man7/feature_test_macros.7

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  25 .TH FEATURE_TEST_MACROS 7 2020-04-11 "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 .PP
  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 The requirement that the macro must be defined before including any
  40 header file exists because header files may freely include one another.
  41 Thus, for example, in the following lines, defining the
  42 .B _GNU_SOURCE
  43 macro may have no effect because the header
  44 .I <abc.h>
  45 itself includes
  46 .I <xyz.h>
  47 (POSIX explicitly allows this):
  48 .PP
  49 .in +4n
  50 .EX
  51 #include <abc.h>
  52 #define _GNU_SOURCE
  53 #include <xys.h>
  54 .EE
  55 .in
  56 .PP
  57 Some feature test macros are useful for creating portable applications,
  58 by preventing nonstandard definitions from being exposed.
  59 Other macros can be used to expose nonstandard definitions that
  60 are not exposed by default.
  61 .PP
  62 The precise effects of each of the feature test macros described below
  63 can be ascertained by inspecting the
  64 .I <features.h>
  65 header file.
  66 .BR Note :
  67 applications do
  68 .I not
  69 need to directly include
  70 .IR <features.h> ;
  71 indeed, doing so is actively discouraged.
  72 See NOTES.
  73 .SS Specification of feature test macro requirements in manual pages
  74 When a function requires that a feature test macro is defined,
  75 the manual page SYNOPSIS typically includes a note of the following form
  76 (this example from the
  77 .BR acct (2)
  78 manual page):
  79 .PP
  80 .RS 8
  81 .B #include <unistd.h>
  82 .PP
  83 .BI "int acct(const char *" filename );
  84 .PP
  85 .EX
  86 .in -4n
  87 Feature Test Macro Requirements for glibc (see
  88 .BR feature_test_macros (7)):
  89 .EE
  90 .in
  91 .PP
  92 .BR acct ():
  93 _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE\ <\ 500)
  94 .RE
  95 .PP
  96 The
  97 .B ||
  98 means that in order to obtain the declaration of
  99 .BR acct (2)
 100 from
 101 .IR <unistd.h> ,
 102 .I either
 103 of the following macro
 104 definitions must be made before including any header files:
 105 .PP
 106 .in +4n
 107 .EX
 108 #define _BSD_SOURCE
 109 #define _XOPEN_SOURCE        /* or any value < 500 */
 110 .EE
 111 .in
 112 .PP
 113 Alternatively, equivalent definitions can be included in the
 114 compilation command:
 115 .PP
 116 .in +4n
 117 .EX
 118 cc \-D_BSD_SOURCE
 119 cc \-D_XOPEN_SOURCE           # Or any value < 500
 120 .EE
 121 .in
 122 .PP
 123 Note that, as described below,
 124 .BR "some feature test macros are defined by default" ,
 125 so that it may not always be necessary to
 126 explicitly specify the feature test macro(s) shown in the
 127 SYNOPSIS.
 128 .PP
 129 In a few cases, manual pages use a shorthand for expressing the
 130 feature test macro requirements (this example from
 131 .BR readahead (2)):
 132 .PP
 133 .in +4n
 134 .EX
 135 .B #define _GNU_SOURCE
 136 .B #include <fcntl.h>
 137 .PP
 138 .BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
 139 .EE
 140 .in
 141 .PP
 142 This format is employed in cases where only a single
 143 feature test macro can be used to expose the function
 144 declaration, and that macro is not defined by default.
 145 .SS Feature test macros understood by glibc
 146 The paragraphs below explain how feature test macros are handled
 147 in Linux glibc 2.\fIx\fP,
 148 .I x
 149 > 0.
 150 .PP
 151 First, though a summary of a few details for the impatient:
 152 .IP * 3
 153 The macros that you most likely need to use in modern source code are
 154 .BR _POSIX_C_SOURCE
 155 (for definitions from various versions of POSIX.1),
 156 .BR _XOPEN_SOURCE
 157 (for definitions from various versions of SUS),
 158 .BR _GNU_SOURCE
 159 (for GNU and/or Linux specific stuff), and
 160 .BR _DEFAULT_SOURCE
 161 (to get definitions that would normally be provided by default).
 162 .IP *
 163 Certain macros are defined with default values.
 164 Thus, although one or more macros may be indicated as being
 165 required in the SYNOPSIS of a man page,
 166 it may not be necessary to define them explicitly.
 167 Full details of the defaults are given later in this man page.
 168 .IP *
 169 Defining
 170 .BR _XOPEN_SOURCE
 171 with a value of 600 or greater produces the same effects as defining
 172 .BR _POSIX_C_SOURCE
 173 with a value of 200112L or greater.
 174 Where one sees
 175 .IP
 176 .in +4n
 177 .EX
 178 _POSIX_C_SOURCE >= 200112L
 179 .EE
 180 .in
 181 .IP
 182 in the feature test macro requirements in the SYNOPSIS of a man page,
 183 it is implicit that the following has the same effect:
 184 .IP
 185 .in +4n
 186 .EX
 187 _XOPEN_SOURCE >= 600
 188 .EE
 189 .in
 190 .IP *
 191 Defining
 192 .BR _XOPEN_SOURCE
 193 with a value of 700 or greater produces the same effects as defining
 194 .BR _POSIX_C_SOURCE
 195 with a value of 200809L or greater.
 196 Where one sees
 197 .IP
 198 .in +4n
 199 .EX
 200 _POSIX_C_SOURCE >= 200809L
 201 .EE
 202 .in
 203 .IP
 204 in the feature test macro requirements in the SYNOPSIS of a man page,
 205 it is implicit that the following has the same effect:
 206 .IP
 207 .in +4n
 208 .EX
 209 _XOPEN_SOURCE >= 700
 210 .EE
 211 .in
 212 .\" The details in glibc 2.0 are simpler, but combining a
 213 .\" a description of them with the details in later glibc versions
 214 .\" would make for a complicated description.
 215 .PP
 216 Linux glibc understands the following feature test macros:
 217 .TP
 218 .B __STRICT_ANSI__
 219 ISO Standard C.
 220 This macro is implicitly defined by
 221 .BR gcc (1)
 222 when invoked with, for example, the
 223 .I -std=c99
 224 or
 225 .I -ansi
 226 flag.
 227 .TP
 228 .B _POSIX_C_SOURCE
 229 Defining this macro causes header files to expose definitions as follows:
 230 .RS
 231 .IP \(bu 3
 232 The value 1 exposes definitions conforming to POSIX.1-1990 and
 233 ISO C (1990).
 234 .IP \(bu
 235 The value 2 or greater additionally exposes
 236 definitions for POSIX.2-1992.
 237 .IP \(bu
 238 The value 199309L or greater additionally exposes
 239 definitions for POSIX.1b (real-time extensions).
 240 .\" 199506L functionality is available only since glibc 2.1
 241 .IP \(bu
 242 The value 199506L or greater additionally exposes
 243 definitions for POSIX.1c (threads).
 244 .IP \(bu
 245 (Since glibc 2.3.3)
 246 The value 200112L or greater additionally exposes definitions corresponding
 247 to the POSIX.1-2001 base specification (excluding the XSI extension).
 248 This value also causes C95 (since glibc 2.12) and
 249 C99 (since glibc 2.10) features to be exposed
 250 (in other words, the equivalent of defining
 251 .BR _ISOC99_SOURCE ).
 252 .IP \(bu
 253 (Since glibc 2.10)
 254 The value 200809L or greater additionally exposes definitions corresponding
 255 to the POSIX.1-2008 base specification (excluding the XSI extension).
 256 .RE
 257 .TP
 258 .B _POSIX_SOURCE
 259 Defining this obsolete macro with any value is equivalent to defining
 260 .B _POSIX_C_SOURCE
 261 with the value 1.
 262 .IP
 263 Since this macro is obsolete,
 264 its usage is generally not documented when discussing
 265 feature test macro requirements in the man pages.
 266 .TP
 267 .B _XOPEN_SOURCE
 268 Defining this macro causes header files to expose definitions as follows:
 269 .RS
 270 .IP \(bu 3
 271 Defining with any value exposes
 272 definitions conforming to POSIX.1, POSIX.2, and XPG4.
 273 .IP \(bu
 274 The value 500 or greater additionally exposes
 275 definitions for SUSv2 (UNIX 98).
 276 .IP \(bu
 277 (Since glibc 2.2) The value 600 or greater additionally exposes
 278 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
 279 plus the XSI extension) and C99 definitions.
 280 .IP \(bu
 281 (Since glibc 2.10) The value 700 or greater additionally exposes
 282 definitions for SUSv4 (i.e., the POSIX.1-2008 base specification
 283 plus the XSI extension).
 284 .RE
 285 .IP
 286 If
 287 .B __STRICT_ANSI__
 288 is not defined, or
 289 .BR _XOPEN_SOURCE
 290 is defined with a value greater than or equal to 500
 291 .I and
 292 neither
 293 .B _POSIX_SOURCE
 294 nor
 295 .B _POSIX_C_SOURCE
 296 is explicitly defined, then
 297 the following macros are implicitly defined:
 298 .RS
 299 .IP \(bu 3
 300 .B _POSIX_SOURCE
 301 is defined with the value 1.
 302 .IP \(bu
 303 .B _POSIX_C_SOURCE
 304 is defined, according to the value of
 305 .BR _XOPEN_SOURCE :
 306 .RS
 307 .TP
 308 .BR _XOPEN_SOURCE " < 500"
 309 .B _POSIX_C_SOURCE
 310 is defined with the value 2.
 311 .TP
 312 .RB "500 <= " _XOPEN_SOURCE " < 600"
 313 .B _POSIX_C_SOURCE
 314 is defined with the value 199506L.
 315 .TP
 316 .RB "600 <= " _XOPEN_SOURCE " < 700"
 317 .B _POSIX_C_SOURCE
 318 is defined with the value 200112L.
 319 .TP
 320 .RB "700 <= " _XOPEN_SOURCE " (since glibc 2.10)"
 321 .B _POSIX_C_SOURCE
 322 is defined with the value 200809L.
 323 .RE
 324 .RE
 325 .IP
 326 In addition, defining
 327 .BR _XOPEN_SOURCE
 328 with a value of 500 or greater produces the same effects as defining
 329 .BR _XOPEN_SOURCE_EXTENDED .
 330 .TP
 331 .B _XOPEN_SOURCE_EXTENDED
 332 If this macro is defined,
 333 .I and
 334 .B _XOPEN_SOURCE
 335 is defined, then expose definitions corresponding to the XPG4v2
 336 (SUSv1) UNIX extensions (UNIX 95).
 337 Defining
 338 .B _XOPEN_SOURCE
 339 with a value of 500 or more also produces the same effect as defining
 340 .BR _XOPEN_SOURCE_EXTENDED .
 341 Use of
 342 .BR _XOPEN_SOURCE_EXTENDED
 343 in new source code should be avoided.
 344 .IP
 345 Since defining
 346 .B _XOPEN_SOURCE
 347 with a value of 500 or more has the same effect as defining
 348 .BR _XOPEN_SOURCE_EXTENDED ,
 349 the latter (obsolete) feature test macro is generally not described in the
 350 SYNOPSIS in man pages.
 351 .TP
 352 .BR _ISOC99_SOURCE " (since glibc 2.1.3)"
 353 Exposes declarations consistent with the ISO C99 standard.
 354 .IP
 355 Earlier glibc 2.1.x versions recognized an equivalent macro named
 356 .B _ISOC9X_SOURCE
 357 (because the C99 standard had not then been finalized).
 358 Although the use of this macro is obsolete, glibc continues
 359 to recognize it for backward compatibility.
 360 .IP
 361 Defining
 362 .B _ISOC99_SOURCE
 363 also exposes ISO C (1990) Amendment 1 ("C95") definitions.
 364 (The primary change in C95 was support for international character sets.)
 365 .IP
 366 Invoking the C compiler with the option
 367 .IR \-std=c99
 368 produces the same effects as defining this macro.
 369 .TP
 370 .BR _ISOC11_SOURCE " (since glibc 2.16)"
 371 Exposes declarations consistent with the ISO C11 standard.
 372 Defining this macro also enables C99 and C95 features (like
 373 .BR _ISOC99_SOURCE ).
 374 .IP
 375 Invoking the C compiler with the option
 376 .IR \-std=c11
 377 produces the same effects as defining this macro.
 378 .TP
 379 .B _LARGEFILE64_SOURCE
 380 Expose definitions for the alternative API specified by the
 381 LFS (Large File Summit) as a "transitional extension" to the
 382 Single UNIX Specification.
 383 (See
 384 .UR http:\:/\:/opengroup.org\:/platform\:/lfs.html
 385 .UE .)
 386 The alternative API consists of a set of new objects
 387 (i.e., functions and types) whose names are suffixed with "64"
 388 (e.g.,
 389 .I off64_t
 390 versus
 391 .IR off_t ,
 392 .BR lseek64 ()
 393 versus
 394 .BR lseek (),
 395 etc.).
 396 New programs should not employ this macro; instead
 397 .I _FILE_OFFSET_BITS=64
 398 should be employed.
 399 .TP
 400 .BR _LARGEFILE_SOURCE
 401 This macro was historically used to expose certain functions (specifically
 402 .BR fseeko (3)
 403 and
 404 .BR ftello (3))
 405 that address limitations of earlier APIs
 406 .RB ( fseek (3)
 407 and
 408 .BR ftell (3))
 409 that use
 410 .IR "long int"
 411 for file offsets.
 412 This macro is implicitly defined if
 413 .BR _XOPEN_SOURCE
 414 is defined with a value greater than or equal to 500.
 415 New programs should not employ this macro;
 416 defining
 417 .BR _XOPEN_SOURCE
 418 as just described or defining
 419 .B _FILE_OFFSET_BITS
 420 with the value 64 is the preferred mechanism to achieve the same result.
 421 .TP
 422 .B _FILE_OFFSET_BITS
 423 Defining this macro with the value 64
 424 automatically converts references to 32-bit functions and data types
 425 related to file I/O and filesystem operations into references to
 426 their 64-bit counterparts.
 427 This is useful for performing I/O on large files (> 2 Gigabytes)
 428 on 32-bit systems.
 429 (Defining this macro permits correctly written programs to use
 430 large files with only a recompilation being required.)
 431 .IP
 432 64-bit systems naturally permit file sizes greater than 2 Gigabytes,
 433 and on those systems this macro has no effect.
 434 .TP
 435 .BR _BSD_SOURCE " (deprecated since glibc 2.20)"
 436 Defining this macro with any value causes header files to expose
 437 BSD-derived definitions.
 438 .IP
 439 In glibc versions up to and including 2.18,
 440 defining this macro also causes BSD definitions to be preferred in
 441 some situations where standards conflict, unless one or more of
 442 .BR _SVID_SOURCE ,
 443 .BR _POSIX_SOURCE ,
 444 .BR _POSIX_C_SOURCE ,
 445 .BR _XOPEN_SOURCE ,
 446 .BR _XOPEN_SOURCE_EXTENDED ,
 447 or
 448 .B _GNU_SOURCE
 449 is defined, in which case BSD definitions are disfavored.
 450 Since glibc 2.19,
 451 .B _BSD_SOURCE
 452 no longer causes BSD definitions to be preferred in case of conflicts.
 453 .IP
 454 Since glibc 2.20, this macro is deprecated.
 455 .\" commit c941736c92fa3a319221f65f6755659b2a5e0a20
 456 .\" commit 498afc54dfee41d33ba519f496e96480badace8e
 457 .\" commit acd7f096d79c181866d56d4aaf3b043e741f1e2c
 458 It now has the same effect as defining
 459 .BR _DEFAULT_SOURCE ,
 460 but generates a compile-time warning (unless
 461 .BR _DEFAULT_SOURCE
 462 .\" commit ade40b10ff5fa59a318cf55b9d8414b758e8df78
 463 is also defined).
 464 Use
 465 .B _DEFAULT_SOURCE
 466 instead.
 467 To allow code that requires
 468 .BR _BSD_SOURCE
 469 in glibc 2.19 and earlier and
 470 .BR _DEFAULT_SOURCE
 471 in glibc 2.20 and later to compile without warnings, define
 472 .I both
 473 .B _BSD_SOURCE
 474 and
 475 .BR _DEFAULT_SOURCE .
 476 .TP
 477 .BR _SVID_SOURCE " (deprecated since glibc 2.20)"
 478 Defining this macro with any value causes header files to expose
 479 System V-derived definitions.
 480 (SVID == System V Interface Definition; see
 481 .BR standards (7).)
 482 .IP
 483 Since glibc 2.20, this macro is deprecated in the same fashion as
 484 .BR _BSD_SOURCE .
 485 .TP
 486 .BR _DEFAULT_SOURCE " (since glibc 2.19)"
 487 This macro can be defined to ensure that the "default"
 488 definitions are provided even when the defaults would otherwise
 489 be disabled,
 490 as happens when individual macros are explicitly defined,
 491 or the compiler is invoked in one of its "standard" modes (e.g.,
 492 .IR "cc\ \-std=c99" ).
 493 Defining
 494 .B _DEFAULT_SOURCE
 495 without defining other individual macros
 496 or invoking the compiler in one of its "standard" modes has no effect.
 497 .IP
 498 The "default" definitions comprise those required by POSIX.1-2008 and ISO C99,
 499 as well as various definitions originally derived from BSD and System V.
 500 On glibc 2.19 and earlier, these defaults were approximately equivalent
 501 to explicitly defining the following:
 502 .IP
 503     cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
 504 .TP
 505 .BR _ATFILE_SOURCE " (since glibc 2.4)"
 506 Defining this macro with any value causes header files to expose
 507 declarations of a range of functions with the suffix "at";
 508 see
 509 .BR openat (2).
 510 Since glibc 2.10, this macro is also implicitly defined if
 511 .BR _POSIX_C_SOURCE
 512 is defined with a value greater than or equal to 200809L.
 513 .TP
 514 .B _GNU_SOURCE
 515 Defining this macro (with any value) implicitly defines
 516 .BR _ATFILE_SOURCE ,
 517 .BR _LARGEFILE64_SOURCE ,
 518 .BR _ISOC99_SOURCE ,
 519 .BR _XOPEN_SOURCE_EXTENDED ,
 520 .BR _POSIX_SOURCE ,
 521 .B _POSIX_C_SOURCE
 522 with the value 200809L
 523 (200112L in glibc versions before 2.10;
 524 199506L in glibc versions before 2.5;
 525 199309L in glibc versions before 2.1)
 526 and
 527 .B _XOPEN_SOURCE
 528 with the value 700
 529 (600 in glibc versions before 2.10;
 530 500 in glibc versions before 2.2).
 531 In addition, various GNU-specific extensions are also exposed.
 532 .IP
 533 Since glibc 2.19, defining
 534 .BR _GNU_SOURCE
 535 also has the effect of implicitly defining
 536 .BR _DEFAULT_SOURCE .
 537 In glibc versions before 2.20, defining
 538 .BR _GNU_SOURCE
 539 also had the effect of implicitly defining
 540 .BR _BSD_SOURCE
 541 and
 542 .BR _SVID_SOURCE .
 543 .TP
 544 .B _REENTRANT
 545 Historically, on various C libraries
 546 it was necessary to define this macro in all
 547 multithreaded code.
 548 .\" Zack Weinberg
 549 .\"     There did once exist C libraries where it was necessary. The ones
 550 .\"     I remember were proprietary Unix vendor libcs from the mid-1990s
 551 .\"     You would get completely unlocked stdio without _REENTRANT.
 552 (Some C libraries may still require this.)
 553 In glibc,
 554 this macro also exposed definitions of certain reentrant functions.
 555 .IP
 556 However, glibc has been thread-safe by default for many years;
 557 since glibc 2.3, the only effect of defining
 558 .BR _REENTRANT
 559 has been to enable one or two of the same declarations that
 560 are also enabled by defining
 561 .BR _POSIX_C_SOURCE
 562 with a value of 199606L or greater.
 563 .IP
 564 .B _REENTRANT
 565 is now obsolete.
 566 In glibc 2.25 and later, defining
 567 .B _REENTRANT
 568 is equivalent to defining
 569 .B _POSIX_C_SOURCE
 570 with the value 199606L.
 571 If a higher POSIX conformance level is
 572 selected by any other means (such as
 573 .B _POSIX_C_SOURCE
 574 itself,
 575 .BR _XOPEN_SOURCE ,
 576 .BR _DEFAULT_SOURCE ,
 577 or
 578 .BR _GNU_SOURCE ),
 579 then defining
 580 .B _REENTRANT
 581 has no effect.
 582 .IP
 583 This macro is automatically defined if one compiles with
 584 .IR "cc\ \-pthread" .
 585 .TP
 586 .B _THREAD_SAFE
 587 Synonym for the (deprecated)
 588 .BR _REENTRANT ,
 589 provided for compatibility with some other implementations.
 590 .TP
 591 .BR _FORTIFY_SOURCE " (since glibc 2.3.4)"
 592 .\" For more detail, see:
 593 .\" http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
 594 .\" [PATCH] Object size checking to prevent (some) buffer overflows
 595 .\" * From: Jakub Jelinek <jakub at redhat dot com>
 596 .\" * To: gcc-patches at gcc dot gnu dot org
 597 .\" * Date: Tue, 21 Sep 2004 04:16:40 -0400
 598 Defining this macro causes some lightweight checks to be performed
 599 to detect some buffer overflow errors when employing
 600 various string and memory manipulation functions (for example,
 601 .BR memcpy (3),
 602 .BR memset (3),
 603 .BR stpcpy (3),
 604 .BR strcpy (3),
 605 .BR strncpy (3),
 606 .BR strcat (3),
 607 .BR strncat (3),
 608 .BR sprintf (3),
 609 .BR snprintf (3),
 610 .BR vsprintf (3),
 611 .BR vsnprintf (3),
 612 .BR gets (3),
 613 and wide character variants thereof).
 614 For some functions, argument consistency is checked;
 615 for example, a check is made that
 616 .BR open (2)
 617 has been supplied with a
 618 .I mode
 619 argument when the specified flags include
 620 .BR O_CREAT .
 621 Not all problems are detected, just some common cases.
 622 .\" Look for __USE_FORTIFY_LEVEL in the header files
 623 .IP
 624 If
 625 .B _FORTIFY_SOURCE
 626 is set to 1, with compiler optimization level 1
 627 .RI ( "gcc\ \-O1" )
 628 and above, checks that shouldn't change the behavior of
 629 conforming programs are performed.
 630 With
 631 .B _FORTIFY_SOURCE
 632 set to 2, some more checking is added, but
 633 some conforming programs might fail.
 634 .\" For example, given the following code
 635 .\"        int d;
 636 .\"        char buf[1000], buf[1000];
 637 .\"        strcpy(fmt, "Hello world\n%n");
 638 .\"        snprintf(buf, sizeof(buf), fmt, &d);
 639 .\"
 640 .\" Compiling with "gcc -D_FORTIFY_SOURCE=2 -O1" and then running will
 641 .\" cause the following diagnostic at run time at the snprintf() call
 642 .\"
 643 .\"        *** %n in writable segment detected ***
 644 .\"        Aborted (core dumped)
 645 .\"
 646 .IP
 647 Some of the checks can be performed at compile time
 648 (via macros logic implemented in header files),
 649 and result in compiler warnings;
 650 other checks take place at run time,
 651 and result in a run-time error if the check fails.
 652 .IP
 653 Use of this macro requires compiler support, available with
 654 .BR gcc (1)
 655 since version 4.0.
 656 .SS Default definitions, implicit definitions, and combining definitions
 657 .PP
 658 If no feature test macros are explicitly defined,
 659 then the following feature test macros are defined by default:
 660 .BR _BSD_SOURCE
 661 (in glibc 2.19 and earlier),
 662 .BR _SVID_SOURCE
 663 (in glibc 2.19 and earlier),
 664 .BR _DEFAULT_SOURCE
 665 (since glibc 2.19),
 666 .BR _POSIX_SOURCE ,
 667 and
 668 .BR _POSIX_C_SOURCE =200809L
 669 (200112L in glibc versions before 2.10;
 670 199506L in glibc versions before 2.4;
 671 199309L in glibc versions before 2.1).
 672 .PP
 673 If any of
 674 .BR __STRICT_ANSI__ ,
 675 .BR _ISOC99_SOURCE ,
 676 .BR _POSIX_SOURCE ,
 677 .BR _POSIX_C_SOURCE  ,
 678 .BR _XOPEN_SOURCE ,
 679 .BR _XOPEN_SOURCE_EXTENDED ,
 680 .BR _BSD_SOURCE
 681 (in glibc 2.19 and earlier),
 682 or
 683 .B _SVID_SOURCE
 684 (in glibc 2.19 and earlier)
 685 is explicitly defined, then
 686 .BR _BSD_SOURCE ,
 687 .BR _SVID_SOURCE ,
 688 and
 689 .BR _DEFAULT_SOURCE
 690 are not defined by default.
 691 .PP
 692 If
 693 .B _POSIX_SOURCE
 694 and
 695 .B _POSIX_C_SOURCE
 696 are not explicitly defined,
 697 and either
 698 .B __STRICT_ANSI__
 699 is not defined or
 700 .B _XOPEN_SOURCE
 701 is defined with a value of 500 or more, then
 702 .IP * 3
 703 .B _POSIX_SOURCE
 704 is defined with the value 1; and
 705 .IP *
 706 .B _POSIX_C_SOURCE
 707 is defined with one of the following values:
 708 .RS 3
 709 .IP \(bu 3
 710 2,
 711 if
 712 .B _XOPEN_SOURCE
 713 is defined with a value less than 500;
 714 .IP \(bu
 715 199506L,
 716 if
 717 .B _XOPEN_SOURCE
 718 is defined with a value greater than or equal to 500 and less than 600;
 719 or
 720 .IP \(bu
 721 (since glibc 2.4) 200112L,
 722 if
 723 .B _XOPEN_SOURCE
 724 is defined with a value greater than or equal to 600 and less than 700.
 725 .IP \(bu
 726 (Since glibc 2.10)
 727 200809L,
 728 if
 729 .B _XOPEN_SOURCE
 730 is defined with a value greater than or equal to 700.
 731 .IP \(bu
 732 Older versions of glibc do not know about the values
 733 200112L and 200809L for
 734 .BR _POSIX_C_SOURCE ,
 735 and the setting of this macro will depend on the glibc version.
 736 .IP \(bu
 737 If
 738 .B _XOPEN_SOURCE
 739 is undefined, then the setting of
 740 .B _POSIX_C_SOURCE
 741 depends on the glibc version:
 742 199506L, in glibc versions before 2.4;
 743 200112L, in glibc 2.4 to 2.9; and
 744 200809L, since glibc 2.10.
 745 .RE
 746 .PP
 747 Multiple macros can be defined; the results are additive.
 748 .SH CONFORMING TO
 749 POSIX.1 specifies
 750 .BR _POSIX_C_SOURCE ,
 751 .BR _POSIX_SOURCE ,
 752 and
 753 .BR _XOPEN_SOURCE .
 754 .PP
 755 .B _XOPEN_SOURCE_EXTENDED
 756 was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2 and later.
 757 .B _FILE_OFFSET_BITS
 758 is not specified by any standard,
 759 but is employed on some other implementations.
 760 .PP
 761 .BR _BSD_SOURCE ,
 762 .BR _SVID_SOURCE ,
 763 .BR _DEFAULT_SOURCE ,
 764 .BR _ATFILE_SOURCE ,
 765 .BR _GNU_SOURCE ,
 766 .BR _FORTIFY_SOURCE ,
 767 .BR _REENTRANT ,
 768 and
 769 .B _THREAD_SAFE
 770 are specific to Linux (glibc).
 771 .SH NOTES
 772 .I <features.h>
 773 is a Linux/glibc-specific header file.
 774 Other systems have an analogous file, but typically with a different name.
 775 This header file is automatically included by other header files as
 776 required: it is not necessary to explicitly include it in order to
 777 employ feature test macros.
 778 .PP
 779 According to which of the above feature test macros are defined,
 780 .I <features.h>
 781 internally defines various other macros that are checked by
 782 other glibc header files.
 783 These macros have names prefixed by two underscores (e.g.,
 784 .BR __USE_MISC ).
 785 Programs should
 786 .I never
 787 define these macros directly:
 788 instead, the appropriate feature test macro(s) from the
 789 list above should be employed.
 790 .SH EXAMPLE
 791 The program below can be used to explore how the various
 792 feature test macros are set depending on the glibc version
 793 and what feature test macros are explicitly set.
 794 The following shell session, on a system with glibc 2.10,
 795 shows some examples of what we would see:
 796 .PP
 797 .in +4n
 798 .EX
 799 $ \fBcc ftm.c\fP
 800 $ \fB./a.out\fP
 801 _POSIX_SOURCE defined
 802 _POSIX_C_SOURCE defined: 200809L
 803 _BSD_SOURCE defined
 804 _SVID_SOURCE defined
 805 _ATFILE_SOURCE defined
 806 $ \fBcc \-D_XOPEN_SOURCE=500 ftm.c\fP
 807 $ \fB./a.out\fP
 808 _POSIX_SOURCE defined
 809 _POSIX_C_SOURCE defined: 199506L
 810 _XOPEN_SOURCE defined: 500
 811 $ \fBcc \-D_GNU_SOURCE ftm.c\fP
 812 $ \fB./a.out\fP
 813 _POSIX_SOURCE defined
 814 _POSIX_C_SOURCE defined: 200809L
 815 _ISOC99_SOURCE defined
 816 _XOPEN_SOURCE defined: 700
 817 _XOPEN_SOURCE_EXTENDED defined
 818 _LARGEFILE64_SOURCE defined
 819 _BSD_SOURCE defined
 820 _SVID_SOURCE defined
 821 _ATFILE_SOURCE defined
 822 _GNU_SOURCE defined
 823 .EE
 824 .in
 825 .SS Program source
 826 \&
 827 .EX
 828 /* ftm.c */
 829
 830 #include <stdio.h>
 831 #include <unistd.h>
 832 #include <stdlib.h>
 833
 834 int
 835 main(int argc, char *argv[])
 836 {
 837 #ifdef _POSIX_SOURCE
 838     printf("_POSIX_SOURCE defined\en");
 839 #endif
 840
 841 #ifdef _POSIX_C_SOURCE
 842     printf("_POSIX_C_SOURCE defined: %ldL\en", (long) _POSIX_C_SOURCE);
 843 #endif
 844
 845 #ifdef _ISOC99_SOURCE
 846     printf("_ISOC99_SOURCE defined\en");
 847 #endif
 848
 849 #ifdef _ISOC11_SOURCE
 850     printf("_ISOC11_SOURCE defined\en");
 851 #endif
 852
 853 #ifdef _XOPEN_SOURCE
 854     printf("_XOPEN_SOURCE defined: %d\en", _XOPEN_SOURCE);
 855 #endif
 856
 857 #ifdef _XOPEN_SOURCE_EXTENDED
 858     printf("_XOPEN_SOURCE_EXTENDED defined\en");
 859 #endif
 860
 861 #ifdef _LARGEFILE64_SOURCE
 862     printf("_LARGEFILE64_SOURCE defined\en");
 863 #endif
 864
 865 #ifdef _FILE_OFFSET_BITS
 866     printf("_FILE_OFFSET_BITS defined: %d\en", _FILE_OFFSET_BITS);
 867 #endif
 868
 869 #ifdef _BSD_SOURCE
 870     printf("_BSD_SOURCE defined\en");
 871 #endif
 872
 873 #ifdef _SVID_SOURCE
 874     printf("_SVID_SOURCE defined\en");
 875 #endif
 876
 877 #ifdef _DEFAULT_SOURCE
 878     printf("_DEFAULT_SOURCE defined\en");
 879 #endif
 880
 881 #ifdef _ATFILE_SOURCE
 882     printf("_ATFILE_SOURCE defined\en");
 883 #endif
 884
 885 #ifdef _GNU_SOURCE
 886     printf("_GNU_SOURCE defined\en");
 887 #endif
 888
 889 #ifdef _REENTRANT
 890     printf("_REENTRANT defined\en");
 891 #endif
 892
 893 #ifdef _THREAD_SAFE
 894     printf("_THREAD_SAFE defined\en");
 895 #endif
 896
 897 #ifdef _FORTIFY_SOURCE
 898     printf("_FORTIFY_SOURCE defined\en");
 899 #endif
 900
 901     exit(EXIT_SUCCESS);
 902 }
 903 .EE
 904 .SH SEE ALSO
 905 .BR libc (7),
 906 .BR standards (7)
 907 .PP
 908 The section "Feature Test Macros" under
 909 .IR "info libc" .
 910 .\" But beware: the info libc document is out of date (Jul 07, mtk)
 911 .PP
 912 .I /usr/include/features.h