))}. | |
2134 | ||
2135 | @node Type Attributes | |
2136 | @section Specifying Attributes of Types | |
2137 | @cindex attribute of types | |
2138 | @cindex type attributes | |
2139 | ||
2140 | The keyword @code{__attribute__} allows you to specify special | |
2141 | attributes of @code{struct} and @code{union} types when you define such | |
2142 | types. This keyword is followed by an attribute specification inside | |
2143 | double parentheses. Three attributes are currently defined for types: | |
2144 | @code{aligned}, @code{packed}, and @code{transparent_union}. Other | |
2145 | attributes are defined for functions (@pxref{Function Attributes}) and | |
2146 | for variables (@pxref{Variable Attributes}). | |
2147 | ||
2148 | You may also specify any one of these attributes with @samp{__} | |
2149 | preceding and following its keyword. This allows you to use these | |
2150 | attributes in header files without being concerned about a possible | |
2151 | macro of the same name. For example, you may use @code{__aligned__} | |
2152 | instead of @code{aligned}. | |
2153 | ||
2154 | You may specify the @code{aligned} and @code{transparent_union} | |
2155 | attributes either in a @code{typedef} declaration or just past the | |
2156 | closing curly brace of a complete enum, struct or union type | |
2157 | @emph{definition} and the @code{packed} attribute only past the closing | |
2158 | brace of a definition. | |
2159 | ||
4051959b JM |
2160 | You may also specify attributes between the enum, struct or union |
2161 | tag and the name of the type rather than after the closing brace. | |
2162 | ||
c1f7febf RK |
2163 | @table @code |
2164 | @cindex @code{aligned} attribute | |
2165 | @item aligned (@var{alignment}) | |
2166 | This attribute specifies a minimum alignment (in bytes) for variables | |
2167 | of the specified type. For example, the declarations: | |
2168 | ||
2169 | @smallexample | |
f69eecfb JL |
2170 | struct S @{ short f[3]; @} __attribute__ ((aligned (8))); |
2171 | typedef int more_aligned_int __attribute__ ((aligned (8))); | |
c1f7febf RK |
2172 | @end smallexample |
2173 | ||
2174 | @noindent | |
d863830b | 2175 | force the compiler to insure (as far as it can) that each variable whose |
c1f7febf RK |
2176 | type is @code{struct S} or @code{more_aligned_int} will be allocated and |
2177 | aligned @emph{at least} on a 8-byte boundary. On a Sparc, having all | |
2178 | variables of type @code{struct S} aligned to 8-byte boundaries allows | |
2179 | the compiler to use the @code{ldd} and @code{std} (doubleword load and | |
2180 | store) instructions when copying one variable of type @code{struct S} to | |
2181 | another, thus improving run-time efficiency. | |
2182 | ||
2183 | Note that the alignment of any given @code{struct} or @code{union} type | |
2184 | is required by the ANSI C standard to be at least a perfect multiple of | |
2185 | the lowest common multiple of the alignments of all of the members of | |
2186 | the @code{struct} or @code{union} in question. This means that you @emph{can} | |
2187 | effectively adjust the alignment of a @code{struct} or @code{union} | |
2188 | type by attaching an @code{aligned} attribute to any one of the members | |
2189 | of such a type, but the notation illustrated in the example above is a | |
2190 | more obvious, intuitive, and readable way to request the compiler to | |
2191 | adjust the alignment of an entire @code{struct} or @code{union} type. | |
2192 | ||
2193 | As in the preceding example, you can explicitly specify the alignment | |
2194 | (in bytes) that you wish the compiler to use for a given @code{struct} | |
2195 | or @code{union} type. Alternatively, you can leave out the alignment factor | |
2196 | and just ask the compiler to align a type to the maximum | |
2197 | useful alignment for the target machine you are compiling for. For | |
2198 | example, you could write: | |
2199 | ||
2200 | @smallexample | |
2201 | struct S @{ short f[3]; @} __attribute__ ((aligned)); | |
2202 | @end smallexample | |
2203 | ||
2204 | Whenever you leave out the alignment factor in an @code{aligned} | |
2205 | attribute specification, the compiler automatically sets the alignment | |
2206 | for the type to the largest alignment which is ever used for any data | |
2207 | type on the target machine you are compiling for. Doing this can often | |
2208 | make copy operations more efficient, because the compiler can use | |
2209 | whatever instructions copy the biggest chunks of memory when performing | |
2210 | copies to or from the variables which have types that you have aligned | |
2211 | this way. | |
2212 | ||
2213 | In the example above, if the size of each @code{short} is 2 bytes, then | |
2214 | the size of the entire @code{struct S} type is 6 bytes. The smallest | |
2215 | power of two which is greater than or equal to that is 8, so the | |
2216 | compiler sets the alignment for the entire @code{struct S} type to 8 | |
2217 | bytes. | |
2218 | ||
2219 | Note that although you can ask the compiler to select a time-efficient | |
2220 | alignment for a given type and then declare only individual stand-alone | |
2221 | objects of that type, the compiler's ability to select a time-efficient | |
2222 | alignment is primarily useful only when you plan to create arrays of | |
2223 | variables having the relevant (efficiently aligned) type. If you | |
2224 | declare or use arrays of variables of an efficiently-aligned type, then | |
2225 | it is likely that your program will also be doing pointer arithmetic (or | |
2226 | subscripting, which amounts to the same thing) on pointers to the | |
2227 | relevant type, and the code that the compiler generates for these | |
2228 | pointer arithmetic operations will often be more efficient for | |
2229 | efficiently-aligned types than for other types. | |
2230 | ||
2231 | The @code{aligned} attribute can only increase the alignment; but you | |
2232 | can decrease it by specifying @code{packed} as well. See below. | |
2233 | ||
2234 | Note that the effectiveness of @code{aligned} attributes may be limited | |
2235 | by inherent limitations in your linker. On many systems, the linker is | |
2236 | only able to arrange for variables to be aligned up to a certain maximum | |
2237 | alignment. (For some linkers, the maximum supported alignment may | |
2238 | be very very small.) If your linker is only able to align variables | |
2239 | up to a maximum of 8 byte alignment, then specifying @code{aligned(16)} | |
2240 | in an @code{__attribute__} will still only provide you with 8 byte | |
2241 | alignment. See your linker documentation for further information. | |
2242 | ||
2243 | @item packed | |
2244 | This attribute, attached to an @code{enum}, @code{struct}, or | |
2245 | @code{union} type definition, specified that the minimum required memory | |
2246 | be used to represent the type. | |
2247 | ||
2248 | Specifying this attribute for @code{struct} and @code{union} types is | |
2249 | equivalent to specifying the @code{packed} attribute on each of the | |
2250 | structure or union members. Specifying the @samp{-fshort-enums} | |
2251 | flag on the line is equivalent to specifying the @code{packed} | |
2252 | attribute on all @code{enum} definitions. | |
2253 | ||
2254 | You may only specify this attribute after a closing curly brace on an | |
1cd4bca9 BK |
2255 | @code{enum} definition, not in a @code{typedef} declaration, unless that |
2256 | declaration also contains the definition of the @code{enum}. | |
c1f7febf RK |
2257 | |
2258 | @item transparent_union | |
2259 | This attribute, attached to a @code{union} type definition, indicates | |
2260 | that any function parameter having that union type causes calls to that | |
2261 | function to be treated in a special way. | |
2262 | ||
2263 | First, the argument corresponding to a transparent union type can be of | |
2264 | any type in the union; no cast is required. Also, if the union contains | |
2265 | a pointer type, the corresponding argument can be a null pointer | |
2266 | constant or a void pointer expression; and if the union contains a void | |
2267 | pointer type, the corresponding argument can be any pointer expression. | |
2268 | If the union member type is a pointer, qualifiers like @code{const} on | |
2269 | the referenced type must be respected, just as with normal pointer | |
2270 | conversions. | |
2271 | ||
2272 | Second, the argument is passed to the function using the calling | |
2273 | conventions of first member of the transparent union, not the calling | |
2274 | conventions of the union itself. All members of the union must have the | |
2275 | same machine representation; this is necessary for this argument passing | |
2276 | to work properly. | |
2277 | ||
2278 | Transparent unions are designed for library functions that have multiple | |
2279 | interfaces for compatibility reasons. For example, suppose the | |
2280 | @code{wait} function must accept either a value of type @code{int *} to | |
2281 | comply with Posix, or a value of type @code{union wait *} to comply with | |
2282 | the 4.1BSD interface. If @code{wait}'s parameter were @code{void *}, | |
2283 | @code{wait} would accept both kinds of arguments, but it would also | |
2284 | accept any other pointer type and this would make argument type checking | |
2285 | less useful. Instead, @code{<sys/wait.h>} might define the interface | |
2286 | as follows: | |
2287 | ||
2288 | @smallexample | |
2289 | typedef union | |
2290 | @{ | |
2291 | int *__ip; | |
2292 | union wait *__up; | |
2293 | @} wait_status_ptr_t __attribute__ ((__transparent_union__)); | |
2294 | ||
2295 | pid_t wait (wait_status_ptr_t); | |
2296 | @end smallexample | |
2297 | ||
2298 | This interface allows either @code{int *} or @code{union wait *} | |
2299 | arguments to be passed, using the @code{int *} calling convention. | |
2300 | The program can call @code{wait} with arguments of either type: | |
2301 | ||
2302 | @example | |
2303 | int w1 () @{ int w; return wait (&w); @} | |
2304 | int w2 () @{ union wait w; return wait (&w); @} | |
2305 | @end example | |
2306 | ||
2307 | With this interface, @code{wait}'s implementation might look like this: | |
2308 | ||
2309 | @example | |
2310 | pid_t wait (wait_status_ptr_t p) | |
2311 | @{ | |
2312 | return waitpid (-1, p.__ip, 0); | |
2313 | @} | |
2314 | @end example | |
d863830b JL |
2315 | |
2316 | @item unused | |
2317 | When attached to a type (including a @code{union} or a @code{struct}), | |
2318 | this attribute means that variables of that type are meant to appear | |
2319 | possibly unused. GNU CC will not produce a warning for any variables of | |
2320 | that type, even if the variable appears to do nothing. This is often | |
2321 | the case with lock or thread classes, which are usually defined and then | |
2322 | not referenced, but contain constructors and destructors that have | |
956d6950 | 2323 | nontrivial bookkeeping functions. |
d863830b | 2324 | |
c1f7febf RK |
2325 | @end table |
2326 | ||
2327 | To specify multiple attributes, separate them by commas within the | |
2328 | double parentheses: for example, @samp{__attribute__ ((aligned (16), | |
2329 | packed))}. | |
2330 | ||
2331 | @node Inline | |
2332 | @section An Inline Function is As Fast As a Macro | |
2333 | @cindex inline functions | |
2334 | @cindex integrating function code | |
2335 | @cindex open coding | |
2336 | @cindex macros, inline alternative | |
2337 | ||
2338 | By declaring a function @code{inline}, you can direct GNU CC to | |
2339 | integrate that function's code into the code for its callers. This | |
2340 | makes execution faster by eliminating the function-call overhead; in | |
2341 | addition, if any of the actual argument values are constant, their known | |
2342 | values may permit simplifications at compile time so that not all of the | |
2343 | inline function's code needs to be included. The effect on code size is | |
2344 | less predictable; object code may be larger or smaller with function | |
2345 | inlining, depending on the particular case. Inlining of functions is an | |
2346 | optimization and it really ``works'' only in optimizing compilation. If | |
2347 | you don't use @samp{-O}, no function is really inline. | |
2348 | ||
2349 | To declare a function inline, use the @code{inline} keyword in its | |
2350 | declaration, like this: | |
2351 | ||
2352 | @example | |
2353 | inline int | |
2354 | inc (int *a) | |
2355 | @{ | |
2356 | (*a)++; | |
2357 | @} | |
2358 | @end example | |
2359 | ||
2360 | (If you are writing a header file to be included in ANSI C programs, write | |
2361 | @code{__inline__} instead of @code{inline}. @xref{Alternate Keywords}.) | |
c1f7febf | 2362 | You can also make all ``simple enough'' functions inline with the option |
247b14bd RH |
2363 | @samp{-finline-functions}. |
2364 | ||
2365 | Note that certain usages in a function definition can make it unsuitable | |
2366 | for inline substitution. Among these usages are: use of varargs, use of | |
2367 | alloca, use of variable sized data types (@pxref{Variable Length}), | |
2368 | use of computed goto (@pxref{Labels as Values}), use of nonlocal goto, | |
2369 | and nested functions (@pxref{Nested Functions}). Using @samp{-Winline} | |
2370 | will warn when a function marked @code{inline} could not be substituted, | |
2371 | and will give the reason for the failure. | |
c1f7febf RK |
2372 | |
2373 | Note that in C and Objective C, unlike C++, the @code{inline} keyword | |
2374 | does not affect the linkage of the function. | |
2375 | ||
2376 | @cindex automatic @code{inline} for C++ member fns | |
2377 | @cindex @code{inline} automatic for C++ member fns | |
2378 | @cindex member fns, automatically @code{inline} | |
2379 | @cindex C++ member fns, automatically @code{inline} | |
2380 | GNU CC automatically inlines member functions defined within the class | |
2381 | body of C++ programs even if they are not explicitly declared | |
2382 | @code{inline}. (You can override this with @samp{-fno-default-inline}; | |
2383 | @pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.) | |
2384 | ||
2385 | @cindex inline functions, omission of | |
2386 | When a function is both inline and @code{static}, if all calls to the | |
2387 | function are integrated into the caller, and the function's address is | |
2388 | never used, then the function's own assembler code is never referenced. | |
2389 | In this case, GNU CC does not actually output assembler code for the | |
2390 | function, unless you specify the option @samp{-fkeep-inline-functions}. | |
2391 | Some calls cannot be integrated for various reasons (in particular, | |
2392 | calls that precede the function's definition cannot be integrated, and | |
2393 | neither can recursive calls within the definition). If there is a | |
2394 | nonintegrated call, then the function is compiled to assembler code as | |
2395 | usual. The function must also be compiled as usual if the program | |
2396 | refers to its address, because that can't be inlined. | |
2397 | ||
2398 | @cindex non-static inline function | |
2399 | When an inline function is not @code{static}, then the compiler must assume | |
2400 | that there may be calls from other source files; since a global symbol can | |
2401 | be defined only once in any program, the function must not be defined in | |
2402 | the other source files, so the calls therein cannot be integrated. | |
2403 | Therefore, a non-@code{static} inline function is always compiled on its | |
2404 | own in the usual fashion. | |
2405 | ||
2406 | If you specify both @code{inline} and @code{extern} in the function | |
2407 | definition, then the definition is used only for inlining. In no case | |
2408 | is the function compiled on its own, not even if you refer to its | |
2409 | address explicitly. Such an address becomes an external reference, as | |
2410 | if you had only declared the function, and had not defined it. | |
2411 | ||
2412 | This combination of @code{inline} and @code{extern} has almost the | |
2413 | effect of a macro. The way to use it is to put a function definition in | |
2414 | a header file with these keywords, and put another copy of the | |
2415 | definition (lacking @code{inline} and @code{extern}) in a library file. | |
2416 | The definition in the header file will cause most calls to the function | |
2417 | to be inlined. If any uses of the function remain, they will refer to | |
2418 | the single copy in the library. | |
2419 | ||
2420 | GNU C does not inline any functions when not optimizing. It is not | |
2421 | clear whether it is better to inline or not, in this case, but we found | |
2422 | that a correct implementation when not optimizing was difficult. So we | |
2423 | did the easy thing, and turned it off. | |
2424 | ||
2425 | @node Extended Asm | |
2426 | @section Assembler Instructions with C Expression Operands | |
2427 | @cindex extended @code{asm} | |
2428 | @cindex @code{asm} expressions | |
2429 | @cindex assembler instructions | |
2430 | @cindex registers | |
2431 | ||
c85f7c16 JL |
2432 | In an assembler instruction using @code{asm}, you can specify the |
2433 | operands of the instruction using C expressions. This means you need not | |
2434 | guess which registers or memory locations will contain the data you want | |
c1f7febf RK |
2435 | to use. |
2436 | ||
c85f7c16 JL |
2437 | You must specify an assembler instruction template much like what |
2438 | appears in a machine description, plus an operand constraint string for | |
2439 | each operand. | |
c1f7febf RK |
2440 | |
2441 | For example, here is how to use the 68881's @code{fsinx} instruction: | |
2442 | ||
2443 | @example | |
2444 | asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); | |
2445 | @end example | |
2446 | ||
2447 | @noindent | |
2448 | Here @code{angle} is the C expression for the input operand while | |
2449 | @code{result} is that of the output operand. Each has @samp{"f"} as its | |
c85f7c16 JL |
2450 | operand constraint, saying that a floating point register is required. |
2451 | The @samp{=} in @samp{=f} indicates that the operand is an output; all | |
2452 | output operands' constraints must use @samp{=}. The constraints use the | |
2453 | same language used in the machine description (@pxref{Constraints}). | |
2454 | ||
2455 | Each operand is described by an operand-constraint string followed by | |
2456 | the C expression in parentheses. A colon separates the assembler | |
2457 | template from the first output operand and another separates the last | |
2458 | output operand from the first input, if any. Commas separate the | |
2459 | operands within each group. The total number of operands is limited to | |
2460 | ten or to the maximum number of operands in any instruction pattern in | |
2461 | the machine description, whichever is greater. | |
2462 | ||
2463 | If there are no output operands but there are input operands, you must | |
2464 | place two consecutive colons surrounding the place where the output | |
c1f7febf RK |
2465 | operands would go. |
2466 | ||
2467 | Output operand expressions must be lvalues; the compiler can check this. | |
c85f7c16 JL |
2468 | The input operands need not be lvalues. The compiler cannot check |
2469 | whether the operands have data types that are reasonable for the | |
2470 | instruction being executed. It does not parse the assembler instruction | |
2471 | template and does not know what it means or even whether it is valid | |
2472 | assembler input. The extended @code{asm} feature is most often used for | |
2473 | machine instructions the compiler itself does not know exist. If | |
2474 | the output expression cannot be directly addressed (for example, it is a | |
2475 | bit field), your constraint must allow a register. In that case, GNU CC | |
2476 | will use the register as the output of the @code{asm}, and then store | |
2477 | that register into the output. | |
2478 | ||
2479 | The ordinary output operands must be write-only; GNU CC will assume that | |
2480 | the values in these operands before the instruction are dead and need | |
2481 | not be generated. Extended asm supports input-output or read-write | |
2482 | operands. Use the constraint character @samp{+} to indicate such an | |
2483 | operand and list it with the output operands. | |
2484 | ||
2485 | When the constraints for the read-write operand (or the operand in which | |
2486 | only some of the bits are to be changed) allows a register, you may, as | |
2487 | an alternative, logically split its function into two separate operands, | |
2488 | one input operand and one write-only output operand. The connection | |
2489 | between them is expressed by constraints which say they need to be in | |
2490 | the same location when the instruction executes. You can use the same C | |
2491 | expression for both operands, or different expressions. For example, | |
2492 | here we write the (fictitious) @samp{combine} instruction with | |
2493 | @code{bar} as its read-only source operand and @code{foo} as its | |
2494 | read-write destination: | |
c1f7febf RK |
2495 | |
2496 | @example | |
2497 | asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); | |
2498 | @end example | |
2499 | ||
2500 | @noindent | |
c85f7c16 JL |
2501 | The constraint @samp{"0"} for operand 1 says that it must occupy the |
2502 | same location as operand 0. A digit in constraint is allowed only in an | |
2503 | input operand and it must refer to an output operand. | |
c1f7febf RK |
2504 | |
2505 | Only a digit in the constraint can guarantee that one operand will be in | |
c85f7c16 JL |
2506 | the same place as another. The mere fact that @code{foo} is the value |
2507 | of both operands is not enough to guarantee that they will be in the | |
2508 | same place in the generated assembler code. The following would not | |
2509 | work reliably: | |
c1f7febf RK |
2510 | |
2511 | @example | |
2512 | asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); | |
2513 | @end example | |
2514 | ||
2515 | Various optimizations or reloading could cause operands 0 and 1 to be in | |
2516 | different registers; GNU CC knows no reason not to do so. For example, the | |
2517 | compiler might find a copy of the value of @code{foo} in one register and | |
2518 | use it for operand 1, but generate the output operand 0 in a different | |
2519 | register (copying it afterward to @code{foo}'s own address). Of course, | |
2520 | since the register for operand 1 is not even mentioned in the assembler | |
2521 | code, the result will not work, but GNU CC can't tell that. | |
2522 | ||
c85f7c16 JL |
2523 | Some instructions clobber specific hard registers. To describe this, |
2524 | write a third colon after the input operands, followed by the names of | |
2525 | the clobbered hard registers (given as strings). Here is a realistic | |
2526 | example for the VAX: | |
c1f7febf RK |
2527 | |
2528 | @example | |
2529 | asm volatile ("movc3 %0,%1,%2" | |
2530 | : /* no outputs */ | |
2531 | : "g" (from), "g" (to), "g" (count) | |
2532 | : "r0", "r1", "r2", "r3", "r4", "r5"); | |
2533 | @end example | |
2534 | ||
c5c76735 JL |
2535 | You may not write a clobber description in a way that overlaps with an |
2536 | input or output operand. For example, you may not have an operand | |
2537 | describing a register class with one member if you mention that register | |
2538 | in the clobber list. There is no way for you to specify that an input | |
2539 | operand is modified without also specifying it as an output | |
2540 | operand. Note that if all the output operands you specify are for this | |
2541 | purpose (and hence unused), you will then also need to specify | |
2542 | @code{volatile} for the @code{asm} construct, as described below, to | |
2543 | prevent GNU CC from deleting the @code{asm} statement as unused. | |
8fe1938e | 2544 | |
c1f7febf | 2545 | If you refer to a particular hardware register from the assembler code, |
c85f7c16 JL |
2546 | you will probably have to list the register after the third colon to |
2547 | tell the compiler the register's value is modified. In some assemblers, | |
2548 | the register names begin with @samp{%}; to produce one @samp{%} in the | |
2549 | assembler code, you must write @samp{%%} in the input. | |
2550 | ||
2551 | If your assembler instruction can alter the condition code register, add | |
2552 | @samp{cc} to the list of clobbered registers. GNU CC on some machines | |
2553 | represents the condition codes as a specific hardware register; | |
2554 | @samp{cc} serves to name this register. On other machines, the | |
2555 | condition code is handled differently, and specifying @samp{cc} has no | |
2556 | effect. But it is valid no matter what the machine. | |
c1f7febf RK |
2557 | |
2558 | If your assembler instruction modifies memory in an unpredictable | |
c85f7c16 JL |
2559 | fashion, add @samp{memory} to the list of clobbered registers. This |
2560 | will cause GNU CC to not keep memory values cached in registers across | |
2561 | the assembler instruction. | |
c1f7febf | 2562 | |
c85f7c16 JL |
2563 | You can put multiple assembler instructions together in a single |
2564 | @code{asm} template, separated either with newlines (written as | |
2565 | @samp{\n}) or with semicolons if the assembler allows such semicolons. | |
2566 | The GNU assembler allows semicolons and most Unix assemblers seem to do | |
2567 | so. The input operands are guaranteed not to use any of the clobbered | |
2568 | registers, and neither will the output operands' addresses, so you can | |
2569 | read and write the clobbered registers as many times as you like. Here | |
2570 | is an example of multiple instructions in a template; it assumes the | |
2571 | subroutine @code{_foo} accepts arguments in registers 9 and 10: | |
c1f7febf RK |
2572 | |
2573 | @example | |
2574 | asm ("movl %0,r9;movl %1,r10;call _foo" | |
2575 | : /* no outputs */ | |
2576 | : "g" (from), "g" (to) | |
2577 | : "r9", "r10"); | |
2578 | @end example | |
2579 | ||
c85f7c16 JL |
2580 | Unless an output operand has the @samp{&} constraint modifier, GNU CC |
2581 | may allocate it in the same register as an unrelated input operand, on | |
2582 | the assumption the inputs are consumed before the outputs are produced. | |
c1f7febf RK |
2583 | This assumption may be false if the assembler code actually consists of |
2584 | more than one instruction. In such a case, use @samp{&} for each output | |
c85f7c16 | 2585 | operand that may not overlap an input. @xref{Modifiers}. |
c1f7febf | 2586 | |
c85f7c16 JL |
2587 | If you want to test the condition code produced by an assembler |
2588 | instruction, you must include a branch and a label in the @code{asm} | |
2589 | construct, as follows: | |
c1f7febf RK |
2590 | |
2591 | @example | |
2592 | asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:" | |
2593 | : "g" (result) | |
2594 | : "g" (input)); | |
2595 | @end example | |
2596 | ||
2597 | @noindent | |
2598 | This assumes your assembler supports local labels, as the GNU assembler | |
2599 | and most Unix assemblers do. | |
2600 | ||
2601 | Speaking of labels, jumps from one @code{asm} to another are not | |
c85f7c16 JL |
2602 | supported. The compiler's optimizers do not know about these jumps, and |
2603 | therefore they cannot take account of them when deciding how to | |
c1f7febf RK |
2604 | optimize. |
2605 | ||
2606 | @cindex macros containing @code{asm} | |
2607 | Usually the most convenient way to use these @code{asm} instructions is to | |
2608 | encapsulate them in macros that look like functions. For example, | |
2609 | ||
2610 | @example | |
2611 | #define sin(x) \ | |
2612 | (@{ double __value, __arg = (x); \ | |
2613 | asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ | |
2614 | __value; @}) | |
2615 | @end example | |
2616 | ||
2617 | @noindent | |
2618 | Here the variable @code{__arg} is used to make sure that the instruction | |
2619 | operates on a proper @code{double} value, and to accept only those | |
2620 | arguments @code{x} which can convert automatically to a @code{double}. | |
2621 | ||
c85f7c16 JL |
2622 | Another way to make sure the instruction operates on the correct data |
2623 | type is to use a cast in the @code{asm}. This is different from using a | |
c1f7febf RK |
2624 | variable @code{__arg} in that it converts more different types. For |
2625 | example, if the desired type were @code{int}, casting the argument to | |
2626 | @code{int} would accept a pointer with no complaint, while assigning the | |
2627 | argument to an @code{int} variable named @code{__arg} would warn about | |
2628 | using a pointer unless the caller explicitly casts it. | |
2629 | ||
2630 | If an @code{asm} has output operands, GNU CC assumes for optimization | |
c85f7c16 JL |
2631 | purposes the instruction has no side effects except to change the output |
2632 | operands. This does not mean instructions with a side effect cannot be | |
2633 | used, but you must be careful, because the compiler may eliminate them | |
2634 | if the output operands aren't used, or move them out of loops, or | |
2635 | replace two with one if they constitute a common subexpression. Also, | |
2636 | if your instruction does have a side effect on a variable that otherwise | |
2637 | appears not to change, the old value of the variable may be reused later | |
2638 | if it happens to be found in a register. | |
c1f7febf RK |
2639 | |
2640 | You can prevent an @code{asm} instruction from being deleted, moved | |
2641 | significantly, or combined, by writing the keyword @code{volatile} after | |
2642 | the @code{asm}. For example: | |
2643 | ||
2644 | @example | |
c85f7c16 JL |
2645 | #define get_and_set_priority(new) \ |
2646 | (@{ int __old; \ | |
2647 | asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \ | |
2648 | __old; @}) | |
24f98470 | 2649 | @end example |
c1f7febf RK |
2650 | |
2651 | @noindent | |
c85f7c16 JL |
2652 | If you write an @code{asm} instruction with no outputs, GNU CC will know |
2653 | the instruction has side-effects and will not delete the instruction or | |
2654 | move it outside of loops. If the side-effects of your instruction are | |
2655 | not purely external, but will affect variables in your program in ways | |
2656 | other than reading the inputs and clobbering the specified registers or | |
2657 | memory, you should write the @code{volatile} keyword to prevent future | |
2658 | versions of GNU CC from moving the instruction around within a core | |
2659 | region. | |
2660 | ||
2661 | An @code{asm} instruction without any operands or clobbers (and ``old | |
2662 | style'' @code{asm}) will not be deleted or moved significantly, | |
2663 | regardless, unless it is unreachable, the same wasy as if you had | |
2664 | written a @code{volatile} keyword. | |
c1f7febf RK |
2665 | |
2666 | Note that even a volatile @code{asm} instruction can be moved in ways | |
2667 | that appear insignificant to the compiler, such as across jump | |
2668 | instructions. You can't expect a sequence of volatile @code{asm} | |
2669 | instructions to remain perfectly consecutive. If you want consecutive | |
2670 | output, use a single @code{asm}. | |
2671 | ||
2672 | It is a natural idea to look for a way to give access to the condition | |
2673 | code left by the assembler instruction. However, when we attempted to | |
2674 | implement this, we found no way to make it work reliably. The problem | |
2675 | is that output operands might need reloading, which would result in | |
2676 | additional following ``store'' instructions. On most machines, these | |
2677 | instructions would alter the condition code before there was time to | |
2678 | test it. This problem doesn't arise for ordinary ``test'' and | |
2679 | ``compare'' instructions because they don't have any output operands. | |
2680 | ||
2681 | If you are writing a header file that should be includable in ANSI C | |
2682 | programs, write @code{__asm__} instead of @code{asm}. @xref{Alternate | |
2683 | Keywords}. | |
2684 | ||
fe0ce426 JH |
2685 | @subsection i386 floating point asm operands |
2686 | ||
2687 | There are several rules on the usage of stack-like regs in | |
2688 | asm_operands insns. These rules apply only to the operands that are | |
2689 | stack-like regs: | |
2690 | ||
2691 | @enumerate | |
2692 | @item | |
2693 | Given a set of input regs that die in an asm_operands, it is | |
2694 | necessary to know which are implicitly popped by the asm, and | |
2695 | which must be explicitly popped by gcc. | |
2696 | ||
2697 | An input reg that is implicitly popped by the asm must be | |
2698 | explicitly clobbered, unless it is constrained to match an | |
2699 | output operand. | |
2700 | ||
2701 | @item | |
2702 | For any input reg that is implicitly popped by an asm, it is | |
2703 | necessary to know how to adjust the stack to compensate for the pop. | |
2704 | If any non-popped input is closer to the top of the reg-stack than | |
2705 | the implicitly popped reg, it would not be possible to know what the | |
2706 | stack looked like --- it's not clear how the rest of the stack ``slides | |
2707 | up''. | |
2708 | ||
2709 | All implicitly popped input regs must be closer to the top of | |
2710 | the reg-stack than any input that is not implicitly popped. | |
2711 | ||
2712 | It is possible that if an input dies in an insn, reload might | |
2713 | use the input reg for an output reload. Consider this example: | |
2714 | ||
2715 | @example | |
2716 | asm ("foo" : "=t" (a) : "f" (b)); | |
2717 | @end example | |
2718 | ||
2719 | This asm says that input B is not popped by the asm, and that | |
2720 | the asm pushes a result onto the reg-stack, ie, the stack is one | |
2721 | deeper after the asm than it was before. But, it is possible that | |
2722 | reload will think that it can use the same reg for both the input and | |
2723 | the output, if input B dies in this insn. | |
2724 | ||
2725 | If any input operand uses the @code{f} constraint, all output reg | |
2726 | constraints must use the @code{&} earlyclobber. | |
2727 | ||
2728 | The asm above would be written as | |
2729 | ||
2730 | @example | |
2731 | asm ("foo" : "=&t" (a) : "f" (b)); | |
2732 | @end example | |
2733 | ||
2734 | @item | |
2735 | Some operands need to be in particular places on the stack. All | |
2736 | output operands fall in this category --- there is no other way to | |
2737 | know which regs the outputs appear in unless the user indicates | |
2738 | this in the constraints. | |
2739 | ||
2740 | Output operands must specifically indicate which reg an output | |
2741 | appears in after an asm. @code{=f} is not allowed: the operand | |
2742 | constraints must select a class with a single reg. | |
2743 | ||
2744 | @item | |
2745 | Output operands may not be ``inserted'' between existing stack regs. | |
2746 | Since no 387 opcode uses a read/write operand, all output operands | |
2747 | are dead before the asm_operands, and are pushed by the asm_operands. | |
2748 | It makes no sense to push anywhere but the top of the reg-stack. | |
2749 | ||
2750 | Output operands must start at the top of the reg-stack: output | |
2751 | operands may not ``skip'' a reg. | |
2752 | ||
2753 | @item | |
2754 | Some asm statements may need extra stack space for internal | |
2755 | calculations. This can be guaranteed by clobbering stack registers | |
2756 | unrelated to the inputs and outputs. | |
2757 | ||
2758 | @end enumerate | |
2759 | ||
2760 | Here are a couple of reasonable asms to want to write. This asm | |
2761 | takes one input, which is internally popped, and produces two outputs. | |
2762 | ||
2763 | @example | |
2764 | asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp)); | |
2765 | @end example | |
2766 | ||
2767 | This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode, | |
2768 | and replaces them with one output. The user must code the @code{st(1)} | |
2769 | clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs. | |
2770 | ||
2771 | @example | |
2772 | asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)"); | |
2773 | @end example | |
2774 | ||
c1f7febf RK |
2775 | @ifclear INTERNALS |
2776 | @c Show the details on constraints if they do not appear elsewhere in | |
2777 | @c the manual | |
2778 | @include md.texi | |
2779 | @end ifclear | |
2780 | ||
2781 | @node Asm Labels | |
2782 | @section Controlling Names Used in Assembler Code | |
2783 | @cindex assembler names for identifiers | |
2784 | @cindex names used in assembler code | |
2785 | @cindex identifiers, names in assembler code | |
2786 | ||
2787 | You can specify the name to be used in the assembler code for a C | |
2788 | function or variable by writing the @code{asm} (or @code{__asm__}) | |
2789 | keyword after the declarator as follows: | |
2790 | ||
2791 | @example | |
2792 | int foo asm ("myfoo") = 2; | |
2793 | @end example | |
2794 | ||
2795 | @noindent | |
2796 | This specifies that the name to be used for the variable @code{foo} in | |
2797 | the assembler code should be @samp{myfoo} rather than the usual | |
2798 | @samp{_foo}. | |
2799 | ||
2800 | On systems where an underscore is normally prepended to the name of a C | |
2801 | function or variable, this feature allows you to define names for the | |
2802 | linker that do not start with an underscore. | |
2803 | ||
2804 | You cannot use @code{asm} in this way in a function @emph{definition}; but | |
2805 | you can get the same effect by writing a declaration for the function | |
2806 | before its definition and putting @code{asm} there, like this: | |
2807 | ||
2808 | @example | |
2809 | extern func () asm ("FUNC"); | |
2810 | ||
2811 | func (x, y) | |
2812 | int x, y; | |
2813 | @dots{} | |
2814 | @end example | |
2815 | ||
2816 | It is up to you to make sure that the assembler names you choose do not | |
2817 | conflict with any other assembler symbols. Also, you must not use a | |
2818 | register name; that would produce completely invalid assembler code. GNU | |
2819 | CC does not as yet have the ability to store static variables in registers. | |
2820 | Perhaps that will be added. | |
2821 | ||
2822 | @node Explicit Reg Vars | |
2823 | @section Variables in Specified Registers | |
2824 | @cindex explicit register variables | |
2825 | @cindex variables in specified registers | |
2826 | @cindex specified registers | |
2827 | @cindex registers, global allocation | |
2828 | ||
2829 | GNU C allows you to put a few global variables into specified hardware | |
2830 | registers. You can also specify the register in which an ordinary | |
2831 | register variable should be allocated. | |
2832 | ||
2833 | @itemize @bullet | |
2834 | @item | |
2835 | Global register variables reserve registers throughout the program. | |
2836 | This may be useful in programs such as programming language | |
2837 | interpreters which have a couple of global variables that are accessed | |
2838 | very often. | |
2839 | ||
2840 | @item | |
2841 | Local register variables in specific registers do not reserve the | |
2842 | registers. The compiler's data flow analysis is capable of determining | |
2843 | where the specified registers contain live values, and where they are | |
8d344fbc | 2844 | available for other uses. Stores into local register variables may be deleted |
0deaf590 JL |
2845 | when they appear to be dead according to dataflow analysis. References |
2846 | to local register variables may be deleted or moved or simplified. | |
c1f7febf RK |
2847 | |
2848 | These local variables are sometimes convenient for use with the extended | |
2849 | @code{asm} feature (@pxref{Extended Asm}), if you want to write one | |
2850 | output of the assembler instruction directly into a particular register. | |
2851 | (This will work provided the register you specify fits the constraints | |
2852 | specified for that operand in the @code{asm}.) | |
2853 | @end itemize | |
2854 | ||
2855 | @menu | |
2856 | * Global Reg Vars:: | |
2857 | * Local Reg Vars:: | |
2858 | @end menu | |
2859 | ||
2860 | @node Global Reg Vars | |
2861 | @subsection Defining Global Register Variables | |
2862 | @cindex global register variables | |
2863 | @cindex registers, global variables in | |
2864 | ||
2865 | You can define a global register variable in GNU C like this: | |
2866 | ||
2867 | @example | |
2868 | register int *foo asm ("a5"); | |
2869 | @end example | |
2870 | ||
2871 | @noindent | |
2872 | Here @code{a5} is the name of the register which should be used. Choose a | |
2873 | register which is normally saved and restored by function calls on your | |
2874 | machine, so that library routines will not clobber it. | |
2875 | ||
2876 | Naturally the register name is cpu-dependent, so you would need to | |
2877 | conditionalize your program according to cpu type. The register | |
2878 | @code{a5} would be a good choice on a 68000 for a variable of pointer | |
2879 | type. On machines with register windows, be sure to choose a ``global'' | |
2880 | register that is not affected magically by the function call mechanism. | |
2881 | ||
2882 | In addition, operating systems on one type of cpu may differ in how they | |
2883 | name the registers; then you would need additional conditionals. For | |
2884 | example, some 68000 operating systems call this register @code{%a5}. | |
2885 | ||
2886 | Eventually there may be a way of asking the compiler to choose a register | |
2887 | automatically, but first we need to figure out how it should choose and | |
2888 | how to enable you to guide the choice. No solution is evident. | |
2889 | ||
2890 | Defining a global register variable in a certain register reserves that | |
2891 | register entirely for this use, at least within the current compilation. | |
2892 | The register will not be allocated for any other purpose in the functions | |
2893 | in the current compilation. The register will not be saved and restored by | |
2894 | these functions. Stores into this register are never deleted even if they | |
2895 | would appear to be dead, but references may be deleted or moved or | |
2896 | simplified. | |
2897 | ||
2898 | It is not safe to access the global register variables from signal | |
2899 | handlers, or from more than one thread of control, because the system | |
2900 | library routines may temporarily use the register for other things (unless | |
2901 | you recompile them specially for the task at hand). | |
2902 | ||
2903 | @cindex @code{qsort}, and global register variables | |
2904 | It is not safe for one function that uses a global register variable to | |
2905 | call another such function @code{foo} by way of a third function | |
2906 | @code{lose} that was compiled without knowledge of this variable (i.e. in a | |
2907 | different source file in which the variable wasn't declared). This is | |
2908 | because @code{lose} might save the register and put some other value there. | |
2909 | For example, you can't expect a global register variable to be available in | |
2910 | the comparison-function that you pass to @code{qsort}, since @code{qsort} | |
2911 | might have put something else in that register. (If you are prepared to | |
2912 | recompile @code{qsort} with the same global register variable, you can | |
2913 | solve this problem.) | |
2914 | ||
2915 | If you want to recompile @code{qsort} or other source files which do not | |
2916 | actually use your global register variable, so that they will not use that | |
2917 | register for any other purpose, then it suffices to specify the compiler | |
2918 | option @samp{-ffixed-@var{reg}}. You need not actually add a global | |
2919 | register declaration to their source code. | |
2920 | ||
2921 | A function which can alter the value of a global register variable cannot | |
2922 | safely be called from a function compiled without this variable, because it | |
2923 | could clobber the value the caller expects to find there on return. | |
2924 | Therefore, the function which is the entry point into the part of the | |
2925 | program that uses the global register variable must explicitly save and | |
2926 | restore the value which belongs to its caller. | |
2927 | ||
2928 | @cindex register variable after @code{longjmp} | |
2929 | @cindex global register after @code{longjmp} | |
2930 | @cindex value after @code{longjmp} | |
2931 | @findex longjmp | |
2932 | @findex setjmp | |
2933 | On most machines, @code{longjmp} will restore to each global register | |
2934 | variable the value it had at the time of the @code{setjmp}. On some | |
2935 | machines, however, @code{longjmp} will not change the value of global | |
2936 | register variables. To be portable, the function that called @code{setjmp} | |
2937 | should make other arrangements to save the values of the global register | |
2938 | variables, and to restore them in a @code{longjmp}. This way, the same | |
2939 | thing will happen regardless of what @code{longjmp} does. | |
2940 | ||
2941 | All global register variable declarations must precede all function | |
2942 | definitions. If such a declaration could appear after function | |
2943 | definitions, the declaration would be too late to prevent the register from | |
2944 | being used for other purposes in the preceding functions. | |
2945 | ||
2946 | Global register variables may not have initial values, because an | |
2947 | executable file has no means to supply initial contents for a register. | |
2948 | ||
2949 | On the Sparc, there are reports that g3 @dots{} g7 are suitable | |
2950 | registers, but certain library functions, such as @code{getwd}, as well | |
2951 | as the subroutines for division and remainder, modify g3 and g4. g1 and | |
2952 | g2 are local temporaries. | |
2953 | ||
2954 | On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7. | |
2955 | Of course, it will not do to use more than a few of those. | |
2956 | ||
2957 | @node Local Reg Vars | |
2958 | @subsection Specifying Registers for Local Variables | |
2959 | @cindex local variables, specifying registers | |
2960 | @cindex specifying registers for local variables | |
2961 | @cindex registers for local variables | |
2962 | ||
2963 | You can define a local register variable with a specified register | |
2964 | like this: | |
2965 | ||
2966 | @example | |
2967 | register int *foo asm ("a5"); | |
2968 | @end example | |
2969 | ||
2970 | @noindent | |
2971 | Here @code{a5} is the name of the register which should be used. Note | |
2972 | that this is the same syntax used for defining global register | |
2973 | variables, but for a local variable it would appear within a function. | |
2974 | ||
2975 | Naturally the register name is cpu-dependent, but this is not a | |
2976 | problem, since specific registers are most often useful with explicit | |
2977 | assembler instructions (@pxref{Extended Asm}). Both of these things | |
2978 | generally require that you conditionalize your program according to | |
2979 | cpu type. | |
2980 | ||
2981 | In addition, operating systems on one type of cpu may differ in how they | |
2982 | name the registers; then you would need additional conditionals. For | |
2983 | example, some 68000 operating systems call this register @code{%a5}. | |
2984 | ||
c1f7febf RK |
2985 | Defining such a register variable does not reserve the register; it |
2986 | remains available for other uses in places where flow control determines | |
2987 | the variable's value is not live. However, these registers are made | |
e5e809f4 JL |
2988 | unavailable for use in the reload pass; excessive use of this feature |
2989 | leaves the compiler too few available registers to compile certain | |
2990 | functions. | |
2991 | ||
2992 | This option does not guarantee that GNU CC will generate code that has | |
2993 | this variable in the register you specify at all times. You may not | |
2994 | code an explicit reference to this register in an @code{asm} statement | |
2995 | and assume it will always refer to this variable. | |
c1f7febf | 2996 | |
8d344fbc | 2997 | Stores into local register variables may be deleted when they appear to be dead |
0deaf590 JL |
2998 | according to dataflow analysis. References to local register variables may |
2999 | be deleted or moved or simplified. | |
3000 | ||
c1f7febf RK |
3001 | @node Alternate Keywords |
3002 | @section Alternate Keywords | |
3003 | @cindex alternate keywords | |
3004 | @cindex keywords, alternate | |
3005 | ||
3006 | The option @samp{-traditional} disables certain keywords; @samp{-ansi} | |
3007 | disables certain others. This causes trouble when you want to use GNU C | |
3008 | extensions, or ANSI C features, in a general-purpose header file that | |
3009 | should be usable by all programs, including ANSI C programs and traditional | |
3010 | ones. The keywords @code{asm}, @code{typeof} and @code{inline} cannot be | |
3011 | used since they won't work in a program compiled with @samp{-ansi}, while | |
3012 | the keywords @code{const}, @code{volatile}, @code{signed}, @code{typeof} | |
3013 | and @code{inline} won't work in a program compiled with | |
3014 | @samp{-traditional}.@refill | |
3015 | ||
3016 | The way to solve these problems is to put @samp{__} at the beginning and | |
3017 | end of each problematical keyword. For example, use @code{__asm__} | |
3018 | instead of @code{asm}, @code{__const__} instead of @code{const}, and | |
3019 | @code{__inline__} instead of @code{inline}. | |
3020 | ||
3021 | Other C compilers won't accept these alternative keywords; if you want to | |
3022 | compile with another compiler, you can define the alternate keywords as | |
3023 | macros to replace them with the customary keywords. It looks like this: | |
3024 | ||
3025 | @example | |
3026 | #ifndef __GNUC__ | |
3027 | #define __asm__ asm | |
3028 | #endif | |
3029 | @end example | |
3030 | ||
6e6b0525 | 3031 | @findex __extension__ |
f1b5ff21 | 3032 | @samp{-pedantic} and other options cause warnings for many GNU C extensions. |
dbe519e0 | 3033 | You can |
c1f7febf RK |
3034 | prevent such warnings within one expression by writing |
3035 | @code{__extension__} before the expression. @code{__extension__} has no | |
3036 | effect aside from this. | |
3037 | ||
3038 | @node Incomplete Enums | |
3039 | @section Incomplete @code{enum} Types | |
3040 | ||
3041 | You can define an @code{enum} tag without specifying its possible values. | |
3042 | This results in an incomplete type, much like what you get if you write | |
3043 | @code{struct foo} without describing the elements. A later declaration | |
3044 | which does specify the possible values completes the type. | |
3045 | ||
3046 | You can't allocate variables or storage using the type while it is | |
3047 | incomplete. However, you can work with pointers to that type. | |
3048 | ||
3049 | This extension may not be very useful, but it makes the handling of | |
3050 | @code{enum} more consistent with the way @code{struct} and @code{union} | |
3051 | are handled. | |
3052 | ||
3053 | This extension is not supported by GNU C++. | |
3054 | ||
3055 | @node Function Names | |
3056 | @section Function Names as Strings | |
3057 | ||
22acfb79 NM |
3058 | GNU CC predefines two magic identifiers to hold the name of the current |
3059 | function. The identifier @code{__FUNCTION__} holds the name of the function | |
3060 | as it appears in the source. The identifier @code{__PRETTY_FUNCTION__} | |
3061 | holds the name of the function pretty printed in a language specific | |
3062 | fashion. | |
c1f7febf RK |
3063 | |
3064 | These names are always the same in a C function, but in a C++ function | |
3065 | they may be different. For example, this program: | |
3066 | ||
3067 | @smallexample | |
3068 | extern "C" @{ | |
3069 | extern int printf (char *, ...); | |
3070 | @} | |
3071 | ||
3072 | class a @{ | |
3073 | public: | |
3074 | sub (int i) | |
3075 | @{ | |
3076 | printf ("__FUNCTION__ = %s\n", __FUNCTION__); | |
3077 | printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); | |
3078 | @} | |
3079 | @}; | |
3080 | ||
3081 | int | |
3082 | main (void) | |
3083 | @{ | |
3084 | a ax; | |
3085 | ax.sub (0); | |
3086 | return 0; | |
3087 | @} | |
3088 | @end smallexample | |
3089 | ||
3090 | @noindent | |
3091 | gives this output: | |
3092 | ||
3093 | @smallexample | |
3094 | __FUNCTION__ = sub | |
3095 | __PRETTY_FUNCTION__ = int a::sub (int) | |
3096 | @end smallexample | |
3097 | ||
22acfb79 NM |
3098 | The compiler automagically replaces the identifiers with a string |
3099 | literal containing the appropriate name. Thus, they are neither | |
3100 | preprocessor macros, like @code{__FILE__} and @code{__LINE__}, nor | |
3101 | variables. This means that they catenate with other string literals, and | |
3102 | that they can be used to initialize char arrays. For example | |
3103 | ||
3104 | @smallexample | |
3105 | char here[] = "Function " __FUNCTION__ " in " __FILE__; | |
3106 | @end smallexample | |
3107 | ||
3108 | On the other hand, @samp{#ifdef __FUNCTION__} does not have any special | |
c1f7febf RK |
3109 | meaning inside a function, since the preprocessor does not do anything |
3110 | special with the identifier @code{__FUNCTION__}. | |
3111 | ||
22acfb79 | 3112 | GNU CC also supports the magic word @code{__func__}, defined by the |
34527c47 | 3113 | ISO standard C-99: |
22acfb79 NM |
3114 | |
3115 | @display | |
3116 | The identifier @code{__func__} is implicitly declared by the translator | |
3117 | as if, immediately following the opening brace of each function | |
3118 | definition, the declaration | |
3119 | ||
3120 | @smallexample | |
3121 | static const char __func__[] = "function-name"; | |
3122 | @end smallexample | |
3123 | ||
3124 | appeared, where function-name is the name of the lexically-enclosing | |
3125 | function. This name is the unadorned name of the function. | |
3126 | @end display | |
3127 | ||
3128 | By this definition, @code{__func__} is a variable, not a string literal. | |
3129 | In particular, @code{__func__} does not catenate with other string | |
3130 | literals. | |
3131 | ||
3132 | In @code{C++}, @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} are | |
3133 | variables, declared in the same way as @code{__func__}. | |
3134 | ||
c1f7febf RK |
3135 | @node Return Address |
3136 | @section Getting the Return or Frame Address of a Function | |
3137 | ||
3138 | These functions may be used to get information about the callers of a | |
3139 | function. | |
3140 | ||
3141 | @table @code | |
185ebd6c | 3142 | @findex __builtin_return_address |
c1f7febf RK |
3143 | @item __builtin_return_address (@var{level}) |
3144 | This function returns the return address of the current function, or of | |
3145 | one of its callers. The @var{level} argument is number of frames to | |
3146 | scan up the call stack. A value of @code{0} yields the return address | |
3147 | of the current function, a value of @code{1} yields the return address | |
3148 | of the caller of the current function, and so forth. | |
3149 | ||
3150 | The @var{level} argument must be a constant integer. | |
3151 | ||
3152 | On some machines it may be impossible to determine the return address of | |
3153 | any function other than the current one; in such cases, or when the top | |
3154 | of the stack has been reached, this function will return @code{0}. | |
3155 | ||
3156 | This function should only be used with a non-zero argument for debugging | |
3157 | purposes. | |
3158 | ||
185ebd6c | 3159 | @findex __builtin_frame_address |
c1f7febf RK |
3160 | @item __builtin_frame_address (@var{level}) |
3161 | This function is similar to @code{__builtin_return_address}, but it | |
3162 | returns the address of the function frame rather than the return address | |
3163 | of the function. Calling @code{__builtin_frame_address} with a value of | |
3164 | @code{0} yields the frame address of the current function, a value of | |
3165 | @code{1} yields the frame address of the caller of the current function, | |
3166 | and so forth. | |
3167 | ||
3168 | The frame is the area on the stack which holds local variables and saved | |
3169 | registers. The frame address is normally the address of the first word | |
3170 | pushed on to the stack by the function. However, the exact definition | |
3171 | depends upon the processor and the calling convention. If the processor | |
3172 | has a dedicated frame pointer register, and the function has a frame, | |
3173 | then @code{__builtin_frame_address} will return the value of the frame | |
3174 | pointer register. | |
3175 | ||
3176 | The caveats that apply to @code{__builtin_return_address} apply to this | |
3177 | function as well. | |
3178 | @end table | |
3179 | ||
185ebd6c RH |
3180 | @node Other Builtins |
3181 | @section Other built-in functions provided by GNU CC | |
3182 | ||
3183 | GNU CC provides a large number of built-in functions other than the ones | |
3184 | mentioned above. Some of these are for internal use in the processing | |
3185 | of exceptions or variable-length argument lists and will not be | |
3186 | documented here because they may change from time to time; we do not | |
3187 | recommend general use of these functions. | |
3188 | ||
3189 | The remaining functions are provided for optimization purposes. | |
3190 | ||
3191 | GNU CC includes builtin versions of many of the functions in the | |
3192 | standard C library. These will always be treated as having the same | |
3193 | meaning as the C library function even if you specify the | |
3194 | @samp{-fno-builtin} (@pxref{C Dialect Options}) option. These functions | |
349c29b5 HPN |
3195 | correspond to the C library functions @code{abort}, @code{abs}, |
3196 | @code{alloca}, @code{cos}, @code{cosf}, @code{cosl}, @code{exit}, | |
3197 | @code{_exit}, @code{fabs}, @code{fabsf}, @code{fabsl}, @code{ffs}, | |
3198 | @code{labs}, @code{memcmp}, @code{memcpy}, @code{memset}, @code{sin}, | |
3199 | @code{sinf}, @code{sinl}, @code{sqrt}, @code{sqrtf}, @code{sqrtl}, | |
3200 | @code{strcmp}, @code{strcpy}, and @code{strlen}. | |
185ebd6c | 3201 | |
994a57cd | 3202 | @table @code |
185ebd6c | 3203 | @findex __builtin_constant_p |
994a57cd | 3204 | @item __builtin_constant_p (@var{exp}) |
185ebd6c RH |
3205 | You can use the builtin function @code{__builtin_constant_p} to |
3206 | determine if a value is known to be constant at compile-time and hence | |
3207 | that GNU CC can perform constant-folding on expressions involving that | |
3208 | value. The argument of the function is the value to test. The function | |
3209 | returns the integer 1 if the argument is known to be a compile-time | |
3210 | constant and 0 if it is not known to be a compile-time constant. A | |
3211 | return of 0 does not indicate that the value is @emph{not} a constant, | |
3212 | but merely that GNU CC cannot prove it is a constant with the specified | |
3213 | value of the @samp{-O} option. | |
3214 | ||
3215 | You would typically use this function in an embedded application where | |
3216 | memory was a critical resource. If you have some complex calculation, | |
3217 | you may want it to be folded if it involves constants, but need to call | |
3218 | a function if it does not. For example: | |
3219 | ||
4d390518 | 3220 | @smallexample |
185ebd6c RH |
3221 | #define Scale_Value(X) \ |
3222 | (__builtin_constant_p (X) ? ((X) * SCALE + OFFSET) : Scale (X)) | |
3223 | @end smallexample | |
3224 | ||
3225 | You may use this builtin function in either a macro or an inline | |
3226 | function. However, if you use it in an inlined function and pass an | |
3227 | argument of the function as the argument to the builtin, GNU CC will | |
3228 | never return 1 when you call the inline function with a string constant | |
3229 | or constructor expression (@pxref{Constructors}) and will not return 1 | |
3230 | when you pass a constant numeric value to the inline function unless you | |
3231 | specify the @samp{-O} option. | |
3232 | ||
994a57cd RH |
3233 | @findex __builtin_expect |
3234 | @item __builtin_expect(@var{exp}, @var{c}) | |
3235 | You may use @code{__builtin_expect} to provide the compiler with | |
3236 | branch prediction information. In general, you should prefer to | |
3237 | use actual profile feedback for this (@samp{-fprofile-arcs}), as | |
3238 | programmers are notoriously bad at predicting how their programs | |
3239 | actually preform. However, there are applications in which this | |
3240 | data is hard to collect. | |
3241 | ||
3242 | The return value is the value of @var{exp}, which should be an | |
3243 | integral expression. The value of @var{c} must be a compile-time | |
3244 | constant. The semantics of the builtin are that it is expected | |
3245 | that @var{exp} == @var{c}. For example: | |
3246 | ||
3247 | @smallexample | |
3248 | if (__builtin_expect (x, 0)) | |
3249 | foo (); | |
3250 | @end smallexample | |
3251 | ||
3252 | @noindent | |
3253 | would indicate that we do not expect to call @code{foo}, since | |
3254 | we expect @code{x} to be zero. Since you are limited to integral | |
3255 | expressions for @var{exp}, you should use constructions such as | |
3256 | ||
3257 | @smallexample | |
3258 | if (__builtin_expect (ptr != NULL, 1)) | |
3259 | error (); | |
3260 | @end smallexample | |
3261 | ||
3262 | @noindent | |
3263 | when testing pointer or floating-point values. | |
3264 | @end table | |
3265 | ||
2de45c06 ML |
3266 | @node Deprecated Features |
3267 | @section Deprecated Features | |
3268 | ||
3269 | In the past, the GNU C++ compiler was extended to experiment with new | |
3270 | features, at a time when the C++ language was still evolving. Now that | |
4a21803f | 3271 | the C++ standard is complete, some of those features are superseded by |
2de45c06 ML |
3272 | superior alternatives. Using the old features might cause a warning in |
3273 | some cases that the feature will be dropped in the future. In other | |
3274 | cases, the feature might be gone already. | |
3275 | ||
3276 | While the list below is not exhaustive, it documents some of the options | |
3277 | that are now deprecated: | |
3278 | ||
69942c20 | 3279 | @table @code |
2de45c06 ML |
3280 | @item -fexternal-templates |
3281 | @itemx -falt-external-templates | |
3282 | These are two of the many ways for g++ to implement template | |
3283 | instantiation. @xref{Template Instantiation}. The C++ standard clearly | |
3284 | defines how template definitions have to be organized across | |
3285 | implementation units. g++ has an implicit instantiation mechanism that | |
3286 | should work just fine for standard-conforming code. | |
3287 | ||
3288 | @end table | |
3289 | ||
c1f7febf RK |
3290 | @node C++ Extensions |
3291 | @chapter Extensions to the C++ Language | |
3292 | @cindex extensions, C++ language | |
3293 | @cindex C++ language extensions | |
3294 | ||
3295 | The GNU compiler provides these extensions to the C++ language (and you | |
3296 | can also use most of the C language extensions in your C++ programs). If you | |
3297 | want to write code that checks whether these features are available, you can | |
3298 | test for the GNU compiler the same way as for C programs: check for a | |
3299 | predefined macro @code{__GNUC__}. You can also use @code{__GNUG__} to | |
3300 | test specifically for GNU C++ (@pxref{Standard Predefined,,Standard | |
3301 | Predefined Macros,cpp.info,The C Preprocessor}). | |
3302 | ||
3303 | @menu | |
3304 | * Naming Results:: Giving a name to C++ function return values. | |
3305 | * Min and Max:: C++ Minimum and maximum operators. | |
02cac427 | 3306 | * Volatiles:: What constitutes an access to a volatile object. |
535233a8 | 3307 | * Restricted Pointers:: C9X restricted pointers and references. |
c1f7febf RK |
3308 | * C++ Interface:: You can use a single C++ header file for both |
3309 | declarations and definitions. | |
3310 | * Template Instantiation:: Methods for ensuring that exactly one copy of | |
3311 | each needed template instantiation is emitted. | |
0ded1f18 JM |
3312 | * Bound member functions:: You can extract a function pointer to the |
3313 | method denoted by a @samp{->*} or @samp{.*} expression. | |
c1f7febf RK |
3314 | @end menu |
3315 | ||
3316 | @node Naming Results | |
3317 | @section Named Return Values in C++ | |
3318 | ||
3319 | @cindex @code{return}, in C++ function header | |
3320 | @cindex return value, named, in C++ | |
3321 | @cindex named return value in C++ | |
3322 | @cindex C++ named return value | |
3323 | GNU C++ extends the function-definition syntax to allow you to specify a | |
3324 | name for the result of a function outside the body of the definition, in | |
3325 | C++ programs: | |
3326 | ||
3327 | @example | |
3328 | @group | |
3329 | @var{type} | |
3330 | @var{functionname} (@var{args}) return @var{resultname}; | |
3331 | @{ | |
3332 | @dots{} | |
3333 | @var{body} | |
3334 | @dots{} | |
3335 | @} | |
3336 | @end group | |
3337 | @end example | |
3338 | ||
3339 | You can use this feature to avoid an extra constructor call when | |
3340 | a function result has a class type. For example, consider a function | |
3341 | @code{m}, declared as @w{@samp{X v = m ();}}, whose result is of class | |
3342 | @code{X}: | |
3343 | ||
3344 | @example | |
3345 | X | |
3346 | m () | |
3347 | @{ | |
3348 | X b; | |
3349 | b.a = 23; | |
3350 | return b; | |
3351 | @} | |
3352 | @end example | |
3353 | ||
3354 | @cindex implicit argument: return value | |
3355 | Although @code{m} appears to have no arguments, in fact it has one implicit | |
3356 | argument: the address of the return value. At invocation, the address | |
3357 | of enough space to hold @code{v} is sent in as the implicit argument. | |
3358 | Then @code{b} is constructed and its @code{a} field is set to the value | |
3359 | 23. Finally, a copy constructor (a constructor of the form @samp{X(X&)}) | |
3360 | is applied to @code{b}, with the (implicit) return value location as the | |
3361 | target, so that @code{v} is now bound to the return value. | |
3362 | ||
3363 | But this is wasteful. The local @code{b} is declared just to hold | |
3364 | something that will be copied right out. While a compiler that | |
3365 | combined an ``elision'' algorithm with interprocedural data flow | |
3366 | analysis could conceivably eliminate all of this, it is much more | |
3367 | practical to allow you to assist the compiler in generating | |
3368 | efficient code by manipulating the return value explicitly, | |
3369 | thus avoiding the local variable and copy constructor altogether. | |
3370 | ||
3371 | Using the extended GNU C++ function-definition syntax, you can avoid the | |
3372 | temporary allocation and copying by naming @code{r} as your return value | |
3373 | at the outset, and assigning to its @code{a} field directly: | |
3374 | ||
3375 | @example | |
3376 | X | |
3377 | m () return r; | |
3378 | @{ | |
3379 | r.a = 23; | |
3380 | @} | |
3381 | @end example | |
3382 | ||
3383 | @noindent | |
3384 | The declaration of @code{r} is a standard, proper declaration, whose effects | |
3385 | are executed @strong{before} any of the body of @code{m}. | |
3386 | ||
3387 | Functions of this type impose no additional restrictions; in particular, | |
3388 | you can execute @code{return} statements, or return implicitly by | |
3389 | reaching the end of the function body (``falling off the edge''). | |
3390 | Cases like | |
3391 | ||
3392 | @example | |
3393 | X | |
3394 | m () return r (23); | |
3395 | @{ | |
3396 | return; | |
3397 | @} | |
3398 | @end example | |
3399 | ||
3400 | @noindent | |
3401 | (or even @w{@samp{X m () return r (23); @{ @}}}) are unambiguous, since | |
3402 | the return value @code{r} has been initialized in either case. The | |
3403 | following code may be hard to read, but also works predictably: | |
3404 | ||
3405 | @example | |
3406 | X | |
3407 | m () return r; | |
3408 | @{ | |
3409 | X b; | |
3410 | return b; | |
3411 | @} | |
3412 | @end example | |
3413 | ||
3414 | The return value slot denoted by @code{r} is initialized at the outset, | |
3415 | but the statement @samp{return b;} overrides this value. The compiler | |
3416 | deals with this by destroying @code{r} (calling the destructor if there | |
3417 | is one, or doing nothing if there is not), and then reinitializing | |
3418 | @code{r} with @code{b}. | |
3419 | ||
3420 | This extension is provided primarily to help people who use overloaded | |
3421 | operators, where there is a great need to control not just the | |
3422 | arguments, but the return values of functions. For classes where the | |
3423 | copy constructor incurs a heavy performance penalty (especially in the | |
3424 | common case where there is a quick default constructor), this is a major | |
3425 | savings. The disadvantage of this extension is that you do not control | |
3426 | when the default constructor for the return value is called: it is | |
3427 | always called at the beginning. | |
3428 | ||
3429 | @node Min and Max | |
3430 | @section Minimum and Maximum Operators in C++ | |
3431 | ||
3432 | It is very convenient to have operators which return the ``minimum'' or the | |
3433 | ``maximum'' of two arguments. In GNU C++ (but not in GNU C), | |
3434 | ||
3435 | @table @code | |
3436 | @item @var{a} <? @var{b} | |
3437 | @findex <? | |
3438 | @cindex minimum operator | |
3439 | is the @dfn{minimum}, returning the smaller of the numeric values | |
3440 | @var{a} and @var{b}; | |
3441 | ||
3442 | @item @var{a} >? @var{b} | |
3443 | @findex >? | |
3444 | @cindex maximum operator | |
3445 | is the @dfn{maximum}, returning the larger of the numeric values @var{a} | |
3446 | and @var{b}. | |
3447 | @end table | |
3448 | ||
3449 | These operations are not primitive in ordinary C++, since you can | |
3450 | use a macro to return the minimum of two things in C++, as in the | |
3451 | following example. | |
3452 | ||
3453 | @example | |
3454 | #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) | |
3455 | @end example | |
3456 | ||
3457 | @noindent | |
3458 | You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to | |
3459 | the minimum value of variables @var{i} and @var{j}. | |
3460 | ||
3461 | However, side effects in @code{X} or @code{Y} may cause unintended | |
3462 | behavior. For example, @code{MIN (i++, j++)} will fail, incrementing | |
3463 | the smaller counter twice. A GNU C extension allows you to write safe | |
3464 | macros that avoid this kind of problem (@pxref{Naming Types,,Naming an | |
3465 | Expression's Type}). However, writing @code{MIN} and @code{MAX} as | |
3466 | macros also forces you to use function-call notation for a | |
3467 | fundamental arithmetic operation. Using GNU C++ extensions, you can | |
3468 | write @w{@samp{int min = i <? j;}} instead. | |
3469 | ||
3470 | Since @code{<?} and @code{>?} are built into the compiler, they properly | |
3471 | handle expressions with side-effects; @w{@samp{int min = i++ <? j++;}} | |
3472 | works correctly. | |
3473 | ||
02cac427 NS |
3474 | @node Volatiles |
3475 | @section When is a Volatile Object Accessed? | |
3476 | @cindex accessing volatiles | |
3477 | @cindex volatile read | |
3478 | @cindex volatile write | |
3479 | @cindex volatile access | |
3480 | ||
3481 | Both the C and C++ standard have the concept of volatile objects. These | |
3482 | are normally accessed by pointers and used for accessing hardware. The | |
3483 | standards encourage compilers to refrain from optimizations on | |
3484 | concerning accesses to volatile objects that it might perform on | |
3485 | non-volatile objects. The C standard leaves it implementation defined | |
3486 | as to what constitutes a volatile access. The C++ standard omits to | |
3487 | specify this, except to say that C++ should behave in a similar manner | |
3488 | to C with respect to volatiles, where possible. The minimum either | |
3489 | standard specifies is that at a sequence point all previous access to | |
3490 | volatile objects have stabilized and no subsequent accesses have | |
3491 | occurred. Thus an implementation is free to reorder and combine | |
3492 | volatile accesses which occur between sequence points, but cannot do so | |
3493 | for accesses across a sequence point. The use of volatiles does not | |
3494 | allow you to violate the restriction on updating objects multiple times | |
3495 | within a sequence point. | |
3496 | ||
3497 | In most expressions, it is intuitively obvious what is a read and what is | |
3498 | a write. For instance | |
3499 | ||
3500 | @example | |
3501 | volatile int *dst = <somevalue>; | |
3502 | volatile int *src = <someothervalue>; | |
3503 | *dst = *src; | |
3504 | @end example | |
3505 | ||
3506 | @noindent | |
3507 | will cause a read of the volatile object pointed to by @var{src} and stores the | |
3508 | value into the volatile object pointed to by @var{dst}. There is no | |
3509 | guarantee that these reads and writes are atomic, especially for objects | |
3510 | larger than @code{int}. | |
3511 | ||
3512 | Less obvious expressions are where something which looks like an access | |
3513 | is used in a void context. An example would be, | |
3514 | ||
3515 | @example | |
3516 | volatile int *src = <somevalue>; | |
3517 | *src; | |
3518 | @end example | |
3519 | ||
3520 | With C, such expressions are rvalues, and as rvalues cause a read of | |
3521 | the object, gcc interprets this as a read of the volatile being pointed | |
3522 | to. The C++ standard specifies that such expressions do not undergo | |
3523 | lvalue to rvalue conversion, and that the type of the dereferenced | |
3524 | object may be incomplete. The C++ standard does not specify explicitly | |
3525 | that it is this lvalue to rvalue conversion which is responsible for | |
3526 | causing an access. However, there is reason to believe that it is, | |
3527 | because otherwise certain simple expressions become undefined. However, | |
3528 | because it would surprise most programmers, g++ treats dereferencing a | |
3529 | pointer to volatile object of complete type in a void context as a read | |
3530 | of the object. When the object has incomplete type, g++ issues a | |
3531 | warning. | |
3532 | ||
3533 | @example | |
3534 | struct S; | |
3535 | struct T @{int m;@}; | |
3536 | volatile S *ptr1 = <somevalue>; | |
3537 | volatile T *ptr2 = <somevalue>; | |
3538 | *ptr1; | |
3539 | *ptr2; | |
3540 | @end example | |
3541 | ||
3542 | In this example, a warning is issued for @code{*ptr1}, and @code{*ptr2} | |
3543 | causes a read of the object pointed to. If you wish to force an error on | |
3544 | the first case, you must force a conversion to rvalue with, for instance | |
3545 | a static cast, @code{static_cast<S>(*ptr1)}. | |
3546 | ||
3547 | When using a reference to volatile, g++ does not treat equivalent | |
3548 | expressions as accesses to volatiles, but instead issues a warning that | |
3549 | no volatile is accessed. The rationale for this is that otherwise it | |
3550 | becomes difficult to determine where volatile access occur, and not | |
3551 | possible to ignore the return value from functions returning volatile | |
3552 | references. Again, if you wish to force a read, cast the reference to | |
3553 | an rvalue. | |
3554 | ||
535233a8 NS |
3555 | @node Restricted Pointers |
3556 | @section Restricting Pointer Aliasing | |
3557 | @cindex restricted pointers | |
3558 | @cindex restricted references | |
3559 | @cindex restricted this pointer | |
3560 | ||
3561 | As with gcc, g++ understands the C9X proposal of restricted pointers, | |
3562 | specified with the @code{__restrict__}, or @code{__restrict} type | |
3563 | qualifier. Because you cannot compile C++ by specifying the -flang-isoc9x | |
3564 | language flag, @code{restrict} is not a keyword in C++. | |
3565 | ||
3566 | In addition to allowing restricted pointers, you can specify restricted | |
3567 | references, which indicate that the reference is not aliased in the local | |
3568 | context. | |
3569 | ||
3570 | @example | |
3571 | void fn (int *__restrict__ rptr, int &__restrict__ rref) | |
3572 | @{ | |
3573 | @dots{} | |
3574 | @} | |
3575 | @end example | |
3576 | ||
3577 | @noindent | |
3578 | In the body of @code{fn}, @var{rptr} points to an unaliased integer and | |
3579 | @var{rref} refers to a (different) unaliased integer. | |
3580 | ||
3581 | You may also specify whether a member function's @var{this} pointer is | |
3582 | unaliased by using @code{__restrict__} as a member function qualifier. | |
3583 | ||
3584 | @example | |
3585 | void T::fn () __restrict__ | |
3586 | @{ | |
3587 | @dots{} | |
3588 | @} | |
3589 | @end example | |
3590 | ||
3591 | @noindent | |
3592 | Within the body of @code{T::fn}, @var{this} will have the effective | |
3593 | definition @code{T *__restrict__ const this}. Notice that the | |
3594 | interpretation of a @code{__restrict__} member function qualifier is | |
3595 | different to that of @code{const} or @code{volatile} qualifier, in that it | |
3596 | is applied to the pointer rather than the object. This is consistent with | |
3597 | other compilers which implement restricted pointers. | |
3598 | ||
3599 | As with all outermost parameter qualifiers, @code{__restrict__} is | |
3600 | ignored in function definition matching. This means you only need to | |
3601 | specify @code{__restrict__} in a function definition, rather than | |
3602 | in a function prototype as well. | |
3603 | ||
c1f7febf RK |
3604 | @node C++ Interface |
3605 | @section Declarations and Definitions in One Header | |
3606 | ||
3607 | @cindex interface and implementation headers, C++ | |
3608 | @cindex C++ interface and implementation headers | |
3609 | C++ object definitions can be quite complex. In principle, your source | |
3610 | code will need two kinds of things for each object that you use across | |
3611 | more than one source file. First, you need an @dfn{interface} | |
3612 | specification, describing its structure with type declarations and | |
3613 | function prototypes. Second, you need the @dfn{implementation} itself. | |
3614 | It can be tedious to maintain a separate interface description in a | |
3615 | header file, in parallel to the actual implementation. It is also | |
3616 | dangerous, since separate interface and implementation definitions may | |
3617 | not remain parallel. | |
3618 | ||
3619 | @cindex pragmas, interface and implementation | |
3620 | With GNU C++, you can use a single header file for both purposes. | |
3621 | ||
3622 | @quotation | |
3623 | @emph{Warning:} The mechanism to specify this is in transition. For the | |
3624 | nonce, you must use one of two @code{#pragma} commands; in a future | |
3625 | release of GNU C++, an alternative mechanism will make these | |
3626 | @code{#pragma} commands unnecessary. | |
3627 | @end quotation | |
3628 | ||
3629 | The header file contains the full definitions, but is marked with | |
3630 | @samp{#pragma interface} in the source code. This allows the compiler | |
3631 | to use the header file only as an interface specification when ordinary | |
3632 | source files incorporate it with @code{#include}. In the single source | |
3633 | file where the full implementation belongs, you can use either a naming | |
3634 | convention or @samp{#pragma implementation} to indicate this alternate | |
3635 | use of the header file. | |
3636 | ||
3637 | @table @code | |
3638 | @item #pragma interface | |
3639 | @itemx #pragma interface "@var{subdir}/@var{objects}.h" | |
3640 | @kindex #pragma interface | |
3641 | Use this directive in @emph{header files} that define object classes, to save | |
3642 | space in most of the object files that use those classes. Normally, | |
3643 | local copies of certain information (backup copies of inline member | |
3644 | functions, debugging information, and the internal tables that implement | |
3645 | virtual functions) must be kept in each object file that includes class | |
3646 | definitions. You can use this pragma to avoid such duplication. When a | |
3647 | header file containing @samp{#pragma interface} is included in a | |
3648 | compilation, this auxiliary information will not be generated (unless | |
3649 | the main input source file itself uses @samp{#pragma implementation}). | |
3650 | Instead, the object files will contain references to be resolved at link | |
3651 | time. | |
3652 | ||
3653 | The second form of this directive is useful for the case where you have | |
3654 | multiple headers with the same name in different directories. If you | |
3655 | use this form, you must specify the same string to @samp{#pragma | |
3656 | implementation}. | |
3657 | ||
3658 | @item #pragma implementation | |
3659 | @itemx #pragma implementation "@var{objects}.h" | |
3660 | @kindex #pragma implementation | |
3661 | Use this pragma in a @emph{main input file}, when you want full output from | |
3662 | included header files to be generated (and made globally visible). The | |
3663 | included header file, in turn, should use @samp{#pragma interface}. | |
3664 | Backup copies of inline member functions, debugging information, and the | |
3665 | internal tables used to implement virtual functions are all generated in | |
3666 | implementation files. | |
3667 | ||
3668 | @cindex implied @code{#pragma implementation} | |
3669 | @cindex @code{#pragma implementation}, implied | |
3670 | @cindex naming convention, implementation headers | |
3671 | If you use @samp{#pragma implementation} with no argument, it applies to | |
3672 | an include file with the same basename@footnote{A file's @dfn{basename} | |
3673 | was the name stripped of all leading path information and of trailing | |
3674 | suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source | |
3675 | file. For example, in @file{allclass.cc}, giving just | |
3676 | @samp{#pragma implementation} | |
3677 | by itself is equivalent to @samp{#pragma implementation "allclass.h"}. | |
3678 | ||
3679 | In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as | |
3680 | an implementation file whenever you would include it from | |
3681 | @file{allclass.cc} even if you never specified @samp{#pragma | |
3682 | implementation}. This was deemed to be more trouble than it was worth, | |
3683 | however, and disabled. | |
3684 | ||
3685 | If you use an explicit @samp{#pragma implementation}, it must appear in | |
3686 | your source file @emph{before} you include the affected header files. | |
3687 | ||
3688 | Use the string argument if you want a single implementation file to | |
3689 | include code from multiple header files. (You must also use | |
3690 | @samp{#include} to include the header file; @samp{#pragma | |
3691 | implementation} only specifies how to use the file---it doesn't actually | |
3692 | include it.) | |
3693 | ||
3694 | There is no way to split up the contents of a single header file into | |
3695 | multiple implementation files. | |
3696 | @end table | |
3697 | ||
3698 | @cindex inlining and C++ pragmas | |
3699 | @cindex C++ pragmas, effect on inlining | |
3700 | @cindex pragmas in C++, effect on inlining | |
3701 | @samp{#pragma implementation} and @samp{#pragma interface} also have an | |
3702 | effect on function inlining. | |
3703 | ||
3704 | If you define a class in a header file marked with @samp{#pragma | |
3705 | interface}, the effect on a function defined in that class is similar to | |
3706 | an explicit @code{extern} declaration---the compiler emits no code at | |
3707 | all to define an independent version of the function. Its definition | |
3708 | is used only for inlining with its callers. | |
3709 | ||
3710 | Conversely, when you include the same header file in a main source file | |
3711 | that declares it as @samp{#pragma implementation}, the compiler emits | |
3712 | code for the function itself; this defines a version of the function | |
3713 | that can be found via pointers (or by callers compiled without | |
3714 | inlining). If all calls to the function can be inlined, you can avoid | |
3715 | emitting the function by compiling with @samp{-fno-implement-inlines}. | |
3716 | If any calls were not inlined, you will get linker errors. | |
3717 | ||
3718 | @node Template Instantiation | |
3719 | @section Where's the Template? | |
3720 | ||
3721 | @cindex template instantiation | |
3722 | ||
3723 | C++ templates are the first language feature to require more | |
3724 | intelligence from the environment than one usually finds on a UNIX | |
3725 | system. Somehow the compiler and linker have to make sure that each | |
3726 | template instance occurs exactly once in the executable if it is needed, | |
3727 | and not at all otherwise. There are two basic approaches to this | |
3728 | problem, which I will refer to as the Borland model and the Cfront model. | |
3729 | ||
3730 | @table @asis | |
3731 | @item Borland model | |
3732 | Borland C++ solved the template instantiation problem by adding the code | |
469b759e JM |
3733 | equivalent of common blocks to their linker; the compiler emits template |
3734 | instances in each translation unit that uses them, and the linker | |
3735 | collapses them together. The advantage of this model is that the linker | |
3736 | only has to consider the object files themselves; there is no external | |
3737 | complexity to worry about. This disadvantage is that compilation time | |
3738 | is increased because the template code is being compiled repeatedly. | |
3739 | Code written for this model tends to include definitions of all | |
3740 | templates in the header file, since they must be seen to be | |
3741 | instantiated. | |
c1f7febf RK |
3742 | |
3743 | @item Cfront model | |
3744 | The AT&T C++ translator, Cfront, solved the template instantiation | |
3745 | problem by creating the notion of a template repository, an | |
469b759e JM |
3746 | automatically maintained place where template instances are stored. A |
3747 | more modern version of the repository works as follows: As individual | |
3748 | object files are built, the compiler places any template definitions and | |
3749 | instantiations encountered in the repository. At link time, the link | |
3750 | wrapper adds in the objects in the repository and compiles any needed | |
3751 | instances that were not previously emitted. The advantages of this | |
3752 | model are more optimal compilation speed and the ability to use the | |
3753 | system linker; to implement the Borland model a compiler vendor also | |
c1f7febf | 3754 | needs to replace the linker. The disadvantages are vastly increased |
469b759e JM |
3755 | complexity, and thus potential for error; for some code this can be |
3756 | just as transparent, but in practice it can been very difficult to build | |
c1f7febf | 3757 | multiple programs in one directory and one program in multiple |
469b759e JM |
3758 | directories. Code written for this model tends to separate definitions |
3759 | of non-inline member templates into a separate file, which should be | |
3760 | compiled separately. | |
c1f7febf RK |
3761 | @end table |
3762 | ||
469b759e | 3763 | When used with GNU ld version 2.8 or later on an ELF system such as |
a4b3b54a JM |
3764 | Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the |
3765 | Borland model. On other systems, g++ implements neither automatic | |
3766 | model. | |
469b759e JM |
3767 | |
3768 | A future version of g++ will support a hybrid model whereby the compiler | |
3769 | will emit any instantiations for which the template definition is | |
3770 | included in the compile, and store template definitions and | |
3771 | instantiation context information into the object file for the rest. | |
3772 | The link wrapper will extract that information as necessary and invoke | |
3773 | the compiler to produce the remaining instantiations. The linker will | |
3774 | then combine duplicate instantiations. | |
3775 | ||
3776 | In the mean time, you have the following options for dealing with | |
3777 | template instantiations: | |
c1f7febf RK |
3778 | |
3779 | @enumerate | |
d863830b JL |
3780 | @item |
3781 | Compile your template-using code with @samp{-frepo}. The compiler will | |
3782 | generate files with the extension @samp{.rpo} listing all of the | |
3783 | template instantiations used in the corresponding object files which | |
3784 | could be instantiated there; the link wrapper, @samp{collect2}, will | |
3785 | then update the @samp{.rpo} files to tell the compiler where to place | |
3786 | those instantiations and rebuild any affected object files. The | |
3787 | link-time overhead is negligible after the first pass, as the compiler | |
3788 | will continue to place the instantiations in the same files. | |
3789 | ||
3790 | This is your best option for application code written for the Borland | |
3791 | model, as it will just work. Code written for the Cfront model will | |
3792 | need to be modified so that the template definitions are available at | |
3793 | one or more points of instantiation; usually this is as simple as adding | |
3794 | @code{#include <tmethods.cc>} to the end of each template header. | |
3795 | ||
3796 | For library code, if you want the library to provide all of the template | |
3797 | instantiations it needs, just try to link all of its object files | |
3798 | together; the link will fail, but cause the instantiations to be | |
3799 | generated as a side effect. Be warned, however, that this may cause | |
3800 | conflicts if multiple libraries try to provide the same instantiations. | |
3801 | For greater control, use explicit instantiation as described in the next | |
3802 | option. | |
3803 | ||
c1f7febf RK |
3804 | @item |
3805 | Compile your code with @samp{-fno-implicit-templates} to disable the | |
3806 | implicit generation of template instances, and explicitly instantiate | |
3807 | all the ones you use. This approach requires more knowledge of exactly | |
3808 | which instances you need than do the others, but it's less | |
3809 | mysterious and allows greater control. You can scatter the explicit | |
3810 | instantiations throughout your program, perhaps putting them in the | |
3811 | translation units where the instances are used or the translation units | |
3812 | that define the templates themselves; you can put all of the explicit | |
3813 | instantiations you need into one big file; or you can create small files | |
3814 | like | |
3815 | ||
3816 | @example | |
3817 | #include "Foo.h" | |
3818 | #include "Foo.cc" | |
3819 | ||
3820 | template class Foo<int>; | |
3821 | template ostream& operator << | |
3822 | (ostream&, const Foo<int>&); | |
3823 | @end example | |
3824 | ||
3825 | for each of the instances you need, and create a template instantiation | |
3826 | library from those. | |
3827 | ||
3828 | If you are using Cfront-model code, you can probably get away with not | |
3829 | using @samp{-fno-implicit-templates} when compiling files that don't | |
3830 | @samp{#include} the member template definitions. | |
3831 | ||
3832 | If you use one big file to do the instantiations, you may want to | |
3833 | compile it without @samp{-fno-implicit-templates} so you get all of the | |
3834 | instances required by your explicit instantiations (but not by any | |
3835 | other files) without having to specify them as well. | |
3836 | ||
3837 | g++ has extended the template instantiation syntax outlined in the | |
03d0f4af MM |
3838 | Working Paper to allow forward declaration of explicit instantiations |
3839 | and instantiation of the compiler support data for a template class | |
3840 | (i.e. the vtable) without instantiating any of its members: | |
c1f7febf RK |
3841 | |
3842 | @example | |
3843 | extern template int max (int, int); | |
c1f7febf RK |
3844 | inline template class Foo<int>; |
3845 | @end example | |
3846 | ||
3847 | @item | |
3848 | Do nothing. Pretend g++ does implement automatic instantiation | |
3849 | management. Code written for the Borland model will work fine, but | |
3850 | each translation unit will contain instances of each of the templates it | |
3851 | uses. In a large program, this can lead to an unacceptable amount of code | |
3852 | duplication. | |
3853 | ||
3854 | @item | |
3855 | Add @samp{#pragma interface} to all files containing template | |
3856 | definitions. For each of these files, add @samp{#pragma implementation | |
3857 | "@var{filename}"} to the top of some @samp{.C} file which | |
3858 | @samp{#include}s it. Then compile everything with | |
3859 | @samp{-fexternal-templates}. The templates will then only be expanded | |
3860 | in the translation unit which implements them (i.e. has a @samp{#pragma | |
3861 | implementation} line for the file where they live); all other files will | |
3862 | use external references. If you're lucky, everything should work | |
3863 | properly. If you get undefined symbol errors, you need to make sure | |
3864 | that each template instance which is used in the program is used in the | |
3865 | file which implements that template. If you don't have any use for a | |
3866 | particular instance in that file, you can just instantiate it | |
3867 | explicitly, using the syntax from the latest C++ working paper: | |
3868 | ||
3869 | @example | |
3870 | template class A<int>; | |
3871 | template ostream& operator << (ostream&, const A<int>&); | |
3872 | @end example | |
3873 | ||
3874 | This strategy will work with code written for either model. If you are | |
3875 | using code written for the Cfront model, the file containing a class | |
3876 | template and the file containing its member templates should be | |
3877 | implemented in the same translation unit. | |
3878 | ||
3879 | A slight variation on this approach is to instead use the flag | |
3880 | @samp{-falt-external-templates}; this flag causes template | |
3881 | instances to be emitted in the translation unit that implements the | |
3882 | header where they are first instantiated, rather than the one which | |
3883 | implements the file where the templates are defined. This header must | |
3884 | be the same in all translation units, or things are likely to break. | |
3885 | ||
3886 | @xref{C++ Interface,,Declarations and Definitions in One Header}, for | |
3887 | more discussion of these pragmas. | |
3888 | @end enumerate | |
3889 | ||
0ded1f18 JM |
3890 | @node Bound member functions |
3891 | @section Extracting the function pointer from a bound pointer to member function | |
3892 | ||
3893 | @cindex pmf | |
3894 | @cindex pointer to member function | |
3895 | @cindex bound pointer to member function | |
3896 | ||
3897 | In C++, pointer to member functions (PMFs) are implemented using a wide | |
3898 | pointer of sorts to handle all the possible call mechanisms; the PMF | |
3899 | needs to store information about how to adjust the @samp{this} pointer, | |
3900 | and if the function pointed to is virtual, where to find the vtable, and | |
3901 | where in the vtable to look for the member function. If you are using | |
3902 | PMFs in an inner loop, you should really reconsider that decision. If | |
3903 | that is not an option, you can extract the pointer to the function that | |
3904 | would be called for a given object/PMF pair and call it directly inside | |
3905 | the inner loop, to save a bit of time. | |
3906 | ||
3907 | Note that you will still be paying the penalty for the call through a | |
3908 | function pointer; on most modern architectures, such a call defeats the | |
3909 | branch prediction features of the CPU. This is also true of normal | |
3910 | virtual function calls. | |
3911 | ||
3912 | The syntax for this extension is | |
3913 | ||
3914 | @example | |
3915 | extern A a; | |
3916 | extern int (A::*fp)(); | |
3917 | typedef int (*fptr)(A *); | |
3918 | ||
3919 | fptr p = (fptr)(a.*fp); | |
3920 | @end example | |
3921 | ||
0fb6bbf5 ML |
3922 | For PMF constants (i.e. expressions of the form @samp{&Klasse::Member}), |
3923 | no object is needed to obtain the address of the function. They can be | |
3924 | converted to function pointers directly: | |
3925 | ||
3926 | @example | |
3927 | fptr p1 = (fptr)(&A::foo); | |
3928 | @end example | |
3929 | ||
0ded1f18 JM |
3930 | You must specify @samp{-Wno-pmf-conversions} to use this extension. |
3931 |