man7/feature_test_macros.7

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  25 .TH FEATURE_TEST_MACROS 7 2012-08-05 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 feature_test_macros \- feature test macros
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <features.h>
  31 .fi
  32 .SH DESCRIPTION
  33 Feature test macros allow the programmer to control the definitions that
  34 are exposed by system header files when a program is compiled.
  35
  36 .B NOTE:
  37 In order to be effective, a feature test macro
  38 .IR "must be defined before including any header files" .
  39 This can be done either in the compilation command
  40 .RI ( "cc \-DMACRO=value" )
  41 or by defining the macro within the source code before
  42 including any headers.
  43
  44 Some feature test macros are useful for creating portable applications,
  45 by preventing nonstandard definitions from being exposed.
  46 Other macros can be used to expose nonstandard definitions that
  47 are not exposed by default.
  48 The precise effects of each of the feature test macros described below
  49 can be ascertained by inspecting the
  50 .I <features.h>
  51 header file.
  52 .SS Specification of feature test macro requirements in manual pages
  53 When a function requires that a feature test macro is defined,
  54 the manual page SYNOPSIS typically includes a note of the following form
  55 (this example from the
  56 .BR acct (2)
  57 manual page):
  58 .RS 8
  59 .sp
  60 .B #include <unistd.h>
  61 .sp
  62 .BI "int acct(const char *" filename );
  63 .sp
  64 .nf
  65 .in -4n
  66 Feature Test Macro Requirements for glibc (see
  67 .BR feature_test_macros (7)):
  68 .fi
  69 .in
  70 .sp
  71 .BR acct ():
  72 _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  73 .RE
  74 .PP
  75 The \fB||\fP means that in order to obtain the declaration of
  76 .BR acct (2)
  77 from
  78 .IR <unistd.h> ,
  79 \fIeither\fP of the following macro
  80 definitions must be made before including any header files:
  81 .RS
  82 .nf
  83
  84 #define _BSD_SOURCE
  85 #define _XOPEN_SOURCE        /* or any value < 500 */
  86 .fi
  87 .RE
  88 .PP
  89 Alternatively, equivalent definitions can be included in the
  90 compilation command:
  91 .RS
  92 .nf
  93
  94 cc \-D_BSD_SOURCE
  95 cc \-D_XOPEN_SOURCE           # Or any value < 500
  96 .fi
  97 .RE
  98 .PP
  99 Note that, as described below,
 100 .BR "some feature test macros are defined by default" ,
 101 so that it may not always be necessary to
 102 explicitly specify the feature test macro(s) shown in the
 103 SYNOPSIS.
 104
 105 In a few cases, manual pages use a shorthand for expressing the
 106 feature test macro requirements (this example from
 107 .BR readahead (2)):
 108 .RS
 109 .nf
 110
 111 .B #define _GNU_SOURCE
 112 .B #include <fcntl.h>
 113 .sp
 114 .BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
 115 .fi
 116 .RE
 117 .PP
 118 This format is employed in cases where only a single
 119 feature test macro can be used to expose the function
 120 declaration, and that macro is not defined by default.
 121 .SS Feature test macros understood by glibc
 122 The following paragraphs explain how feature test macros are handled
 123 in Linux glibc 2.\fIx\fP, \fIx\fP > 0.
 124 .\" The details in glibc 2.0 are simpler, but combining a
 125 .\" a description of them with the details in later glibc versions
 126 .\" would make for a complicated description.
 127
 128 Linux glibc understands the following feature test macros:
 129 .TP
 130 .B __STRICT_ANSI__
 131 ISO Standard C.
 132 This macro is implicitly defined by
 133 .BR gcc (1)
 134 when invoked with, for example, the
 135 .I -std=c99
 136 or
 137 .I -ansi
 138 flag.
 139 .TP
 140 .B _POSIX_C_SOURCE
 141 Defining this macro causes header files to expose definitions as follows:
 142 .RS
 143 .IP \(bu 3
 144 The value 1 exposes definitions conforming to POSIX.1-1990 and
 145 ISO C (1990).
 146 .IP \(bu
 147 The value 2 or greater additionally exposes
 148 definitions for POSIX.2-1992.
 149 .IP \(bu
 150 The value 199309L or greater additionally exposes
 151 definitions for POSIX.1b (real-time extensions).
 152 .\" 199506L functionality is only available since glibc 2.1
 153 .IP \(bu
 154 The value 199506L or greater additionally exposes
 155 definitions for POSIX.1c (threads).
 156 .IP \(bu
 157 (Since glibc 2.3.3)
 158 The value 200112L or greater exposes definitions corresponding
 159 to the POSIX.1-2001 base specification (excluding the XSI extension).
 160 .IP \(bu
 161 (Since glibc 2.10)
 162 The value 200809L or greater exposes definitions corresponding
 163 to the POSIX.1-2008 base specification (excluding the XSI extension).
 164 .RE
 165 .TP
 166 .B _POSIX_SOURCE
 167 Defining this obsolete macro with any value is equivalent to defining
 168 .B _POSIX_C_SOURCE
 169 with the value 1.
 170 .TP
 171 .B _XOPEN_SOURCE
 172 Defining this macro causes header files to expose definitions as follows:
 173 .RS
 174 .IP \(bu 3
 175 Defining with any value exposes
 176 definitions conforming to POSIX.1, POSIX.2, and XPG4.
 177 .IP \(bu
 178 The value 500 or greater additionally exposes
 179 definitions for SUSv2 (UNIX 98).
 180 .IP \(bu
 181 (Since glibc 2.2) The value 600 or greater additionally exposes
 182 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
 183 plus the XSI extension) and C99 definitions.
 184 .IP \(bu
 185 (Since glibc 2.10) The value 700 or greater additionally exposes
 186 definitions for SUSv4 (i.e., the POSIX.1-2008 base specification
 187 plus the XSI extension).
 188 .RE
 189 .TP
 190 .B _XOPEN_SOURCE_EXTENDED
 191 If this macro is defined, and
 192 .B _XOPEN_SOURCE
 193 is defined, then expose definitions corresponding to the XPG4v2
 194 (SUSv1) UNIX extensions (UNIX 95).
 195 This macro is also implicitly defined if
 196 .B _XOPEN_SOURCE
 197 is defined with a value of 500 or more.
 198 .TP
 199 .B _ISOC95_SOURCE
 200 Exposes ISO C (1990) Amendment 1 definitions (also known as C95).
 201 This macro is recognized since glibc 2.12.
 202 The primary change in C95 was support for international character sets.
 203 The C95 changes were included in the subsequent C99 standard
 204 (in other words,
 205 .B _ISOC99_SOURCE
 206 implies
 207 .BR _ISOC95_SOURCE ).
 208 .TP
 209 .B _ISOC99_SOURCE
 210 Exposes C99 extensions to ISO C (1990).
 211 This macro is recognized since glibc 2.1.3;
 212 earlier glibc 2.1.x versions recognized an equivalent macro named
 213 .B _ISOC9X_SOURCE
 214 (because the C99 standard had not then been finalized).
 215 Although the use of the latter macro is obsolete, glibc continues
 216 to recognize it for backward compatibility.
 217 .TP
 218 .B _ISOC11_SOURCE
 219 Exposes declarations consistent with the ISO C11 standard.
 220 This macro is recognized since glibc 2.16.
 221 .TP
 222 .B _LARGEFILE64_SOURCE
 223 Expose definitions for the alternative API specified by the
 224 LFS (Large File Summit) as a "transitional extension" to the
 225 Single UNIX Specification.
 226 (See
 227 .UR http:\:/\:/opengroup.org\:/platform\:/lfs.html
 228 .UE )
 229 The alternative API consists of a set of new objects
 230 (i.e., functions and types) whose names are suffixed with "64"
 231 (e.g.,
 232 .I off64_t
 233 versus
 234 .IR off_t ,
 235 .BR lseek64 ()
 236 versus
 237 .BR lseek (),
 238 etc.).
 239 New programs should not employ this interface; instead
 240 .I _FILE_OFFSET_BITS=64
 241 should be employed.
 242 .TP
 243 .B _FILE_OFFSET_BITS
 244 Defining this macro with the value 64
 245 automatically converts references to 32-bit functions and data types
 246 related to file I/O and file system operations into references to
 247 their 64-bit counterparts.
 248 This is useful for performing I/O on large files (> 2 Gigabytes)
 249 on 32-bit systems.
 250 (Defining this macro permits correctly written programs to use
 251 large files with only a recompilation being required.)
 252 64-bit systems naturally permit file sizes greater than 2 Gigabytes,
 253 and on those systems this macro has no effect.
 254 .TP
 255 .B _BSD_SOURCE
 256 Defining this macro with any value causes header files to expose
 257 BSD-derived definitions.
 258 Defining this macro also causes BSD definitions to be preferred in
 259 some situations where standards conflict, unless one or more of
 260 .BR _SVID_SOURCE ,
 261 .BR _POSIX_SOURCE ,
 262 .BR _POSIX_C_SOURCE ,
 263 .BR _XOPEN_SOURCE ,
 264 .BR _XOPEN_SOURCE_EXTENDED ,
 265 or
 266 .B _GNU_SOURCE
 267 is defined, in which case BSD definitions are disfavored.
 268 .TP
 269 .B _SVID_SOURCE
 270 Defining this macro with any value causes header files to expose
 271 System V-derived definitions.
 272 (SVID == System V Interface Definition; see
 273 .BR standards (7).)
 274 .TP
 275 .BR _ATFILE_SOURCE " (since glibc 2.4)"
 276 Defining this macro with any value causes header files to expose
 277 declarations of a range of functions with the suffix "at";
 278 see
 279 .BR openat (2).
 280 Since glibc 2.10, this macro is also implicitly defined if
 281 .BR _POSIX_C_SOURCE
 282 is defined with a value greater than or equal to 200809L.
 283 .TP
 284 .B _GNU_SOURCE
 285 Defining this macro (with any value) is equivalent to defining
 286 .BR _BSD_SOURCE ,
 287 .BR _SVID_SOURCE ,
 288 .BR _ATFILE_SOURCE ,
 289 .BR _LARGEFILE64_SOURCE ,
 290 .BR _ISOC99_SOURCE ,
 291 .BR _XOPEN_SOURCE_EXTENDED ,
 292 .BR _POSIX_SOURCE ,
 293 .B _POSIX_C_SOURCE
 294 with the value 200809L
 295 (200112L in glibc versions before 2.10;
 296 199506L in glibc versions before 2.5;
 297 199309L in glibc versions before 2.1)
 298 and
 299 .B _XOPEN_SOURCE
 300 with the value 700
 301 (600 in glibc versions before 2.10;
 302 500 in glibc versions before 2.2).
 303 In addition, various GNU-specific extensions are also exposed.
 304 Where standards conflict, BSD definitions are disfavored.
 305 .TP
 306 .B _REENTRANT
 307 Defining this macro exposes definitions of certain reentrant functions.
 308 For multithreaded programs, use
 309 .I "cc\ \-pthread"
 310 instead.
 311 .TP
 312 .B _THREAD_SAFE
 313 Synonym for
 314 .BR _REENTRANT ,
 315 provided for compatibility with some other implementations.
 316 .TP
 317 .BR _FORTIFY_SOURCE " (since glibc 2.3.4)"
 318 .\" For more detail, see:
 319 .\" http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
 320 .\" [PATCH] Object size checking to prevent (some) buffer overflows
 321 .\" * From: Jakub Jelinek <jakub at redhat dot com>
 322 .\" * To: gcc-patches at gcc dot gnu dot org
 323 .\" * Date: Tue, 21 Sep 2004 04:16:40 -0400
 324 Defining this macro causes some lightweight checks to be performed
 325 to detect some buffer overflow errors when employing
 326 various string and memory manipulation functions.
 327 Not all buffer overflows are detected, just some common cases.
 328 In the current implementation checks are added for
 329 calls to
 330 .BR memcpy (3),
 331 .BR mempcpy (3),
 332 .BR memmove (3),
 333 .BR memset (3),
 334 .BR stpcpy (3),
 335 .BR strcpy (3),
 336 .BR strncpy (3),
 337 .BR strcat (3),
 338 .BR strncat (3),
 339 .BR sprintf (3),
 340 .BR snprintf (3),
 341 .BR vsprintf (3),
 342 .BR vsnprintf (3),
 343 and
 344 .BR gets (3).
 345 If
 346 .B _FORTIFY_SOURCE
 347 is set to 1, with compiler optimization level 1
 348 .RI ( "gcc\ \-O1" )
 349 and above, checks that shouldn't change the behavior of
 350 conforming programs are performed.
 351 With
 352 .B _FORTIFY_SOURCE
 353 set to 2 some more checking is added, but
 354 some conforming programs might fail.
 355 Some of the checks can be performed at compile time,
 356 and result in compiler warnings;
 357 other checks take place at run time,
 358 and result in a run-time error if the check fails.
 359 Use of this macro requires compiler support, available with
 360 .BR gcc (1)
 361 since version 4.0.
 362 .SS Default definitions, implicit definitions, and combining definitions
 363 .PP
 364 If no feature test macros are explicitly defined,
 365 then the following feature test macros are defined by default:
 366 .BR _BSD_SOURCE ,
 367 .BR _SVID_SOURCE ,
 368 .BR _POSIX_SOURCE ,
 369 and
 370 .BR _POSIX_C_SOURCE =200809L
 371 (200112L in glibc versions before 2.10;
 372 199506L in glibc versions before 2.4;
 373 199309L in glibc versions before 2.1).
 374 .PP
 375 If any of
 376 .BR __STRICT_ANSI__ ,
 377 .BR _ISOC99_SOURCE ,
 378 .BR _POSIX_SOURCE ,
 379 .BR _POSIX_C_SOURCE  ,
 380 .BR _XOPEN_SOURCE ,
 381 .BR _XOPEN_SOURCE_EXTENDED ,
 382 .BR _BSD_SOURCE ,
 383 or
 384 .B _SVID_SOURCE
 385 is explicitly defined, then
 386 .BR _BSD_SOURCE ,
 387 and
 388 .B _SVID_SOURCE
 389 are not defined by default.
 390
 391 If
 392 .B _POSIX_SOURCE
 393 and
 394 .B _POSIX_C_SOURCE
 395 are not explicitly defined,
 396 and either
 397 .B __STRICT_ANSI__
 398 is not defined or
 399 .B _XOPEN_SOURCE
 400 is defined with a value of 500 or more, then
 401 .RS 3
 402 .IP * 3
 403 .B _POSIX_SOURCE
 404 is defined with the value 1; and
 405 .IP *
 406 .B _POSIX_C_SOURCE
 407 is defined with one of the following values:
 408 .RS 6
 409 .IP \(bu 3
 410 2,
 411 if
 412 .B XOPEN_SOURCE
 413 is defined with a value less than 500;
 414 .IP \(bu
 415 199506L,
 416 if
 417 .B XOPEN_SOURCE
 418 is defined with a value greater than or equal to 500 and less than 600;
 419 or
 420 .IP \(bu
 421 (since glibc 2.4) 200112L,
 422 if
 423 .B XOPEN_SOURCE
 424 is defined with a value greater than or equal to 600 and less than 700.
 425 .IP \(bu
 426 (Since glibc 2.10)
 427 200809L,
 428 if
 429 .B XOPEN_SOURCE
 430 is defined with a value greater than or equal to 700.
 431 .IP \(bu
 432 Older versions of glibc do not know about the values
 433 200112L and 200809L for
 434 .BR _POSIX_C_SOURCE ,
 435 and the setting of this macro will depend on the glibc version.
 436 .IP \(bu
 437 If
 438 .B _XOPEN_SOURCE
 439 is undefined, then the setting of
 440 .B _POSIX_C_SOURCE
 441 depends on the glibc version:
 442 199506L, in glibc versions before 2.4;
 443 200112L, in glibc 2.4 to 2.9; and
 444 200809L, since glibc 2.10.
 445 .RE
 446 .RE
 447 .PP
 448 Multiple macros can be defined; the results are additive.
 449 .SH CONFORMING TO
 450 POSIX.1 specifies
 451 .BR _POSIX_C_SOURCE ,
 452 .BR _POSIX_SOURCE ,
 453 and
 454 .BR _XOPEN_SOURCE .
 455 .B _XOPEN_SOURCE_EXTENDED
 456 was specified by XPG4v2 (aka SUSv1).
 457
 458 .B _FILE_OFFSET_BITS
 459 is not specified by any standard,
 460 but is employed on some other implementations.
 461
 462 .BR _BSD_SOURCE ,
 463 .BR _SVID_SOURCE ,
 464 .BR _ATFILE_SOURCE ,
 465 .BR _GNU_SOURCE ,
 466 .BR _FORTIFY_SOURCE ,
 467 .BR _REENTRANT ,
 468 and
 469 .B _THREAD_SAFE
 470 are specific to Linux (glibc).
 471 .SH NOTES
 472 .I <features.h>
 473 is a Linux/glibc-specific header file.
 474 Other systems have an analogous file, but typically with a different name.
 475 This header file is automatically included by other header files as
 476 required: it is not necessary to explicitly include it in order to
 477 employ feature test macros.
 478
 479 According to which of the above feature test macros are defined,
 480 .I <features.h>
 481 internally defines various other macros that are checked by
 482 other glibc header files.
 483 These macros have names prefixed by two underscores (e.g.,
 484 .BR __USE_MISC ).
 485 Programs should \fInever\fP define these macros directly:
 486 instead, the appropriate feature test macro(s) from the
 487 list above should be employed.
 488 .SH EXAMPLE
 489 The program below can be used to explore how the various
 490 feature test macros are set depending on the glibc version
 491 and what feature test macros are explicitly set.
 492 The following shell session, on a system with glibc 2.10,
 493 shows some examples of what we would see:
 494 .in +4n
 495 .nf
 496
 497 $ \fBcc ftm.c\fP
 498 $ \fB./a.out\fP
 499 _POSIX_SOURCE defined
 500 _POSIX_C_SOURCE defined: 200809L
 501 _BSD_SOURCE defined
 502 _SVID_SOURCE defined
 503 _ATFILE_SOURCE defined
 504 $ \fBcc \-D_XOPEN_SOURCE=500 ftm.c\fP
 505 $ \fB./a.out\fP
 506 _POSIX_SOURCE defined
 507 _POSIX_C_SOURCE defined: 199506L
 508 _XOPEN_SOURCE defined: 500
 509 $ \fBcc \-D_GNU_SOURCE ftm.c\fP
 510 $ \fB./a.out\fP
 511 _POSIX_SOURCE defined
 512 _POSIX_C_SOURCE defined: 200809L
 513 _ISOC99_SOURCE defined
 514 _XOPEN_SOURCE defined: 700
 515 _XOPEN_SOURCE_EXTENDED defined
 516 _LARGEFILE64_SOURCE defined
 517 _BSD_SOURCE defined
 518 _SVID_SOURCE defined
 519 _ATFILE_SOURCE defined
 520 _GNU_SOURCE defined
 521 .fi
 522 .in
 523 .SS Program source
 524 \&
 525 .nf
 526 /* ftm.c */
 527
 528 #include <stdio.h>
 529 #include <unistd.h>
 530 #include <stdlib.h>
 531
 532 int
 533 main(int argc, char *argv[])
 534 {
 535 #ifdef _POSIX_SOURCE
 536     printf("_POSIX_SOURCE defined\\n");
 537 #endif
 538
 539 #ifdef _POSIX_C_SOURCE
 540     printf("_POSIX_C_SOURCE defined: %ldL\\n", (long) _POSIX_C_SOURCE);
 541 #endif
 542
 543 #ifdef _ISOC99_SOURCE
 544     printf("_ISOC99_SOURCE defined\\n");
 545 #endif
 546
 547 #ifdef _XOPEN_SOURCE
 548     printf("_XOPEN_SOURCE defined: %d\\n", _XOPEN_SOURCE);
 549 #endif
 550
 551 #ifdef _XOPEN_SOURCE_EXTENDED
 552     printf("_XOPEN_SOURCE_EXTENDED defined\\n");
 553 #endif
 554
 555 #ifdef _LARGEFILE64_SOURCE
 556     printf("_LARGEFILE64_SOURCE defined\\n");
 557 #endif
 558
 559 #ifdef _FILE_OFFSET_BITS
 560     printf("_FILE_OFFSET_BITS defined: %d\\n", _FILE_OFFSET_BITS);
 561 #endif
 562
 563 #ifdef _BSD_SOURCE
 564     printf("_BSD_SOURCE defined\\n");
 565 #endif
 566
 567 #ifdef _SVID_SOURCE
 568     printf("_SVID_SOURCE defined\\n");
 569 #endif
 570
 571 #ifdef _ATFILE_SOURCE
 572     printf("_ATFILE_SOURCE defined\\n");
 573 #endif
 574
 575 #ifdef _GNU_SOURCE
 576     printf("_GNU_SOURCE defined\\n");
 577 #endif
 578
 579 #ifdef _REENTRANT
 580     printf("_REENTRANT defined\\n");
 581 #endif
 582
 583 #ifdef _THREAD_SAFE
 584     printf("_THREAD_SAFE defined\\n");
 585 #endif
 586
 587 #ifdef _FORTIFY_SOURCE
 588     printf("_FORTIFY_SOURCE defined\\n");
 589 #endif
 590
 591     exit(EXIT_SUCCESS);
 592 }
 593 .fi
 594 .SH SEE ALSO
 595 .BR libc (7),
 596 .BR standards (7)
 597
 598 The section "Feature Test Macros" under
 599 .IR "info libc" .
 600 .\" But beware: the info libc document is out of date (Jul 07, mtk)
 601
 602 .I /usr/include/features.h