man7/feature_test_macros.7

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  25 .TH FEATURE_TEST_MACROS 7 2016-07-17 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 feature_test_macros \- feature test macros
  28 .SH DESCRIPTION
  29 Feature test macros allow the programmer to control the definitions that
  30 are exposed by system header files when a program is compiled.
  31
  32 .B NOTE:
  33 In order to be effective, a feature test macro
  34 .IR "must be defined before including any header files" .
  35 This can be done either in the compilation command
  36 .RI ( "cc \-DMACRO=value" )
  37 or by defining the macro within the source code before
  38 including any headers.
  39
  40 Some feature test macros are useful for creating portable applications,
  41 by preventing nonstandard definitions from being exposed.
  42 Other macros can be used to expose nonstandard definitions that
  43 are not exposed by default.
  44
  45 The precise effects of each of the feature test macros described below
  46 can be ascertained by inspecting the
  47 .I <features.h>
  48 header file.
  49 .BR Note :
  50 applications do
  51 .I not
  52 need to directly include
  53 .IR <features.h> ;
  54 indeed, doing so is actively discouraged.
  55 See NOTES.
  56 .SS Specification of feature test macro requirements in manual pages
  57 When a function requires that a feature test macro is defined,
  58 the manual page SYNOPSIS typically includes a note of the following form
  59 (this example from the
  60 .BR acct (2)
  61 manual page):
  62 .RS 8
  63 .sp
  64 .B #include <unistd.h>
  65 .sp
  66 .BI "int acct(const char *" filename );
  67 .sp
  68 .nf
  69 .in -4n
  70 Feature Test Macro Requirements for glibc (see
  71 .BR feature_test_macros (7)):
  72 .fi
  73 .in
  74 .sp
  75 .BR acct ():
  76 _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  77 .RE
  78 .PP
  79 The
  80 .B ||
  81 means that in order to obtain the declaration of
  82 .BR acct (2)
  83 from
  84 .IR <unistd.h> ,
  85 .I either
  86 of the following macro
  87 definitions must be made before including any header files:
  88 .RS
  89 .nf
  90
  91 #define _BSD_SOURCE
  92 #define _XOPEN_SOURCE        /* or any value < 500 */
  93 .fi
  94 .RE
  95 .PP
  96 Alternatively, equivalent definitions can be included in the
  97 compilation command:
  98 .RS
  99 .nf
 100
 101 cc \-D_BSD_SOURCE
 102 cc \-D_XOPEN_SOURCE           # Or any value < 500
 103 .fi
 104 .RE
 105 .PP
 106 Note that, as described below,
 107 .BR "some feature test macros are defined by default" ,
 108 so that it may not always be necessary to
 109 explicitly specify the feature test macro(s) shown in the
 110 SYNOPSIS.
 111
 112 In a few cases, manual pages use a shorthand for expressing the
 113 feature test macro requirements (this example from
 114 .BR readahead (2)):
 115 .RS
 116 .nf
 117
 118 .B #define _GNU_SOURCE
 119 .B #include <fcntl.h>
 120 .sp
 121 .BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
 122 .fi
 123 .RE
 124 .PP
 125 This format is employed in cases where only a single
 126 feature test macro can be used to expose the function
 127 declaration, and that macro is not defined by default.
 128 .SS Feature test macros understood by glibc
 129 The paragraphs below explain how feature test macros are handled
 130 in Linux glibc 2.\fIx\fP,
 131 .I x
 132 > 0.
 133
 134 First, though a summary of a few details for the impatient:
 135 .IP * 3
 136 The macros that you most likely need to use in modern source code are
 137 .BR _POSIX_C_SOURCE
 138 (for definitions from various versions of POSIX.1),
 139 .BR _XOPEN_SOURCE
 140 (for definitions from various versions of SUS),
 141 .BR _GNU_SOURCE
 142 (for GNU and/or Linux specific stuff), and
 143 .BR _DEFAULT_SOURCE
 144 (to get definitions that would normally be provided by default).
 145 .IP *
 146 Certain macros are defined with default values.
 147 Thus, although one or more macros may be indicated as being
 148 required in the SYNOPSIS of a man page,
 149 it may not be necessary to define them explicitly.
 150 Full details of the defaults are given later in this man page.
 151 .IP *
 152 Defining
 153 .BR _XOPEN_SOURCE
 154 with a value of 600 or greater produces the same effects as defining
 155 .BR _POSIX_C_SOURCE
 156 with a value of 200112L or greater.
 157 Where one sees
 158 .RS
 159 .nf
 160
 161     _POSIX_C_SOURCE >= 200112L
 162
 163 .fi
 164 .RE
 165 .IP
 166 in the feature test macro requirements in the SYNOPSIS of a man page,
 167 it is implicit that the following has the same effect:
 168 .RS
 169 .nf
 170
 171     _XOPEN_SOURCE >= 600
 172
 173 .fi
 174 .RE
 175 .IP *
 176 Defining
 177 .BR _XOPEN_SOURCE
 178 with a value of 700 or greater produces the same effects as defining
 179 .BR _POSIX_C_SOURCE
 180 with a value of 200809L or greater.
 181 Where one sees
 182 .RS
 183 .nf
 184
 185     _POSIX_C_SOURCE >= 200809L
 186
 187 .fi
 188 .RE
 189 .IP
 190 in the feature test macro requirements in the SYNOPSIS of a man page,
 191 it is implicit that the following has the same effect:
 192 .RS
 193 .nf
 194
 195     _XOPEN_SOURCE >= 700
 196
 197 .fi
 198 .RE
 199 .\" The details in glibc 2.0 are simpler, but combining a
 200 .\" a description of them with the details in later glibc versions
 201 .\" would make for a complicated description.
 202
 203 Linux glibc understands the following feature test macros:
 204 .TP
 205 .B __STRICT_ANSI__
 206 ISO Standard C.
 207 This macro is implicitly defined by
 208 .BR gcc (1)
 209 when invoked with, for example, the
 210 .I -std=c99
 211 or
 212 .I -ansi
 213 flag.
 214 .TP
 215 .B _POSIX_C_SOURCE
 216 Defining this macro causes header files to expose definitions as follows:
 217 .RS
 218 .IP \(bu 3
 219 The value 1 exposes definitions conforming to POSIX.1-1990 and
 220 ISO C (1990).
 221 .IP \(bu
 222 The value 2 or greater additionally exposes
 223 definitions for POSIX.2-1992.
 224 .IP \(bu
 225 The value 199309L or greater additionally exposes
 226 definitions for POSIX.1b (real-time extensions).
 227 .\" 199506L functionality is available only since glibc 2.1
 228 .IP \(bu
 229 The value 199506L or greater additionally exposes
 230 definitions for POSIX.1c (threads).
 231 .IP \(bu
 232 (Since glibc 2.3.3)
 233 The value 200112L or greater additionally exposes definitions corresponding
 234 to the POSIX.1-2001 base specification (excluding the XSI extension).
 235 This value also causes C95 (since glibc 2.12) and
 236 C99 (since glibc 2.10) features to be exposed
 237 (in other words, the equivalent of defining
 238 .BR _ISOC99_SOURCE ).
 239 .IP \(bu
 240 (Since glibc 2.10)
 241 The value 200809L or greater additionally exposes definitions corresponding
 242 to the POSIX.1-2008 base specification (excluding the XSI extension).
 243 .RE
 244 .TP
 245 .B _POSIX_SOURCE
 246 Defining this obsolete macro with any value is equivalent to defining
 247 .B _POSIX_C_SOURCE
 248 with the value 1.
 249
 250 Since this macro is obsolete,
 251 its usage is generally not documented when discussing
 252 feature test macro requirements in the man pages.
 253 .TP
 254 .B _XOPEN_SOURCE
 255 Defining this macro causes header files to expose definitions as follows:
 256 .RS
 257 .IP \(bu 3
 258 Defining with any value exposes
 259 definitions conforming to POSIX.1, POSIX.2, and XPG4.
 260 .IP \(bu
 261 The value 500 or greater additionally exposes
 262 definitions for SUSv2 (UNIX 98).
 263 .IP \(bu
 264 (Since glibc 2.2) The value 600 or greater additionally exposes
 265 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
 266 plus the XSI extension) and C99 definitions.
 267 .IP \(bu
 268 (Since glibc 2.10) The value 700 or greater additionally exposes
 269 definitions for SUSv4 (i.e., the POSIX.1-2008 base specification
 270 plus the XSI extension).
 271 .RE
 272 .IP
 273 If
 274 .B __STRICT_ANSI__
 275 is not defined, or
 276 .BR _XOPEN_SOURCE
 277 is defined with a value greater than or equal to 500
 278 .I and
 279 neither
 280 .B _POSIX_SOURCE
 281 nor
 282 .B _POSIX_C_SOURCE
 283 is explicitly defined, then
 284 the following macros are implicitly defined:
 285 .RS
 286 .IP \(bu 3
 287 .B _POSIX_SOURCE
 288 is defined with the value 1.
 289 .IP \(bu
 290 .B _POSIX_C_SOURCE
 291 is defined, according to the value of
 292 .BR _XOPEN_SOURCE :
 293 .RS 7
 294 .TP
 295 .BR _XOPEN_SOURCE " < 500"
 296 .B _POSIX_C_SOURCE
 297 is defined with the value 2.
 298 .TP
 299 .RB "500 <= " _XOPEN_SOURCE " < 600"
 300 .B _POSIX_C_SOURCE
 301 is defined with the value 199506L.
 302 .TP
 303 .RB "600 <= " _XOPEN_SOURCE " < 700"
 304 .B _POSIX_C_SOURCE
 305 is defined with the value 200112L.
 306 .TP
 307 .RB "700 <= " _XOPEN_SOURCE " (since glibc 2.10)"
 308 .B _POSIX_C_SOURCE
 309 is defined with the value 200809L.
 310 .RE
 311 .RE
 312 .IP
 313 In addition, defining
 314 .BR _XOPEN_SOURCE
 315 with a value of 500 or greater produces the same effects as defining
 316 .BR _XOPEN_SOURCE_EXTENDED .
 317 .TP
 318 .B _XOPEN_SOURCE_EXTENDED
 319 If this macro is defined,
 320 .I and
 321 .B _XOPEN_SOURCE
 322 is defined, then expose definitions corresponding to the XPG4v2
 323 (SUSv1) UNIX extensions (UNIX 95).
 324 Defining
 325 .B _XOPEN_SOURCE
 326 with a value of 500 or more also produces the same effect as defining
 327 .BR _XOPEN_SOURCE_EXTENDED .
 328 Use of
 329 .BR _XOPEN_SOURCE_EXTENDED
 330 in new source code should be avoided.
 331
 332 Since defining
 333 .B _XOPEN_SOURCE
 334 with a value of 500 or more has the same effect as defining
 335 .BR _XOPEN_SOURCE_EXTENDED ,
 336 the latter (obsolete) feature test macro is generally not described in the
 337 SYNOPSIS in man pages.
 338 .TP
 339 .BR _ISOC99_SOURCE " (since glibc 2.1.3)"
 340 Exposes declarations consistent with the ISO C99 standard.
 341
 342 Earlier glibc 2.1.x versions recognized an equivalent macro named
 343 .B _ISOC9X_SOURCE
 344 (because the C99 standard had not then been finalized).
 345 Although the use of this macro is obsolete, glibc continues
 346 to recognize it for backward compatibility.
 347
 348 Defining
 349 .B _ISOC99_SOURCE
 350 also exposes ISO C (1990) Amendment 1 ("C95") definitions.
 351 (The primary change in C95 was support for international character sets.)
 352
 353 Invoking the C compiler with the option
 354 .IR \-std=c99
 355 produces the same effects as defining this macro.
 356 .TP
 357 .BR _ISOC11_SOURCE " (since glibc 2.16)"
 358 Exposes declarations consistent with the ISO C11 standard.
 359 Defining this macro also enables C99 and C95 features (like
 360 .BR _ISOC99_SOURCE ).
 361
 362 Invoking the C compiler with the option
 363 .IR \-std=c11
 364 produces the same effects as defining this macro.
 365 .TP
 366 .B _LARGEFILE64_SOURCE
 367 Expose definitions for the alternative API specified by the
 368 LFS (Large File Summit) as a "transitional extension" to the
 369 Single UNIX Specification.
 370 (See
 371 .UR http:\:/\:/opengroup.org\:/platform\:/lfs.html
 372 .UE .)
 373 The alternative API consists of a set of new objects
 374 (i.e., functions and types) whose names are suffixed with "64"
 375 (e.g.,
 376 .I off64_t
 377 versus
 378 .IR off_t ,
 379 .BR lseek64 ()
 380 versus
 381 .BR lseek (),
 382 etc.).
 383 New programs should not employ this macro; instead
 384 .I _FILE_OFFSET_BITS=64
 385 should be employed.
 386 .TP
 387 .BR _LARGEFILE_SOURCE
 388 This macro was historically used to expose certain functions (specifically
 389 .BR fseeko (3)
 390 and
 391 .BR ftello (3))
 392 that address limitations of earlier APIs
 393 .RB ( fseek (3)
 394 and
 395 .BR ftell (3))
 396 that use
 397 .IR "long int"
 398 for file offsets.
 399 This macro is implicitly defined if
 400 .BR _XOPEN_SOURCE
 401 is defined with a value greater than or equal to 500.
 402 New programs should not employ this macro;
 403 defining
 404 .BR _XOPEN_SOURCE
 405 as just described or defining
 406 .B _FILE_OFFSET_BITS
 407 with the value 64 is the preferred mechanism to achieve the same result.
 408 .TP
 409 .B _FILE_OFFSET_BITS
 410 Defining this macro with the value 64
 411 automatically converts references to 32-bit functions and data types
 412 related to file I/O and filesystem operations into references to
 413 their 64-bit counterparts.
 414 This is useful for performing I/O on large files (> 2 Gigabytes)
 415 on 32-bit systems.
 416 (Defining this macro permits correctly written programs to use
 417 large files with only a recompilation being required.)
 418
 419 64-bit systems naturally permit file sizes greater than 2 Gigabytes,
 420 and on those systems this macro has no effect.
 421 .TP
 422 .BR _BSD_SOURCE " (deprecated since glibc 2.20)"
 423 Defining this macro with any value causes header files to expose
 424 BSD-derived definitions.
 425
 426 In glibc versions up to and including 2.18,
 427 defining this macro also causes BSD definitions to be preferred in
 428 some situations where standards conflict, unless one or more of
 429 .BR _SVID_SOURCE ,
 430 .BR _POSIX_SOURCE ,
 431 .BR _POSIX_C_SOURCE ,
 432 .BR _XOPEN_SOURCE ,
 433 .BR _XOPEN_SOURCE_EXTENDED ,
 434 or
 435 .B _GNU_SOURCE
 436 is defined, in which case BSD definitions are disfavored.
 437 Since glibc 2.19,
 438 .B _BSD_SOURCE
 439 no longer causes BSD definitions to be preferred in case of conflicts.
 440
 441 Since glibc 2.20, this macro is deprecated.
 442 .\" commit c941736c92fa3a319221f65f6755659b2a5e0a20
 443 .\" commit 498afc54dfee41d33ba519f496e96480badace8e
 444 .\" commit acd7f096d79c181866d56d4aaf3b043e741f1e2c
 445 It now has the same effect as defining
 446 .BR _DEFAULT_SOURCE ,
 447 but generates a compile-time warning (unless
 448 .BR _DEFAULT_SOURCE
 449 .\" commit ade40b10ff5fa59a318cf55b9d8414b758e8df78
 450 is also defined).
 451 Use
 452 .B _DEFAULT_SOURCE
 453 instead.
 454 To allow code that requires
 455 .BR _BSD_SOURCE
 456 in glibc 2.19 and earlier and
 457 .BR _DEFAULT_SOURCE
 458 in glibc 2.20 and later to compile without warnings, define
 459 .I both
 460 .B _BSD_SOURCE
 461 and
 462 .BR _DEFAULT_SOURCE .
 463 .TP
 464 .BR _SVID_SOURCE " (deprecated since glibc 2.20)"
 465 Defining this macro with any value causes header files to expose
 466 System V-derived definitions.
 467 (SVID == System V Interface Definition; see
 468 .BR standards (7).)
 469
 470 Since glibc 2.20, this macro is deprecated in the same fashion as
 471 .BR _BSD_SOURCE .
 472 .TP
 473 .BR _DEFAULT_SOURCE " (since glibc 2.19)"
 474 This macro can be defined to ensure that the "default"
 475 definitions are provided even when the defaults would otherwise
 476 be disabled,
 477 as happens when individual macros are explicitly defined,
 478 or the compiler is invoked in one of its "standard" modes (e.g.,
 479 .IR "cc\ \-std=c99" ).
 480 Defining
 481 .B _DEFAULT_SOURCE
 482 without defining other individual macros
 483 or invoking the compiler in one of its "standard" modes has no effect.
 484
 485 The "default" definitions comprise those required by POSIX.1-2008 and ISO C99,
 486 as well as various definitions originally derived from BSD and System V.
 487 On glibc 2.19 and earlier, these defaults were approximately equivalent
 488 to explicitly defining the following:
 489
 490     cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
 491 .TP
 492 .BR _ATFILE_SOURCE " (since glibc 2.4)"
 493 Defining this macro with any value causes header files to expose
 494 declarations of a range of functions with the suffix "at";
 495 see
 496 .BR openat (2).
 497 Since glibc 2.10, this macro is also implicitly defined if
 498 .BR _POSIX_C_SOURCE
 499 is defined with a value greater than or equal to 200809L.
 500 .TP
 501 .B _GNU_SOURCE
 502 Defining this macro (with any value) implicitly defines
 503 .BR _ATFILE_SOURCE ,
 504 .BR _LARGEFILE64_SOURCE ,
 505 .BR _ISOC99_SOURCE ,
 506 .BR _XOPEN_SOURCE_EXTENDED ,
 507 .BR _POSIX_SOURCE ,
 508 .B _POSIX_C_SOURCE
 509 with the value 200809L
 510 (200112L in glibc versions before 2.10;
 511 199506L in glibc versions before 2.5;
 512 199309L in glibc versions before 2.1)
 513 and
 514 .B _XOPEN_SOURCE
 515 with the value 700
 516 (600 in glibc versions before 2.10;
 517 500 in glibc versions before 2.2).
 518 In addition, various GNU-specific extensions are also exposed.
 519
 520 Since glibc 2.19, defining
 521 .BR _GNU_SOURCE
 522 also has the effect of implicitly defining
 523 .BR _DEFAULT_SOURCE .
 524 In glibc versions before 2.20, defining
 525 .BR _GNU_SOURCE
 526 also had the effect of implicitly defining
 527 .BR _BSD_SOURCE
 528 and
 529 .BR _SVID_SOURCE .
 530 .TP
 531 .B _REENTRANT
 532 Defining this macro exposes definitions of certain reentrant functions.
 533 For multithreaded programs, use
 534 .I "cc\ \-pthread"
 535 instead.
 536 .TP
 537 .B _THREAD_SAFE
 538 Synonym for
 539 .BR _REENTRANT ,
 540 provided for compatibility with some other implementations.
 541 .TP
 542 .BR _FORTIFY_SOURCE " (since glibc 2.3.4)"
 543 .\" For more detail, see:
 544 .\" http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
 545 .\" [PATCH] Object size checking to prevent (some) buffer overflows
 546 .\" * From: Jakub Jelinek <jakub at redhat dot com>
 547 .\" * To: gcc-patches at gcc dot gnu dot org
 548 .\" * Date: Tue, 21 Sep 2004 04:16:40 -0400
 549 Defining this macro causes some lightweight checks to be performed
 550 to detect some buffer overflow errors when employing
 551 various string and memory manipulation functions (for example,
 552 .BR memcpy (3),
 553 .BR memset (3),
 554 .BR stpcpy (3),
 555 .BR strcpy (3),
 556 .BR strncpy (3),
 557 .BR strcat (3),
 558 .BR strncat (3),
 559 .BR sprintf (3),
 560 .BR snprintf (3),
 561 .BR vsprintf (3),
 562 .BR vsnprintf (3),
 563 .BR gets (3),
 564 and wide character variants thereof).
 565 For some functions, argument consistency is checked;
 566 for example, a check is made that
 567 .BR open (2)
 568 has been supplied with a
 569 .I mode
 570 argument when the specified flags include
 571 .BR O_CREAT .
 572 Not all problems are detected, just some common cases.
 573 .\" Look for __USE_FORTIFY_LEVEL in the header files
 574
 575 If
 576 .B _FORTIFY_SOURCE
 577 is set to 1, with compiler optimization level 1
 578 .RI ( "gcc\ \-O1" )
 579 and above, checks that shouldn't change the behavior of
 580 conforming programs are performed.
 581 With
 582 .B _FORTIFY_SOURCE
 583 set to 2, some more checking is added, but
 584 some conforming programs might fail.
 585 .\" For example, given the following code
 586 .\"        int d;
 587 .\"        char buf[1000], buf[1000];
 588 .\"        strcpy(fmt, "Hello world\n%n");
 589 .\"        snprintf(buf, sizeof(buf), fmt, &d);
 590 .\"
 591 .\" Compiling with "gcc -D_FORTIFY_SOURCE=2 -O1" and then running will
 592 .\" cause the following diagnostic at runtime at the snprintf() call
 593 .\"
 594 .\"        *** %n in writable segment detected ***
 595 .\"        Aborted (core dumped)
 596 .\"
 597
 598 Some of the checks can be performed at compile time
 599 (via macros logic implemented in header files),
 600 and result in compiler warnings;
 601 other checks take place at run time,
 602 and result in a run-time error if the check fails.
 603
 604 Use of this macro requires compiler support, available with
 605 .BR gcc (1)
 606 since version 4.0.
 607 .SS Default definitions, implicit definitions, and combining definitions
 608 .PP
 609 If no feature test macros are explicitly defined,
 610 then the following feature test macros are defined by default:
 611 .BR _BSD_SOURCE
 612 (in glibc 2.19 and earlier),
 613 .BR _SVID_SOURCE
 614 (in glibc 2.19 and earlier),
 615 .BR _DEFAULT_SOURCE
 616 (since glibc 2.19),
 617 .BR _POSIX_SOURCE ,
 618 and
 619 .BR _POSIX_C_SOURCE =200809L
 620 (200112L in glibc versions before 2.10;
 621 199506L in glibc versions before 2.4;
 622 199309L in glibc versions before 2.1).
 623 .PP
 624 If any of
 625 .BR __STRICT_ANSI__ ,
 626 .BR _ISOC99_SOURCE ,
 627 .BR _POSIX_SOURCE ,
 628 .BR _POSIX_C_SOURCE  ,
 629 .BR _XOPEN_SOURCE ,
 630 .BR _XOPEN_SOURCE_EXTENDED ,
 631 .BR _BSD_SOURCE
 632 (in glibc 2.19 and earlier),
 633 or
 634 .B _SVID_SOURCE
 635 (in glibc 2.19 and earlier)
 636 is explicitly defined, then
 637 .BR _BSD_SOURCE ,
 638 .BR _SVID_SOURCE ,
 639 and
 640 .BR _DEFAULT_SOURCE
 641 are not defined by default.
 642
 643 If
 644 .B _POSIX_SOURCE
 645 and
 646 .B _POSIX_C_SOURCE
 647 are not explicitly defined,
 648 and either
 649 .B __STRICT_ANSI__
 650 is not defined or
 651 .B _XOPEN_SOURCE
 652 is defined with a value of 500 or more, then
 653 .IP * 3
 654 .B _POSIX_SOURCE
 655 is defined with the value 1; and
 656 .IP *
 657 .B _POSIX_C_SOURCE
 658 is defined with one of the following values:
 659 .RS 3
 660 .IP \(bu 3
 661 2,
 662 if
 663 .B _XOPEN_SOURCE
 664 is defined with a value less than 500;
 665 .IP \(bu
 666 199506L,
 667 if
 668 .B _XOPEN_SOURCE
 669 is defined with a value greater than or equal to 500 and less than 600;
 670 or
 671 .IP \(bu
 672 (since glibc 2.4) 200112L,
 673 if
 674 .B _XOPEN_SOURCE
 675 is defined with a value greater than or equal to 600 and less than 700.
 676 .IP \(bu
 677 (Since glibc 2.10)
 678 200809L,
 679 if
 680 .B _XOPEN_SOURCE
 681 is defined with a value greater than or equal to 700.
 682 .IP \(bu
 683 Older versions of glibc do not know about the values
 684 200112L and 200809L for
 685 .BR _POSIX_C_SOURCE ,
 686 and the setting of this macro will depend on the glibc version.
 687 .IP \(bu
 688 If
 689 .B _XOPEN_SOURCE
 690 is undefined, then the setting of
 691 .B _POSIX_C_SOURCE
 692 depends on the glibc version:
 693 199506L, in glibc versions before 2.4;
 694 200112L, in glibc 2.4 to 2.9; and
 695 200809L, since glibc 2.10.
 696 .RE
 697 .PP
 698 Multiple macros can be defined; the results are additive.
 699 .SH CONFORMING TO
 700 POSIX.1 specifies
 701 .BR _POSIX_C_SOURCE ,
 702 .BR _POSIX_SOURCE ,
 703 and
 704 .BR _XOPEN_SOURCE .
 705
 706 .B _XOPEN_SOURCE_EXTENDED
 707 was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2 and later.
 708 .B _FILE_OFFSET_BITS
 709 is not specified by any standard,
 710 but is employed on some other implementations.
 711
 712 .BR _BSD_SOURCE ,
 713 .BR _SVID_SOURCE ,
 714 .BR _DEFAULT_SOURCE ,
 715 .BR _ATFILE_SOURCE ,
 716 .BR _GNU_SOURCE ,
 717 .BR _FORTIFY_SOURCE ,
 718 .BR _REENTRANT ,
 719 and
 720 .B _THREAD_SAFE
 721 are specific to Linux (glibc).
 722 .SH NOTES
 723 .I <features.h>
 724 is a Linux/glibc-specific header file.
 725 Other systems have an analogous file, but typically with a different name.
 726 This header file is automatically included by other header files as
 727 required: it is not necessary to explicitly include it in order to
 728 employ feature test macros.
 729
 730 According to which of the above feature test macros are defined,
 731 .I <features.h>
 732 internally defines various other macros that are checked by
 733 other glibc header files.
 734 These macros have names prefixed by two underscores (e.g.,
 735 .BR __USE_MISC ).
 736 Programs should
 737 .I never
 738 define these macros directly:
 739 instead, the appropriate feature test macro(s) from the
 740 list above should be employed.
 741 .SH EXAMPLE
 742 The program below can be used to explore how the various
 743 feature test macros are set depending on the glibc version
 744 and what feature test macros are explicitly set.
 745 The following shell session, on a system with glibc 2.10,
 746 shows some examples of what we would see:
 747 .in +4n
 748 .nf
 749
 750 $ \fBcc ftm.c\fP
 751 $ \fB./a.out\fP
 752 _POSIX_SOURCE defined
 753 _POSIX_C_SOURCE defined: 200809L
 754 _BSD_SOURCE defined
 755 _SVID_SOURCE defined
 756 _ATFILE_SOURCE defined
 757 $ \fBcc \-D_XOPEN_SOURCE=500 ftm.c\fP
 758 $ \fB./a.out\fP
 759 _POSIX_SOURCE defined
 760 _POSIX_C_SOURCE defined: 199506L
 761 _XOPEN_SOURCE defined: 500
 762 $ \fBcc \-D_GNU_SOURCE ftm.c\fP
 763 $ \fB./a.out\fP
 764 _POSIX_SOURCE defined
 765 _POSIX_C_SOURCE defined: 200809L
 766 _ISOC99_SOURCE defined
 767 _XOPEN_SOURCE defined: 700
 768 _XOPEN_SOURCE_EXTENDED defined
 769 _LARGEFILE64_SOURCE defined
 770 _BSD_SOURCE defined
 771 _SVID_SOURCE defined
 772 _ATFILE_SOURCE defined
 773 _GNU_SOURCE defined
 774 .fi
 775 .in
 776 .SS Program source
 777 \&
 778 .nf
 779 /* ftm.c */
 780
 781 #include <stdio.h>
 782 #include <unistd.h>
 783 #include <stdlib.h>
 784
 785 int
 786 main(int argc, char *argv[])
 787 {
 788 #ifdef _POSIX_SOURCE
 789     printf("_POSIX_SOURCE defined\\n");
 790 #endif
 791
 792 #ifdef _POSIX_C_SOURCE
 793     printf("_POSIX_C_SOURCE defined: %ldL\\n", (long) _POSIX_C_SOURCE);
 794 #endif
 795
 796 #ifdef _ISOC99_SOURCE
 797     printf("_ISOC99_SOURCE defined\\n");
 798 #endif
 799
 800 #ifdef _ISOC11_SOURCE
 801     printf("_ISOC11_SOURCE defined\\n");
 802 #endif
 803
 804 #ifdef _XOPEN_SOURCE
 805     printf("_XOPEN_SOURCE defined: %d\\n", _XOPEN_SOURCE);
 806 #endif
 807
 808 #ifdef _XOPEN_SOURCE_EXTENDED
 809     printf("_XOPEN_SOURCE_EXTENDED defined\\n");
 810 #endif
 811
 812 #ifdef _LARGEFILE64_SOURCE
 813     printf("_LARGEFILE64_SOURCE defined\\n");
 814 #endif
 815
 816 #ifdef _FILE_OFFSET_BITS
 817     printf("_FILE_OFFSET_BITS defined: %d\\n", _FILE_OFFSET_BITS);
 818 #endif
 819
 820 #ifdef _BSD_SOURCE
 821     printf("_BSD_SOURCE defined\\n");
 822 #endif
 823
 824 #ifdef _SVID_SOURCE
 825     printf("_SVID_SOURCE defined\\n");
 826 #endif
 827
 828 #ifdef _DEFAULT_SOURCE
 829     printf("_DEFAULT_SOURCE defined\\n");
 830 #endif
 831
 832 #ifdef _ATFILE_SOURCE
 833     printf("_ATFILE_SOURCE defined\\n");
 834 #endif
 835
 836 #ifdef _GNU_SOURCE
 837     printf("_GNU_SOURCE defined\\n");
 838 #endif
 839
 840 #ifdef _REENTRANT
 841     printf("_REENTRANT defined\\n");
 842 #endif
 843
 844 #ifdef _THREAD_SAFE
 845     printf("_THREAD_SAFE defined\\n");
 846 #endif
 847
 848 #ifdef _FORTIFY_SOURCE
 849     printf("_FORTIFY_SOURCE defined\\n");
 850 #endif
 851
 852     exit(EXIT_SUCCESS);
 853 }
 854 .fi
 855 .SH SEE ALSO
 856 .BR libc (7),
 857 .BR standards (7)
 858
 859 The section "Feature Test Macros" under
 860 .IR "info libc" .
 861 .\" But beware: the info libc document is out of date (Jul 07, mtk)
 862
 863 .I /usr/include/features.h