man7/feature_test_macros.7

   1 .\" This manpage is Copyright (C) 2006, Michael Kerrisk
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .TH FEATURE_TEST_MACROS 7 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
   6 .SH NAME
   7 feature_test_macros \- feature test macros
   8 .SH DESCRIPTION
   9 Feature test macros allow the programmer to control the definitions that
  10 are exposed by system header files when a program is compiled.
  11 .PP
  12 .B NOTE:
  13 In order to be effective, a feature test macro
  14 .IR "must be defined before including any header files" .
  15 This can be done either in the compilation command
  16 .RI ( "cc \-DMACRO=value" )
  17 or by defining the macro within the source code before
  18 including any headers.
  19 The requirement that the macro must be defined before including any
  20 header file exists because header files may freely include one another.
  21 Thus, for example, in the following lines, defining the
  22 .B _GNU_SOURCE
  23 macro may have no effect because the header
  24 .I <abc.h>
  25 itself includes
  26 .I <xyz.h>
  27 (POSIX explicitly allows this):
  28 .PP
  29 .in +4n
  30 .EX
  31 #include <abc.h>
  32 #define _GNU_SOURCE
  33 #include <xyz.h>
  34 .EE
  35 .in
  36 .PP
  37 Some feature test macros are useful for creating portable applications,
  38 by preventing nonstandard definitions from being exposed.
  39 Other macros can be used to expose nonstandard definitions that
  40 are not exposed by default.
  41 .PP
  42 The precise effects of each of the feature test macros described below
  43 can be ascertained by inspecting the
  44 .I <features.h>
  45 header file.
  46 .BR Note :
  47 applications do
  48 .I not
  49 need to directly include
  50 .IR <features.h> ;
  51 indeed, doing so is actively discouraged.
  52 See NOTES.
  53 .SS Specification of feature test macro requirements in manual pages
  54 When a function requires that a feature test macro is defined,
  55 the manual page SYNOPSIS typically includes a note of the following form
  56 (this example from the
  57 .BR acct (2)
  58 manual page):
  59 .PP
  60 .RS
  61 .B #include <unistd.h>
  62 .PP
  63 .BI "int acct(const char *" filename );
  64 .PP
  65 .RS -4
  66 .EX
  67 Feature Test Macro Requirements for glibc (see
  68 .BR feature_test_macros (7)):
  69 .EE
  70 .RE
  71 .PP
  72 .BR acct ():
  73 _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  74 .RE
  75 .PP
  76 The
  77 .B ||
  78 means that in order to obtain the declaration of
  79 .BR acct (2)
  80 from
  81 .IR <unistd.h> ,
  82 .I either
  83 of the following macro
  84 definitions must be made before including any header files:
  85 .PP
  86 .in +4n
  87 .EX
  88 #define _BSD_SOURCE
  89 #define _XOPEN_SOURCE        /* or any value < 500 */
  90 .EE
  91 .in
  92 .PP
  93 Alternatively, equivalent definitions can be included in the
  94 compilation command:
  95 .PP
  96 .in +4n
  97 .EX
  98 cc \-D_BSD_SOURCE
  99 cc \-D_XOPEN_SOURCE           # Or any value < 500
 100 .EE
 101 .in
 102 .PP
 103 Note that, as described below,
 104 .BR "some feature test macros are defined by default" ,
 105 so that it may not always be necessary to
 106 explicitly specify the feature test macro(s) shown in the
 107 SYNOPSIS.
 108 .PP
 109 In a few cases, manual pages use a shorthand for expressing the
 110 feature test macro requirements (this example from
 111 .BR readahead (2)):
 112 .PP
 113 .RS +4
 114 .EX
 115 .B #define _GNU_SOURCE
 116 .B #include <fcntl.h>
 117 .PP
 118 .BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
 119 .EE
 120 .RE
 121 .PP
 122 This format is employed in cases where only a single
 123 feature test macro can be used to expose the function
 124 declaration, and that macro is not defined by default.
 125 .SS Feature test macros understood by glibc
 126 The paragraphs below explain how feature test macros are handled
 127 in glibc 2.\fIx\fP,
 128 .I x
 129 > 0.
 130 .PP
 131 First, though, a summary of a few details for the impatient:
 132 .IP * 3
 133 The macros that you most likely need to use in modern source code are
 134 .B _POSIX_C_SOURCE
 135 (for definitions from various versions of POSIX.1),
 136 .B _XOPEN_SOURCE
 137 (for definitions from various versions of SUS),
 138 .B _GNU_SOURCE
 139 (for GNU and/or Linux specific stuff), and
 140 .B _DEFAULT_SOURCE
 141 (to get definitions that would normally be provided by default).
 142 .IP *
 143 Certain macros are defined with default values.
 144 Thus, although one or more macros may be indicated as being
 145 required in the SYNOPSIS of a man page,
 146 it may not be necessary to define them explicitly.
 147 Full details of the defaults are given later in this man page.
 148 .IP *
 149 Defining
 150 .B _XOPEN_SOURCE
 151 with a value of 600 or greater produces the same effects as defining
 152 .B _POSIX_C_SOURCE
 153 with a value of 200112L or greater.
 154 Where one sees
 155 .IP
 156 .in +4n
 157 .EX
 158 _POSIX_C_SOURCE >= 200112L
 159 .EE
 160 .in
 161 .IP
 162 in the feature test macro requirements in the SYNOPSIS of a man page,
 163 it is implicit that the following has the same effect:
 164 .IP
 165 .in +4n
 166 .EX
 167 _XOPEN_SOURCE >= 600
 168 .EE
 169 .in
 170 .IP *
 171 Defining
 172 .B _XOPEN_SOURCE
 173 with a value of 700 or greater produces the same effects as defining
 174 .B _POSIX_C_SOURCE
 175 with a value of 200809L or greater.
 176 Where one sees
 177 .IP
 178 .in +4n
 179 .EX
 180 _POSIX_C_SOURCE >= 200809L
 181 .EE
 182 .in
 183 .IP
 184 in the feature test macro requirements in the SYNOPSIS of a man page,
 185 it is implicit that the following has the same effect:
 186 .IP
 187 .in +4n
 188 .EX
 189 _XOPEN_SOURCE >= 700
 190 .EE
 191 .in
 192 .\" The details in glibc 2.0 are simpler, but combining a
 193 .\" a description of them with the details in later glibc versions
 194 .\" would make for a complicated description.
 195 .PP
 196 Glibc understands the following feature test macros:
 197 .TP
 198 .B __STRICT_ANSI__
 199 ISO Standard C.
 200 This macro is implicitly defined by
 201 .BR gcc (1)
 202 when invoked with, for example, the
 203 .I \-std=c99
 204 or
 205 .I \-ansi
 206 flag.
 207 .TP
 208 .B _POSIX_C_SOURCE
 209 Defining this macro causes header files to expose definitions as follows:
 210 .RS
 211 .IP \(bu 3
 212 The value 1 exposes definitions conforming to POSIX.1-1990 and
 213 ISO C (1990).
 214 .IP \(bu
 215 The value 2 or greater additionally exposes
 216 definitions for POSIX.2-1992.
 217 .IP \(bu
 218 The value 199309L or greater additionally exposes
 219 definitions for POSIX.1b (real-time extensions).
 220 .\" 199506L functionality is available only since glibc 2.1
 221 .IP \(bu
 222 The value 199506L or greater additionally exposes
 223 definitions for POSIX.1c (threads).
 224 .IP \(bu
 225 (Since glibc 2.3.3)
 226 The value 200112L or greater additionally exposes definitions corresponding
 227 to the POSIX.1-2001 base specification (excluding the XSI extension).
 228 This value also causes C95 (since glibc 2.12) and
 229 C99 (since glibc 2.10) features to be exposed
 230 (in other words, the equivalent of defining
 231 .BR _ISOC99_SOURCE ).
 232 .IP \(bu
 233 (Since glibc 2.10)
 234 The value 200809L or greater additionally exposes definitions corresponding
 235 to the POSIX.1-2008 base specification (excluding the XSI extension).
 236 .RE
 237 .TP
 238 .B _POSIX_SOURCE
 239 Defining this obsolete macro with any value is equivalent to defining
 240 .B _POSIX_C_SOURCE
 241 with the value 1.
 242 .IP
 243 Since this macro is obsolete,
 244 its usage is generally not documented when discussing
 245 feature test macro requirements in the man pages.
 246 .TP
 247 .B _XOPEN_SOURCE
 248 Defining this macro causes header files to expose definitions as follows:
 249 .RS
 250 .IP \(bu 3
 251 Defining with any value exposes
 252 definitions conforming to POSIX.1, POSIX.2, and XPG4.
 253 .IP \(bu
 254 The value 500 or greater additionally exposes
 255 definitions for SUSv2 (UNIX 98).
 256 .IP \(bu
 257 (Since glibc 2.2) The value 600 or greater additionally exposes
 258 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
 259 plus the XSI extension) and C99 definitions.
 260 .IP \(bu
 261 (Since glibc 2.10) The value 700 or greater additionally exposes
 262 definitions for SUSv4 (i.e., the POSIX.1-2008 base specification
 263 plus the XSI extension).
 264 .RE
 265 .IP
 266 If
 267 .B __STRICT_ANSI__
 268 is not defined, or
 269 .B _XOPEN_SOURCE
 270 is defined with a value greater than or equal to 500
 271 .I and
 272 neither
 273 .B _POSIX_SOURCE
 274 nor
 275 .B _POSIX_C_SOURCE
 276 is explicitly defined, then
 277 the following macros are implicitly defined:
 278 .RS
 279 .IP \(bu 3
 280 .B _POSIX_SOURCE
 281 is defined with the value 1.
 282 .IP \(bu
 283 .B _POSIX_C_SOURCE
 284 is defined, according to the value of
 285 .BR _XOPEN_SOURCE :
 286 .RS
 287 .TP
 288 .BR _XOPEN_SOURCE " < 500"
 289 .B _POSIX_C_SOURCE
 290 is defined with the value 2.
 291 .TP
 292 .RB "500 <= " _XOPEN_SOURCE " < 600"
 293 .B _POSIX_C_SOURCE
 294 is defined with the value 199506L.
 295 .TP
 296 .RB "600 <= " _XOPEN_SOURCE " < 700"
 297 .B _POSIX_C_SOURCE
 298 is defined with the value 200112L.
 299 .TP
 300 .RB "700 <= " _XOPEN_SOURCE " (since glibc 2.10)"
 301 .B _POSIX_C_SOURCE
 302 is defined with the value 200809L.
 303 .RE
 304 .RE
 305 .IP
 306 In addition, defining
 307 .B _XOPEN_SOURCE
 308 with a value of 500 or greater produces the same effects as defining
 309 .BR _XOPEN_SOURCE_EXTENDED .
 310 .TP
 311 .B _XOPEN_SOURCE_EXTENDED
 312 If this macro is defined,
 313 .I and
 314 .B _XOPEN_SOURCE
 315 is defined, then expose definitions corresponding to the XPG4v2
 316 (SUSv1) UNIX extensions (UNIX 95).
 317 Defining
 318 .B _XOPEN_SOURCE
 319 with a value of 500 or more also produces the same effect as defining
 320 .BR _XOPEN_SOURCE_EXTENDED .
 321 Use of
 322 .B _XOPEN_SOURCE_EXTENDED
 323 in new source code should be avoided.
 324 .IP
 325 Since defining
 326 .B _XOPEN_SOURCE
 327 with a value of 500 or more has the same effect as defining
 328 .BR _XOPEN_SOURCE_EXTENDED ,
 329 the latter (obsolete) feature test macro is generally not described in the
 330 SYNOPSIS in man pages.
 331 .TP
 332 .BR _ISOC99_SOURCE " (since glibc 2.1.3)"
 333 Exposes declarations consistent with the ISO C99 standard.
 334 .IP
 335 Earlier glibc 2.1.x versions recognized an equivalent macro named
 336 .B _ISOC9X_SOURCE
 337 (because the C99 standard had not then been finalized).
 338 Although the use of this macro is obsolete, glibc continues
 339 to recognize it for backward compatibility.
 340 .IP
 341 Defining
 342 .B _ISOC99_SOURCE
 343 also exposes ISO C (1990) Amendment 1 ("C95") definitions.
 344 (The primary change in C95 was support for international character sets.)
 345 .IP
 346 Invoking the C compiler with the option
 347 .I \-std=c99
 348 produces the same effects as defining this macro.
 349 .TP
 350 .BR _ISOC11_SOURCE " (since glibc 2.16)"
 351 Exposes declarations consistent with the ISO C11 standard.
 352 Defining this macro also enables C99 and C95 features (like
 353 .BR _ISOC99_SOURCE ).
 354 .IP
 355 Invoking the C compiler with the option
 356 .I \-std=c11
 357 produces the same effects as defining this macro.
 358 .TP
 359 .B _LARGEFILE64_SOURCE
 360 Expose definitions for the alternative API specified by the
 361 LFS (Large File Summit) as a "transitional extension" to the
 362 Single UNIX Specification.
 363 (See
 364 .UR http:\:/\:/opengroup.org\:/platform\:/lfs.html
 365 .UE .)
 366 The alternative API consists of a set of new objects
 367 (i.e., functions and types) whose names are suffixed with "64"
 368 (e.g.,
 369 .I off64_t
 370 versus
 371 .IR off_t ,
 372 .BR lseek64 ()
 373 versus
 374 .BR lseek (),
 375 etc.).
 376 New programs should not employ this macro; instead
 377 .I _FILE_OFFSET_BITS=64
 378 should be employed.
 379 .TP
 380 .B _LARGEFILE_SOURCE
 381 This macro was historically used to expose certain functions (specifically
 382 .BR fseeko (3)
 383 and
 384 .BR ftello (3))
 385 that address limitations of earlier APIs
 386 .RB ( fseek (3)
 387 and
 388 .BR ftell (3))
 389 that use
 390 .I long
 391 for file offsets.
 392 This macro is implicitly defined if
 393 .B _XOPEN_SOURCE
 394 is defined with a value greater than or equal to 500.
 395 New programs should not employ this macro;
 396 defining
 397 .B _XOPEN_SOURCE
 398 as just described or defining
 399 .B _FILE_OFFSET_BITS
 400 with the value 64 is the preferred mechanism to achieve the same result.
 401 .TP
 402 .B _FILE_OFFSET_BITS
 403 Defining this macro with the value 64
 404 automatically converts references to 32-bit functions and data types
 405 related to file I/O and filesystem operations into references to
 406 their 64-bit counterparts.
 407 This is useful for performing I/O on large files (> 2 Gigabytes)
 408 on 32-bit systems.
 409 (Defining this macro permits correctly written programs to use
 410 large files with only a recompilation being required.)
 411 .IP
 412 64-bit systems naturally permit file sizes greater than 2 Gigabytes,
 413 and on those systems this macro has no effect.
 414 .TP
 415 .BR _BSD_SOURCE " (deprecated since glibc 2.20)"
 416 Defining this macro with any value causes header files to expose
 417 BSD-derived definitions.
 418 .IP
 419 In glibc versions up to and including 2.18,
 420 defining this macro also causes BSD definitions to be preferred in
 421 some situations where standards conflict, unless one or more of
 422 .BR _SVID_SOURCE ,
 423 .BR _POSIX_SOURCE ,
 424 .BR _POSIX_C_SOURCE ,
 425 .BR _XOPEN_SOURCE ,
 426 .BR _XOPEN_SOURCE_EXTENDED ,
 427 or
 428 .B _GNU_SOURCE
 429 is defined, in which case BSD definitions are disfavored.
 430 Since glibc 2.19,
 431 .B _BSD_SOURCE
 432 no longer causes BSD definitions to be preferred in case of conflicts.
 433 .IP
 434 Since glibc 2.20, this macro is deprecated.
 435 .\" commit c941736c92fa3a319221f65f6755659b2a5e0a20
 436 .\" commit 498afc54dfee41d33ba519f496e96480badace8e
 437 .\" commit acd7f096d79c181866d56d4aaf3b043e741f1e2c
 438 It now has the same effect as defining
 439 .BR _DEFAULT_SOURCE ,
 440 but generates a compile-time warning (unless
 441 .B _DEFAULT_SOURCE
 442 .\" commit ade40b10ff5fa59a318cf55b9d8414b758e8df78
 443 is also defined).
 444 Use
 445 .B _DEFAULT_SOURCE
 446 instead.
 447 To allow code that requires
 448 .B _BSD_SOURCE
 449 in glibc 2.19 and earlier and
 450 .B _DEFAULT_SOURCE
 451 in glibc 2.20 and later to compile without warnings, define
 452 .I both
 453 .B _BSD_SOURCE
 454 and
 455 .BR _DEFAULT_SOURCE .
 456 .TP
 457 .BR _SVID_SOURCE " (deprecated since glibc 2.20)"
 458 Defining this macro with any value causes header files to expose
 459 System V-derived definitions.
 460 (SVID == System V Interface Definition; see
 461 .BR standards (7).)
 462 .IP
 463 Since glibc 2.20, this macro is deprecated in the same fashion as
 464 .BR _BSD_SOURCE .
 465 .TP
 466 .BR _DEFAULT_SOURCE " (since glibc 2.19)"
 467 This macro can be defined to ensure that the "default"
 468 definitions are provided even when the defaults would otherwise
 469 be disabled,
 470 as happens when individual macros are explicitly defined,
 471 or the compiler is invoked in one of its "standard" modes (e.g.,
 472 .IR cc\~\-std=c99 ).
 473 Defining
 474 .B _DEFAULT_SOURCE
 475 without defining other individual macros
 476 or invoking the compiler in one of its "standard" modes has no effect.
 477 .IP
 478 The "default" definitions comprise those required by POSIX.1-2008 and ISO C99,
 479 as well as various definitions originally derived from BSD and System V.
 480 On glibc 2.19 and earlier, these defaults were approximately equivalent
 481 to explicitly defining the following:
 482 .IP
 483 .in +4n
 484 .EX
 485 cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
 486 .EE
 487 .in
 488 .TP
 489 .BR _ATFILE_SOURCE " (since glibc 2.4)"
 490 Defining this macro with any value causes header files to expose
 491 declarations of a range of functions with the suffix "at";
 492 see
 493 .BR openat (2).
 494 Since glibc 2.10, this macro is also implicitly defined if
 495 .B _POSIX_C_SOURCE
 496 is defined with a value greater than or equal to 200809L.
 497 .TP
 498 .B _GNU_SOURCE
 499 Defining this macro (with any value) implicitly defines
 500 .BR _ATFILE_SOURCE ,
 501 .BR _LARGEFILE64_SOURCE ,
 502 .BR _ISOC99_SOURCE ,
 503 .BR _XOPEN_SOURCE_EXTENDED ,
 504 .BR _POSIX_SOURCE ,
 505 .B _POSIX_C_SOURCE
 506 with the value 200809L
 507 (200112L in glibc versions before 2.10;
 508 199506L in glibc versions before 2.5;
 509 199309L in glibc versions before 2.1)
 510 and
 511 .B _XOPEN_SOURCE
 512 with the value 700
 513 (600 in glibc versions before 2.10;
 514 500 in glibc versions before 2.2).
 515 In addition, various GNU-specific extensions are also exposed.
 516 .IP
 517 Since glibc 2.19, defining
 518 .B _GNU_SOURCE
 519 also has the effect of implicitly defining
 520 .BR _DEFAULT_SOURCE .
 521 In glibc versions before 2.20, defining
 522 .B _GNU_SOURCE
 523 also had the effect of implicitly defining
 524 .B _BSD_SOURCE
 525 and
 526 .BR _SVID_SOURCE .
 527 .TP
 528 .B _REENTRANT
 529 Historically, on various C libraries
 530 it was necessary to define this macro in all
 531 multithreaded code.
 532 .\" Zack Weinberg
 533 .\"     There did once exist C libraries where it was necessary. The ones
 534 .\"     I remember were proprietary Unix vendor libcs from the mid-1990s
 535 .\"     You would get completely unlocked stdio without _REENTRANT.
 536 (Some C libraries may still require this.)
 537 In glibc,
 538 this macro also exposed definitions of certain reentrant functions.
 539 .IP
 540 However, glibc has been thread-safe by default for many years;
 541 since glibc 2.3, the only effect of defining
 542 .B _REENTRANT
 543 has been to enable one or two of the same declarations that
 544 are also enabled by defining
 545 .B _POSIX_C_SOURCE
 546 with a value of 199606L or greater.
 547 .IP
 548 .B _REENTRANT
 549 is now obsolete.
 550 In glibc 2.25 and later, defining
 551 .B _REENTRANT
 552 is equivalent to defining
 553 .B _POSIX_C_SOURCE
 554 with the value 199606L.
 555 If a higher POSIX conformance level is
 556 selected by any other means (such as
 557 .B _POSIX_C_SOURCE
 558 itself,
 559 .BR _XOPEN_SOURCE ,
 560 .BR _DEFAULT_SOURCE ,
 561 or
 562 .BR _GNU_SOURCE ),
 563 then defining
 564 .B _REENTRANT
 565 has no effect.
 566 .IP
 567 This macro is automatically defined if one compiles with
 568 .IR cc\~\-pthread .
 569 .TP
 570 .B _THREAD_SAFE
 571 Synonym for the (deprecated)
 572 .BR _REENTRANT ,
 573 provided for compatibility with some other implementations.
 574 .TP
 575 .BR _FORTIFY_SOURCE " (since glibc 2.3.4)"
 576 .\" For more detail, see:
 577 .\" http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
 578 .\" [PATCH] Object size checking to prevent (some) buffer overflows
 579 .\" * From: Jakub Jelinek <jakub at redhat dot com>
 580 .\" * To: gcc-patches at gcc dot gnu dot org
 581 .\" * Date: Tue, 21 Sep 2004 04:16:40 -0400
 582 Defining this macro causes some lightweight checks to be performed
 583 to detect some buffer overflow errors when employing
 584 various string and memory manipulation functions (for example,
 585 .BR memcpy (3),
 586 .BR memset (3),
 587 .BR stpcpy (3),
 588 .BR strcpy (3),
 589 .BR strncpy (3),
 590 .BR strcat (3),
 591 .BR strncat (3),
 592 .BR sprintf (3),
 593 .BR snprintf (3),
 594 .BR vsprintf (3),
 595 .BR vsnprintf (3),
 596 .BR gets (3),
 597 and wide character variants thereof).
 598 For some functions, argument consistency is checked;
 599 for example, a check is made that
 600 .BR open (2)
 601 has been supplied with a
 602 .I mode
 603 argument when the specified flags include
 604 .BR O_CREAT .
 605 Not all problems are detected, just some common cases.
 606 .\" Look for __USE_FORTIFY_LEVEL in the header files
 607 .IP
 608 If
 609 .B _FORTIFY_SOURCE
 610 is set to 1, with compiler optimization level 1
 611 .RI ( "gcc\ \-O1" )
 612 and above, checks that shouldn't change the behavior of
 613 conforming programs are performed.
 614 With
 615 .B _FORTIFY_SOURCE
 616 set to 2, some more checking is added, but
 617 some conforming programs might fail.
 618 .\" For example, given the following code
 619 .\"        int d;
 620 .\"        char buf[1000], buf[1000];
 621 .\"        strcpy(fmt, "Hello world\n%n");
 622 .\"        snprintf(buf, sizeof(buf), fmt, &d);
 623 .\"
 624 .\" Compiling with "gcc -D_FORTIFY_SOURCE=2 -O1" and then running will
 625 .\" cause the following diagnostic at run time at the snprintf() call
 626 .\"
 627 .\"        *** %n in writable segment detected ***
 628 .\"        Aborted (core dumped)
 629 .\"
 630 .IP
 631 Some of the checks can be performed at compile time
 632 (via macros logic implemented in header files),
 633 and result in compiler warnings;
 634 other checks take place at run time,
 635 and result in a run-time error if the check fails.
 636 .IP
 637 Use of this macro requires compiler support, available with
 638 .BR gcc (1)
 639 since version 4.0.
 640 .SS Default definitions, implicit definitions, and combining definitions
 641 If no feature test macros are explicitly defined,
 642 then the following feature test macros are defined by default:
 643 .B _BSD_SOURCE
 644 (in glibc 2.19 and earlier),
 645 .B _SVID_SOURCE
 646 (in glibc 2.19 and earlier),
 647 .B _DEFAULT_SOURCE
 648 (since glibc 2.19),
 649 .BR _POSIX_SOURCE ,
 650 and
 651 .BR _POSIX_C_SOURCE =200809L
 652 (200112L in glibc versions before 2.10;
 653 199506L in glibc versions before 2.4;
 654 199309L in glibc versions before 2.1).
 655 .PP
 656 If any of
 657 .BR __STRICT_ANSI__ ,
 658 .BR _ISOC99_SOURCE ,
 659 .B _ISOC11_SOURCE
 660 (since glibc 2.18),
 661 .BR _POSIX_SOURCE ,
 662 .BR _POSIX_C_SOURCE  ,
 663 .BR _XOPEN_SOURCE ,
 664 .B _XOPEN_SOURCE_EXTENDED
 665 (in glibc 2.11 and earlier),
 666 .B _BSD_SOURCE
 667 (in glibc 2.19 and earlier),
 668 or
 669 .B _SVID_SOURCE
 670 (in glibc 2.19 and earlier)
 671 is explicitly defined, then
 672 .BR _BSD_SOURCE ,
 673 .BR _SVID_SOURCE ,
 674 and
 675 .B _DEFAULT_SOURCE
 676 are not defined by default.
 677 .PP
 678 If
 679 .B _POSIX_SOURCE
 680 and
 681 .B _POSIX_C_SOURCE
 682 are not explicitly defined,
 683 and either
 684 .B __STRICT_ANSI__
 685 is not defined or
 686 .B _XOPEN_SOURCE
 687 is defined with a value of 500 or more, then
 688 .IP * 3
 689 .B _POSIX_SOURCE
 690 is defined with the value 1; and
 691 .IP *
 692 .B _POSIX_C_SOURCE
 693 is defined with one of the following values:
 694 .RS 3
 695 .IP \(bu 3
 696 2,
 697 if
 698 .B _XOPEN_SOURCE
 699 is defined with a value less than 500;
 700 .IP \(bu
 701 199506L,
 702 if
 703 .B _XOPEN_SOURCE
 704 is defined with a value greater than or equal to 500 and less than 600;
 705 or
 706 .IP \(bu
 707 (since glibc 2.4) 200112L,
 708 if
 709 .B _XOPEN_SOURCE
 710 is defined with a value greater than or equal to 600 and less than 700.
 711 .IP \(bu
 712 (Since glibc 2.10)
 713 200809L,
 714 if
 715 .B _XOPEN_SOURCE
 716 is defined with a value greater than or equal to 700.
 717 .IP \(bu
 718 Older versions of glibc do not know about the values
 719 200112L and 200809L for
 720 .BR _POSIX_C_SOURCE ,
 721 and the setting of this macro will depend on the glibc version.
 722 .IP \(bu
 723 If
 724 .B _XOPEN_SOURCE
 725 is undefined, then the setting of
 726 .B _POSIX_C_SOURCE
 727 depends on the glibc version:
 728 199506L, in glibc versions before 2.4;
 729 200112L, in glibc 2.4 to 2.9; and
 730 200809L, since glibc 2.10.
 731 .RE
 732 .PP
 733 Multiple macros can be defined; the results are additive.
 734 .SH STANDARDS
 735 POSIX.1 specifies
 736 .BR _POSIX_C_SOURCE ,
 737 .BR _POSIX_SOURCE ,
 738 and
 739 .BR _XOPEN_SOURCE .
 740 .PP
 741 .B _XOPEN_SOURCE_EXTENDED
 742 was specified by XPG4v2 (aka SUSv1), but is not present in SUSv2 and later.
 743 .B _FILE_OFFSET_BITS
 744 is not specified by any standard,
 745 but is employed on some other implementations.
 746 .PP
 747 .BR _BSD_SOURCE ,
 748 .BR _SVID_SOURCE ,
 749 .BR _DEFAULT_SOURCE ,
 750 .BR _ATFILE_SOURCE ,
 751 .BR _GNU_SOURCE ,
 752 .BR _FORTIFY_SOURCE ,
 753 .BR _REENTRANT ,
 754 and
 755 .B _THREAD_SAFE
 756 are specific to Linux (glibc).
 757 .SH NOTES
 758 .I <features.h>
 759 is a Linux/glibc-specific header file.
 760 Other systems have an analogous file, but typically with a different name.
 761 This header file is automatically included by other header files as
 762 required: it is not necessary to explicitly include it in order to
 763 employ feature test macros.
 764 .PP
 765 According to which of the above feature test macros are defined,
 766 .I <features.h>
 767 internally defines various other macros that are checked by
 768 other glibc header files.
 769 These macros have names prefixed by two underscores (e.g.,
 770 .BR __USE_MISC ).
 771 Programs should
 772 .I never
 773 define these macros directly:
 774 instead, the appropriate feature test macro(s) from the
 775 list above should be employed.
 776 .SH EXAMPLES
 777 The program below can be used to explore how the various
 778 feature test macros are set depending on the glibc version
 779 and what feature test macros are explicitly set.
 780 The following shell session, on a system with glibc 2.10,
 781 shows some examples of what we would see:
 782 .PP
 783 .in +4n
 784 .EX
 785 $ \fBcc ftm.c\fP
 786 $ \fB./a.out\fP
 787 _POSIX_SOURCE defined
 788 _POSIX_C_SOURCE defined: 200809L
 789 _BSD_SOURCE defined
 790 _SVID_SOURCE defined
 791 _ATFILE_SOURCE defined
 792 $ \fBcc \-D_XOPEN_SOURCE=500 ftm.c\fP
 793 $ \fB./a.out\fP
 794 _POSIX_SOURCE defined
 795 _POSIX_C_SOURCE defined: 199506L
 796 _XOPEN_SOURCE defined: 500
 797 $ \fBcc \-D_GNU_SOURCE ftm.c\fP
 798 $ \fB./a.out\fP
 799 _POSIX_SOURCE defined
 800 _POSIX_C_SOURCE defined: 200809L
 801 _ISOC99_SOURCE defined
 802 _XOPEN_SOURCE defined: 700
 803 _XOPEN_SOURCE_EXTENDED defined
 804 _LARGEFILE64_SOURCE defined
 805 _BSD_SOURCE defined
 806 _SVID_SOURCE defined
 807 _ATFILE_SOURCE defined
 808 _GNU_SOURCE defined
 809 .EE
 810 .in
 811 .SS Program source
 812 \&
 813 .EX
 814 /* ftm.c */
 815
 816 #include <stdint.h>
 817 #include <stdio.h>
 818 #include <unistd.h>
 819 #include <stdlib.h>
 820
 821 int
 822 main(int argc, char *argv[])
 823 {
 824 #ifdef _POSIX_SOURCE
 825     printf("_POSIX_SOURCE defined\en");
 826 #endif
 827
 828 #ifdef _POSIX_C_SOURCE
 829     printf("_POSIX_C_SOURCE defined: %jdL\en",
 830             (intmax_t) _POSIX_C_SOURCE);
 831 #endif
 832
 833 #ifdef _ISOC99_SOURCE
 834     printf("_ISOC99_SOURCE defined\en");
 835 #endif
 836
 837 #ifdef _ISOC11_SOURCE
 838     printf("_ISOC11_SOURCE defined\en");
 839 #endif
 840
 841 #ifdef _XOPEN_SOURCE
 842     printf("_XOPEN_SOURCE defined: %d\en", _XOPEN_SOURCE);
 843 #endif
 844
 845 #ifdef _XOPEN_SOURCE_EXTENDED
 846     printf("_XOPEN_SOURCE_EXTENDED defined\en");
 847 #endif
 848
 849 #ifdef _LARGEFILE64_SOURCE
 850     printf("_LARGEFILE64_SOURCE defined\en");
 851 #endif
 852
 853 #ifdef _FILE_OFFSET_BITS
 854     printf("_FILE_OFFSET_BITS defined: %d\en", _FILE_OFFSET_BITS);
 855 #endif
 856
 857 #ifdef _BSD_SOURCE
 858     printf("_BSD_SOURCE defined\en");
 859 #endif
 860
 861 #ifdef _SVID_SOURCE
 862     printf("_SVID_SOURCE defined\en");
 863 #endif
 864
 865 #ifdef _DEFAULT_SOURCE
 866     printf("_DEFAULT_SOURCE defined\en");
 867 #endif
 868
 869 #ifdef _ATFILE_SOURCE
 870     printf("_ATFILE_SOURCE defined\en");
 871 #endif
 872
 873 #ifdef _GNU_SOURCE
 874     printf("_GNU_SOURCE defined\en");
 875 #endif
 876
 877 #ifdef _REENTRANT
 878     printf("_REENTRANT defined\en");
 879 #endif
 880
 881 #ifdef _THREAD_SAFE
 882     printf("_THREAD_SAFE defined\en");
 883 #endif
 884
 885 #ifdef _FORTIFY_SOURCE
 886     printf("_FORTIFY_SOURCE defined\en");
 887 #endif
 888
 889     exit(EXIT_SUCCESS);
 890 }
 891 .EE
 892 .SH SEE ALSO
 893 .BR libc (7),
 894 .BR standards (7),
 895 .BR system_data_types (7)
 896 .PP
 897 The section "Feature Test Macros" under
 898 .IR "info libc" .
 899 .\" But beware: the info libc document is out of date (Jul 07, mtk)
 900 .PP
 901 .I /usr/include/features.h