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1 | /* Compile-time assert-like macros. |
2 | ||
3 | Copyright (C) 2005-2006, 2009-2012 Free Software Foundation, Inc. | |
4 | ||
5 | This program is free software: you can redistribute it and/or modify | |
6 | it under the terms of the GNU General Public License as published by | |
7 | the Free Software Foundation; either version 3 of the License, or | |
8 | (at your option) any later version. | |
9 | ||
10 | This program is distributed in the hope that it will be useful, | |
11 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
13 | GNU General Public License for more details. | |
14 | ||
15 | You should have received a copy of the GNU General Public License | |
16 | along with this program. If not, see <http://www.gnu.org/licenses/>. */ | |
17 | ||
18 | /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */ | |
19 | ||
20 | #ifndef _GL_VERIFY_H | |
21 | # define _GL_VERIFY_H | |
22 | ||
23 | ||
24 | /* Define _GL_HAVE__STATIC_ASSERT to 1 if _Static_assert works as per C11. | |
25 | This is supported by GCC 4.6.0 and later, in C mode, and its use | |
26 | here generates easier-to-read diagnostics when verify (R) fails. | |
27 | ||
28 | Define _GL_HAVE_STATIC_ASSERT to 1 if static_assert works as per C++11. | |
29 | This will likely be supported by future GCC versions, in C++ mode. | |
30 | ||
31 | Use this only with GCC. If we were willing to slow 'configure' | |
32 | down we could also use it with other compilers, but since this | |
33 | affects only the quality of diagnostics, why bother? */ | |
34 | # if (4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)) && !defined __cplusplus | |
35 | # define _GL_HAVE__STATIC_ASSERT 1 | |
36 | # endif | |
37 | /* The condition (99 < __GNUC__) is temporary, until we know about the | |
38 | first G++ release that supports static_assert. */ | |
39 | # if (99 < __GNUC__) && defined __cplusplus | |
40 | # define _GL_HAVE_STATIC_ASSERT 1 | |
41 | # endif | |
42 | ||
43 | /* Each of these macros verifies that its argument R is nonzero. To | |
44 | be portable, R should be an integer constant expression. Unlike | |
45 | assert (R), there is no run-time overhead. | |
46 | ||
47 | If _Static_assert works, verify (R) uses it directly. Similarly, | |
48 | _GL_VERIFY_TRUE works by packaging a _Static_assert inside a struct | |
49 | that is an operand of sizeof. | |
50 | ||
51 | The code below uses several ideas for C++ compilers, and for C | |
52 | compilers that do not support _Static_assert: | |
53 | ||
54 | * The first step is ((R) ? 1 : -1). Given an expression R, of | |
55 | integral or boolean or floating-point type, this yields an | |
56 | expression of integral type, whose value is later verified to be | |
57 | constant and nonnegative. | |
58 | ||
59 | * Next this expression W is wrapped in a type | |
60 | struct _gl_verify_type { | |
61 | unsigned int _gl_verify_error_if_negative: W; | |
62 | }. | |
63 | If W is negative, this yields a compile-time error. No compiler can | |
64 | deal with a bit-field of negative size. | |
65 | ||
66 | One might think that an array size check would have the same | |
67 | effect, that is, that the type struct { unsigned int dummy[W]; } | |
68 | would work as well. However, inside a function, some compilers | |
69 | (such as C++ compilers and GNU C) allow local parameters and | |
70 | variables inside array size expressions. With these compilers, | |
71 | an array size check would not properly diagnose this misuse of | |
72 | the verify macro: | |
73 | ||
74 | void function (int n) { verify (n < 0); } | |
75 | ||
76 | * For the verify macro, the struct _gl_verify_type will need to | |
77 | somehow be embedded into a declaration. To be portable, this | |
78 | declaration must declare an object, a constant, a function, or a | |
79 | typedef name. If the declared entity uses the type directly, | |
80 | such as in | |
81 | ||
82 | struct dummy {...}; | |
83 | typedef struct {...} dummy; | |
84 | extern struct {...} *dummy; | |
85 | extern void dummy (struct {...} *); | |
86 | extern struct {...} *dummy (void); | |
87 | ||
88 | two uses of the verify macro would yield colliding declarations | |
89 | if the entity names are not disambiguated. A workaround is to | |
90 | attach the current line number to the entity name: | |
91 | ||
92 | #define _GL_CONCAT0(x, y) x##y | |
93 | #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) | |
94 | extern struct {...} * _GL_CONCAT (dummy, __LINE__); | |
95 | ||
96 | But this has the problem that two invocations of verify from | |
97 | within the same macro would collide, since the __LINE__ value | |
98 | would be the same for both invocations. (The GCC __COUNTER__ | |
99 | macro solves this problem, but is not portable.) | |
100 | ||
101 | A solution is to use the sizeof operator. It yields a number, | |
102 | getting rid of the identity of the type. Declarations like | |
103 | ||
104 | extern int dummy [sizeof (struct {...})]; | |
105 | extern void dummy (int [sizeof (struct {...})]); | |
106 | extern int (*dummy (void)) [sizeof (struct {...})]; | |
107 | ||
108 | can be repeated. | |
109 | ||
110 | * Should the implementation use a named struct or an unnamed struct? | |
111 | Which of the following alternatives can be used? | |
112 | ||
113 | extern int dummy [sizeof (struct {...})]; | |
114 | extern int dummy [sizeof (struct _gl_verify_type {...})]; | |
115 | extern void dummy (int [sizeof (struct {...})]); | |
116 | extern void dummy (int [sizeof (struct _gl_verify_type {...})]); | |
117 | extern int (*dummy (void)) [sizeof (struct {...})]; | |
118 | extern int (*dummy (void)) [sizeof (struct _gl_verify_type {...})]; | |
119 | ||
120 | In the second and sixth case, the struct type is exported to the | |
121 | outer scope; two such declarations therefore collide. GCC warns | |
122 | about the first, third, and fourth cases. So the only remaining | |
123 | possibility is the fifth case: | |
124 | ||
125 | extern int (*dummy (void)) [sizeof (struct {...})]; | |
126 | ||
127 | * GCC warns about duplicate declarations of the dummy function if | |
a512b375 | 128 | -Wredundant-decls is used. GCC 4.3 and later have a builtin |
8690e634 JK |
129 | __COUNTER__ macro that can let us generate unique identifiers for |
130 | each dummy function, to suppress this warning. | |
131 | ||
132 | * This implementation exploits the fact that older versions of GCC, | |
133 | which do not support _Static_assert, also do not warn about the | |
134 | last declaration mentioned above. | |
135 | ||
a512b375 JB |
136 | * GCC warns if -Wnested-externs is enabled and verify() is used |
137 | within a function body; but inside a function, you can always | |
138 | arrange to use verify_expr() instead. | |
139 | ||
8690e634 JK |
140 | * In C++, any struct definition inside sizeof is invalid. |
141 | Use a template type to work around the problem. */ | |
142 | ||
143 | /* Concatenate two preprocessor tokens. */ | |
144 | # define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y) | |
145 | # define _GL_CONCAT0(x, y) x##y | |
146 | ||
147 | /* _GL_COUNTER is an integer, preferably one that changes each time we | |
148 | use it. Use __COUNTER__ if it works, falling back on __LINE__ | |
149 | otherwise. __LINE__ isn't perfect, but it's better than a | |
150 | constant. */ | |
151 | # if defined __COUNTER__ && __COUNTER__ != __COUNTER__ | |
152 | # define _GL_COUNTER __COUNTER__ | |
153 | # else | |
154 | # define _GL_COUNTER __LINE__ | |
155 | # endif | |
156 | ||
157 | /* Generate a symbol with the given prefix, making it unique if | |
158 | possible. */ | |
159 | # define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER) | |
160 | ||
161 | /* Verify requirement R at compile-time, as an integer constant expression | |
162 | that returns 1. If R is false, fail at compile-time, preferably | |
163 | with a diagnostic that includes the string-literal DIAGNOSTIC. */ | |
164 | ||
165 | # define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \ | |
166 | (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC))) | |
167 | ||
168 | # ifdef __cplusplus | |
169 | # if !GNULIB_defined_struct__gl_verify_type | |
170 | template <int w> | |
171 | struct _gl_verify_type { | |
172 | unsigned int _gl_verify_error_if_negative: w; | |
173 | }; | |
174 | # define GNULIB_defined_struct__gl_verify_type 1 | |
175 | # endif | |
176 | # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ | |
177 | _gl_verify_type<(R) ? 1 : -1> | |
178 | # elif defined _GL_HAVE__STATIC_ASSERT | |
179 | # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ | |
180 | struct { \ | |
181 | _Static_assert (R, DIAGNOSTIC); \ | |
182 | int _gl_dummy; \ | |
183 | } | |
184 | # else | |
185 | # define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \ | |
186 | struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; } | |
187 | # endif | |
188 | ||
189 | /* Verify requirement R at compile-time, as a declaration without a | |
190 | trailing ';'. If R is false, fail at compile-time, preferably | |
191 | with a diagnostic that includes the string-literal DIAGNOSTIC. | |
192 | ||
193 | Unfortunately, unlike C11, this implementation must appear as an | |
194 | ordinary declaration, and cannot appear inside struct { ... }. */ | |
195 | ||
196 | # ifdef _GL_HAVE__STATIC_ASSERT | |
197 | # define _GL_VERIFY _Static_assert | |
198 | # else | |
199 | # define _GL_VERIFY(R, DIAGNOSTIC) \ | |
200 | extern int (*_GL_GENSYM (_gl_verify_function) (void)) \ | |
201 | [_GL_VERIFY_TRUE (R, DIAGNOSTIC)] | |
202 | # endif | |
203 | ||
204 | /* _GL_STATIC_ASSERT_H is defined if this code is copied into assert.h. */ | |
205 | # ifdef _GL_STATIC_ASSERT_H | |
206 | # if !defined _GL_HAVE__STATIC_ASSERT && !defined _Static_assert | |
207 | # define _Static_assert(R, DIAGNOSTIC) _GL_VERIFY (R, DIAGNOSTIC) | |
208 | # endif | |
209 | # if !defined _GL_HAVE_STATIC_ASSERT && !defined static_assert | |
210 | # define static_assert _Static_assert /* C11 requires this #define. */ | |
211 | # endif | |
212 | # endif | |
213 | ||
214 | /* @assert.h omit start@ */ | |
215 | ||
216 | /* Each of these macros verifies that its argument R is nonzero. To | |
217 | be portable, R should be an integer constant expression. Unlike | |
218 | assert (R), there is no run-time overhead. | |
219 | ||
220 | There are two macros, since no single macro can be used in all | |
221 | contexts in C. verify_true (R) is for scalar contexts, including | |
222 | integer constant expression contexts. verify (R) is for declaration | |
223 | contexts, e.g., the top level. */ | |
224 | ||
225 | /* Verify requirement R at compile-time, as an integer constant expression. | |
226 | Return 1. This is equivalent to verify_expr (R, 1). | |
227 | ||
228 | verify_true is obsolescent; please use verify_expr instead. */ | |
229 | ||
230 | # define verify_true(R) _GL_VERIFY_TRUE (R, "verify_true (" #R ")") | |
231 | ||
232 | /* Verify requirement R at compile-time. Return the value of the | |
233 | expression E. */ | |
234 | ||
235 | # define verify_expr(R, E) \ | |
236 | (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E)) | |
237 | ||
238 | /* Verify requirement R at compile-time, as a declaration without a | |
239 | trailing ';'. */ | |
240 | ||
241 | # define verify(R) _GL_VERIFY (R, "verify (" #R ")") | |
242 | ||
243 | /* @assert.h omit end@ */ | |
244 | ||
245 | #endif |