1FEATURE_TEST_MACROS(7)     Linux Programmer's Manual    FEATURE_TEST_MACROS(7)
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NAME

6       feature_test_macros - feature test macros
7

DESCRIPTION

9       Feature  test  macros  allow  the programmer to control the definitions
10       that are exposed by system header files when a program is compiled.
11
12       NOTE: In order to be effective, a feature test macro  must  be  defined
13       before including any header files.  This can be done either in the com‐
14       pilation command (cc -DMACRO=value) or by defining the macro within the
15       source  code  before  including  any headers.  The requirement that the
16       macro must be defined before including any header file  exists  because
17       header files may freely include one another.  Thus, for example, in the
18       following lines, defining the _GNU_SOURCE macro may have no effect  be‐
19       cause  the header <abc.h> itself includes <xyz.h> (POSIX explicitly al‐
20       lows this):
21
22           #include <abc.h>
23           #define _GNU_SOURCE
24           #include <xyz.h>
25
26       Some feature test macros are useful for creating portable applications,
27       by preventing nonstandard definitions from being exposed.  Other macros
28       can be used to expose nonstandard definitions that are not  exposed  by
29       default.
30
31       The  precise effects of each of the feature test macros described below
32       can be ascertained by inspecting the <features.h> header  file.   Note:
33       applications  do not need to directly include <features.h>; indeed, do‐
34       ing so is actively discouraged.  See NOTES.
35
36   Specification of feature test macro requirements in manual pages
37       When a function requires that a feature test macro is defined, the man‐
38       ual page SYNOPSIS typically includes a note of the following form (this
39       example from the acct(2) manual page):
40
41              #include <unistd.h>
42
43              int acct(const char *filename);
44
45          Feature Test Macro Requirements for glibc (see
46          feature_test_macros(7)):
47
48              acct(): _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
49
50       The || means that in order to obtain the declaration  of  acct(2)  from
51       <unistd.h>,  either of the following macro definitions must be made be‐
52       fore including any header files:
53
54           #define _BSD_SOURCE
55           #define _XOPEN_SOURCE        /* or any value < 500 */
56
57       Alternatively, equivalent definitions can be included in  the  compila‐
58       tion command:
59
60           cc -D_BSD_SOURCE
61           cc -D_XOPEN_SOURCE           # Or any value < 500
62
63       Note  that, as described below, some feature test macros are defined by
64       default, so that it may not always be necessary to  explicitly  specify
65       the feature test macro(s) shown in the SYNOPSIS.
66
67       In a few cases, manual pages use a shorthand for expressing the feature
68       test macro requirements (this example from readahead(2)):
69
70           #define _GNU_SOURCE
71           #include <fcntl.h>
72
73           ssize_t readahead(int fd, off64_t *offset, size_t count);
74
75       This format is employed in cases where only a single feature test macro
76       can  be  used to expose the function declaration, and that macro is not
77       defined by default.
78
79   Feature test macros understood by glibc
80       The paragraphs below explain how feature test  macros  are  handled  in
81       glibc 2.x, x > 0.
82
83       First, though, a summary of a few details for the impatient:
84
85       *  The  macros  that  you most likely need to use in modern source code
86          are  _POSIX_C_SOURCE  (for  definitions  from  various  versions  of
87          POSIX.1),  _XOPEN_SOURCE  (for  definitions from various versions of
88          SUS), _GNU_SOURCE (for GNU and/or Linux specific  stuff),  and  _DE‐
89          FAULT_SOURCE  (to get definitions that would normally be provided by
90          default).
91
92       *  Certain macros are defined with default values.  Thus, although  one
93          or more macros may be indicated as being required in the SYNOPSIS of
94          a man page, it may not be necessary to define them explicitly.  Full
95          details of the defaults are given later in this man page.
96
97       *  Defining  _XOPEN_SOURCE  with a value of 600 or greater produces the
98          same effects as defining _POSIX_C_SOURCE with a value of 200112L  or
99          greater.  Where one sees
100
101              _POSIX_C_SOURCE >= 200112L
102
103          in  the  feature  test  macro  requirements in the SYNOPSIS of a man
104          page, it is implicit that the following has the same effect:
105
106              _XOPEN_SOURCE >= 600
107
108       *  Defining _XOPEN_SOURCE with a value of 700 or greater  produces  the
109          same  effects as defining _POSIX_C_SOURCE with a value of 200809L or
110          greater.  Where one sees
111
112              _POSIX_C_SOURCE >= 200809L
113
114          in the feature test macro requirements in  the  SYNOPSIS  of  a  man
115          page, it is implicit that the following has the same effect:
116
117              _XOPEN_SOURCE >= 700
118
119       Glibc understands the following feature test macros:
120
121       __STRICT_ANSI__
122              ISO Standard C.  This macro is implicitly defined by gcc(1) when
123              invoked with, for example, the -std=c99 or -ansi flag.
124
125       _POSIX_C_SOURCE
126              Defining this macro causes header files to expose definitions as
127              follows:
128
129              •  The  value  1  exposes definitions conforming to POSIX.1-1990
130                 and ISO C (1990).
131
132              •  The value 2 or greater additionally exposes  definitions  for
133                 POSIX.2-1992.
134
135              •  The value 199309L or greater additionally exposes definitions
136                 for POSIX.1b (real-time extensions).
137
138              •  The value 199506L or greater additionally exposes definitions
139                 for POSIX.1c (threads).
140
141              •  (Since glibc 2.3.3) The value 200112L or greater additionally
142                 exposes definitions corresponding to  the  POSIX.1-2001  base
143                 specification (excluding the XSI extension).  This value also
144                 causes C95 (since glibc 2.12) and C99 (since glibc 2.10) fea‐
145                 tures to be exposed (in other words, the equivalent of defin‐
146                 ing _ISOC99_SOURCE).
147
148              •  (Since glibc 2.10) The value 200809L or greater  additionally
149                 exposes  definitions  corresponding  to the POSIX.1-2008 base
150                 specification (excluding the XSI extension).
151
152       _POSIX_SOURCE
153              Defining this obsolete macro with any  value  is  equivalent  to
154              defining _POSIX_C_SOURCE with the value 1.
155
156              Since  this  macro is obsolete, its usage is generally not docu‐
157              mented when discussing feature test macro  requirements  in  the
158              man pages.
159
160       _XOPEN_SOURCE
161              Defining this macro causes header files to expose definitions as
162              follows:
163
164              •  Defining with any value  exposes  definitions  conforming  to
165                 POSIX.1, POSIX.2, and XPG4.
166
167              •  The value 500 or greater additionally exposes definitions for
168                 SUSv2 (UNIX 98).
169
170              •  (Since glibc 2.2) The value 600 or greater  additionally  ex‐
171                 poses  definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001
172                 base specification plus the XSI extension)  and  C99  defini‐
173                 tions.
174
175              •  (Since  glibc 2.10) The value 700 or greater additionally ex‐
176                 poses definitions for  SUSv4  (i.e.,  the  POSIX.1-2008  base
177                 specification plus the XSI extension).
178
179              If  __STRICT_ANSI__  is not defined, or _XOPEN_SOURCE is defined
180              with  a  value  greater  than  or  equal  to  500  and   neither
181              _POSIX_SOURCE  nor  _POSIX_C_SOURCE  is explicitly defined, then
182              the following macros are implicitly defined:
183
184_POSIX_SOURCE is defined with the value 1.
185
186_POSIX_C_SOURCE  is  defined,  according  to  the  value   of
187                 _XOPEN_SOURCE:
188
189                 _XOPEN_SOURCE < 500
190                        _POSIX_C_SOURCE is defined with the value 2.
191
192                 500 <= _XOPEN_SOURCE < 600
193                        _POSIX_C_SOURCE is defined with the value 199506L.
194
195                 600 <= _XOPEN_SOURCE < 700
196                        _POSIX_C_SOURCE is defined with the value 200112L.
197
198                 700 <= _XOPEN_SOURCE (since glibc 2.10)
199                        _POSIX_C_SOURCE is defined with the value 200809L.
200
201              In  addition,  defining  _XOPEN_SOURCE  with  a  value of 500 or
202              greater produces the same effects as defining  _XOPEN_SOURCE_EX‐
203              TENDED.
204
205       _XOPEN_SOURCE_EXTENDED
206              If this macro is defined, and _XOPEN_SOURCE is defined, then ex‐
207              pose definitions corresponding to the XPG4v2 (SUSv1) UNIX exten‐
208              sions  (UNIX 95).  Defining _XOPEN_SOURCE with a value of 500 or
209              more also produces the same effect as defining _XOPEN_SOURCE_EX‐
210              TENDED.  Use of _XOPEN_SOURCE_EXTENDED in new source code should
211              be avoided.
212
213              Since defining _XOPEN_SOURCE with a value of 500 or more has the
214              same  effect as defining _XOPEN_SOURCE_EXTENDED, the latter (ob‐
215              solete) feature test macro is generally  not  described  in  the
216              SYNOPSIS in man pages.
217
218       _ISOC99_SOURCE (since glibc 2.1.3)
219              Exposes declarations consistent with the ISO C99 standard.
220
221              Earlier  glibc  2.1.x  versions  recognized  an equivalent macro
222              named _ISOC9X_SOURCE (because the C99 standard had not then been
223              finalized).   Although  the use of this macro is obsolete, glibc
224              continues to recognize it for backward compatibility.
225
226              Defining _ISOC99_SOURCE also exposes ISO C  (1990)  Amendment  1
227              ("C95") definitions.  (The primary change in C95 was support for
228              international character sets.)
229
230              Invoking the C compiler with the option  -std=c99  produces  the
231              same effects as defining this macro.
232
233       _ISOC11_SOURCE (since glibc 2.16)
234              Exposes  declarations  consistent  with  the  ISO  C11 standard.
235              Defining this macro also enables  C99  and  C95  features  (like
236              _ISOC99_SOURCE).
237
238              Invoking  the  C  compiler with the option -std=c11 produces the
239              same effects as defining this macro.
240
241       _LARGEFILE64_SOURCE
242              Expose definitions for the alternative API specified by the  LFS
243              (Large  File Summit) as a "transitional extension" to the Single
244              UNIX   Specification.     (See    ⟨http://opengroup.org/platform
245              /lfs.html⟩.)   The  alternative API consists of a set of new ob‐
246              jects (i.e., functions and types) whose names are suffixed  with
247              "64"  (e.g.,  off64_t  versus  off_t,  lseek64() versus lseek(),
248              etc.).  New programs  should  not  employ  this  macro;  instead
249              _FILE_OFFSET_BITS=64 should be employed.
250
251       _LARGEFILE_SOURCE
252              This  macro  was  historically  used to expose certain functions
253              (specifically fseeko(3) and ftello(3)) that address  limitations
254              of  earlier  APIs (fseek(3) and ftell(3)) that use long for file
255              offsets.  This macro is implicitly defined if  _XOPEN_SOURCE  is
256              defined with a value greater than or equal to 500.  New programs
257              should not employ this macro; defining _XOPEN_SOURCE as just de‐
258              scribed  or  defining _FILE_OFFSET_BITS with the value 64 is the
259              preferred mechanism to achieve the same result.
260
261       _FILE_OFFSET_BITS
262              Defining this macro with the  value  64  automatically  converts
263              references  to  32-bit  functions and data types related to file
264              I/O and filesystem operations into references  to  their  64-bit
265              counterparts.   This is useful for performing I/O on large files
266              (> 2 Gigabytes) on 32-bit systems.  (Defining this macro permits
267              correctly written programs to use large files with only a recom‐
268              pilation being required.)
269
270              64-bit systems naturally permit file sizes greater than 2  Giga‐
271              bytes, and on those systems this macro has no effect.
272
273       _BSD_SOURCE (deprecated since glibc 2.20)
274              Defining this macro with any value causes header files to expose
275              BSD-derived definitions.
276
277              In glibc versions up to and including 2.18, defining this  macro
278              also  causes  BSD definitions to be preferred in some situations
279              where standards conflict, unless one or  more  of  _SVID_SOURCE,
280              _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EX‐
281              TENDED, or _GNU_SOURCE is defined, in which case BSD definitions
282              are  disfavored.  Since glibc 2.19, _BSD_SOURCE no longer causes
283              BSD definitions to be preferred in case of conflicts.
284
285              Since glibc 2.20, this macro is deprecated.  It now has the same
286              effect as defining _DEFAULT_SOURCE, but generates a compile-time
287              warning (unless _DEFAULT_SOURCE  is  also  defined).   Use  _DE‐
288              FAULT_SOURCE  instead.   To allow code that requires _BSD_SOURCE
289              in glibc 2.19 and earlier and _DEFAULT_SOURCE in glibc 2.20  and
290              later  to  compile without warnings, define both _BSD_SOURCE and
291              _DEFAULT_SOURCE.
292
293       _SVID_SOURCE (deprecated since glibc 2.20)
294              Defining this macro with any value causes header files to expose
295              System V-derived definitions.  (SVID == System V Interface Defi‐
296              nition; see standards(7).)
297
298              Since glibc 2.20, this macro is deprecated in the  same  fashion
299              as _BSD_SOURCE.
300
301       _DEFAULT_SOURCE (since glibc 2.19)
302              This  macro  can be defined to ensure that the "default" defini‐
303              tions are provided even when the  defaults  would  otherwise  be
304              disabled,  as  happens when individual macros are explicitly de‐
305              fined, or the compiler is invoked in one of its "standard" modes
306              (e.g.,  cc -std=c99).  Defining _DEFAULT_SOURCE without defining
307              other individual macros or invoking the compiler in one  of  its
308              "standard" modes has no effect.
309
310              The   "default"   definitions   comprise   those   required   by
311              POSIX.1-2008 and ISO C99, as well as various definitions  origi‐
312              nally derived from BSD and System V.  On glibc 2.19 and earlier,
313              these  defaults  were  approximately  equivalent  to  explicitly
314              defining the following:
315
316                  cc -D_BSD_SOURCE -D_SVID_SOURCE -D_POSIX_C_SOURCE=200809
317
318       _ATFILE_SOURCE (since glibc 2.4)
319              Defining this macro with any value causes header files to expose
320              declarations of a range of functions with the suffix  "at";  see
321              openat(2).   Since glibc 2.10, this macro is also implicitly de‐
322              fined if _POSIX_C_SOURCE is defined with a value greater than or
323              equal to 200809L.
324
325       _GNU_SOURCE
326              Defining  this  macro  (with  any value) implicitly defines _AT‐
327              FILE_SOURCE,        _LARGEFILE64_SOURCE,         _ISOC99_SOURCE,
328              _XOPEN_SOURCE_EXTENDED,  _POSIX_SOURCE, _POSIX_C_SOURCE with the
329              value 200809L (200112L in glibc versions before 2.10; 199506L in
330              glibc versions before 2.5; 199309L in glibc versions before 2.1)
331              and _XOPEN_SOURCE with the value 700 (600 in glibc versions  be‐
332              fore 2.10; 500 in glibc versions before 2.2).  In addition, var‐
333              ious GNU-specific extensions are also exposed.
334
335              Since glibc 2.19, defining _GNU_SOURCE also has  the  effect  of
336              implicitly  defining  _DEFAULT_SOURCE.  In glibc versions before
337              2.20, defining _GNU_SOURCE also had  the  effect  of  implicitly
338              defining _BSD_SOURCE and _SVID_SOURCE.
339
340       _REENTRANT
341              Historically,  on various C libraries it was necessary to define
342              this macro in all multithreaded code.   (Some  C  libraries  may
343              still  require this.)  In glibc, this macro also exposed defini‐
344              tions of certain reentrant functions.
345
346              However, glibc has been thread-safe by default for  many  years;
347              since glibc 2.3, the only effect of defining _REENTRANT has been
348              to enable one or two of the same declarations that are also  en‐
349              abled  by  defining  _POSIX_C_SOURCE  with a value of 199606L or
350              greater.
351
352              _REENTRANT is now obsolete.  In glibc 2.25 and  later,  defining
353              _REENTRANT  is  equivalent  to defining _POSIX_C_SOURCE with the
354              value 199606L.  If a higher POSIX conformance level is  selected
355              by   any   other   means   (such   as   _POSIX_C_SOURCE  itself,
356              _XOPEN_SOURCE, _DEFAULT_SOURCE, or _GNU_SOURCE),  then  defining
357              _REENTRANT has no effect.
358
359              This  macro  is  automatically  defined  if  one  compiles  with
360              cc -pthread.
361
362       _THREAD_SAFE
363              Synonym for the (deprecated) _REENTRANT, provided  for  compati‐
364              bility with some other implementations.
365
366       _FORTIFY_SOURCE (since glibc 2.3.4)
367              Defining  this  macro  causes some lightweight checks to be per‐
368              formed to detect some buffer overflow errors when employing var‐
369              ious string and memory manipulation functions (for example, mem‐
370              cpy(3), memset(3), stpcpy(3), strcpy(3), strncpy(3),  strcat(3),
371              strncat(3),  sprintf(3), snprintf(3), vsprintf(3), vsnprintf(3),
372              gets(3), and wide character variants thereof).  For  some  func‐
373              tions,  argument consistency is checked; for example, a check is
374              made that open(2) has been supplied with a  mode  argument  when
375              the  specified  flags include O_CREAT.  Not all problems are de‐
376              tected, just some common cases.
377
378              If _FORTIFY_SOURCE is set to 1, with compiler optimization level
379              1 (gcc -O1) and above, checks that shouldn't change the behavior
380              of conforming programs are performed.  With _FORTIFY_SOURCE  set
381              to  2, some more checking is added, but some conforming programs
382              might fail.
383
384              Some of the checks can be performed at compile time (via  macros
385              logic implemented in header files), and result in compiler warn‐
386              ings; other checks take place at run time, and result in a  run-
387              time error if the check fails.
388
389              Use  of  this  macro  requires  compiler support, available with
390              gcc(1) since version 4.0.
391
392   Default definitions, implicit definitions, and combining definitions
393       If no feature test macros are explicitly defined,  then  the  following
394       feature  test macros are defined by default: _BSD_SOURCE (in glibc 2.19
395       and earlier), _SVID_SOURCE (in glibc 2.19 and earlier), _DEFAULT_SOURCE
396       (since glibc 2.19), _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L
397       in glibc versions before 2.10; 199506L in glibc  versions  before  2.4;
398       199309L in glibc versions before 2.1).
399
400       If  any of __STRICT_ANSI__, _ISOC99_SOURCE, _ISOC11_SOURCE (since glibc
401       2.18), _POSIX_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EX‐
402       TENDED (in glibc 2.11 and earlier), _BSD_SOURCE (in glibc 2.19 and ear‐
403       lier), or _SVID_SOURCE (in glibc 2.19 and earlier)  is  explicitly  de‐
404       fined,  then _BSD_SOURCE, _SVID_SOURCE, and _DEFAULT_SOURCE are not de‐
405       fined by default.
406
407       If _POSIX_SOURCE and _POSIX_C_SOURCE are not  explicitly  defined,  and
408       either  __STRICT_ANSI__ is not defined or _XOPEN_SOURCE is defined with
409       a value of 500 or more, then
410
411       *  _POSIX_SOURCE is defined with the value 1; and
412
413       *  _POSIX_C_SOURCE is defined with one of the following values:
414
415          •  2, if _XOPEN_SOURCE is defined with a value less than 500;
416
417          •  199506L, if _XOPEN_SOURCE is defined with a value greater than or
418             equal to 500 and less than 600; or
419
420          •  (since  glibc  2.4)  200112L,  if _XOPEN_SOURCE is defined with a
421             value greater than or equal to 600 and less than 700.
422
423          •  (Since glibc 2.10) 200809L, if _XOPEN_SOURCE is  defined  with  a
424             value greater than or equal to 700.
425
426          •  Older  versions of glibc do not know about the values 200112L and
427             200809L for _POSIX_C_SOURCE, and the setting of this  macro  will
428             depend on the glibc version.
429
430          •  If    _XOPEN_SOURCE   is   undefined,   then   the   setting   of
431             _POSIX_C_SOURCE depends on the glibc version: 199506L,  in  glibc
432             versions  before  2.4; 200112L, in glibc 2.4 to 2.9; and 200809L,
433             since glibc 2.10.
434
435       Multiple macros can be defined; the results are additive.
436

CONFORMING TO

438       POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.
439
440       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is  not
441       present  in SUSv2 and later.  _FILE_OFFSET_BITS is not specified by any
442       standard, but is employed on some other implementations.
443
444       _BSD_SOURCE,     _SVID_SOURCE,     _DEFAULT_SOURCE,     _ATFILE_SOURCE,
445       _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are specific
446       to Linux (glibc).
447

NOTES

449       <features.h> is a Linux/glibc-specific header file.  Other systems have
450       an  analogous  file,  but typically with a different name.  This header
451       file is automatically included by other header files as required: it is
452       not  necessary to explicitly include it in order to employ feature test
453       macros.
454
455       According to which of the above feature test macros are defined,  <fea‐
456       tures.h>  internally  defines  various other macros that are checked by
457       other glibc header files.  These macros have names prefixed by two  un‐
458       derscores  (e.g.,  __USE_MISC).   Programs  should  never  define these
459       macros directly: instead, the appropriate feature  test  macro(s)  from
460       the list above should be employed.
461

EXAMPLES

463       The  program  below can be used to explore how the various feature test
464       macros are set depending on the glibc version  and  what  feature  test
465       macros  are  explicitly  set.  The following shell session, on a system
466       with glibc 2.10, shows some examples of what we would see:
467
468           $ cc ftm.c
469           $ ./a.out
470           _POSIX_SOURCE defined
471           _POSIX_C_SOURCE defined: 200809L
472           _BSD_SOURCE defined
473           _SVID_SOURCE defined
474           _ATFILE_SOURCE defined
475           $ cc -D_XOPEN_SOURCE=500 ftm.c
476           $ ./a.out
477           _POSIX_SOURCE defined
478           _POSIX_C_SOURCE defined: 199506L
479           _XOPEN_SOURCE defined: 500
480           $ cc -D_GNU_SOURCE ftm.c
481           $ ./a.out
482           _POSIX_SOURCE defined
483           _POSIX_C_SOURCE defined: 200809L
484           _ISOC99_SOURCE defined
485           _XOPEN_SOURCE defined: 700
486           _XOPEN_SOURCE_EXTENDED defined
487           _LARGEFILE64_SOURCE defined
488           _BSD_SOURCE defined
489           _SVID_SOURCE defined
490           _ATFILE_SOURCE defined
491           _GNU_SOURCE defined
492
493   Program source
494
495       /* ftm.c */
496
497       #include <stdint.h>
498       #include <stdio.h>
499       #include <unistd.h>
500       #include <stdlib.h>
501
502       int
503       main(int argc, char *argv[])
504       {
505       #ifdef _POSIX_SOURCE
506           printf("_POSIX_SOURCE defined\n");
507       #endif
508
509       #ifdef _POSIX_C_SOURCE
510           printf("_POSIX_C_SOURCE defined: %jdL\n",
511                   (intmax_t) _POSIX_C_SOURCE);
512       #endif
513
514       #ifdef _ISOC99_SOURCE
515           printf("_ISOC99_SOURCE defined\n");
516       #endif
517
518       #ifdef _ISOC11_SOURCE
519           printf("_ISOC11_SOURCE defined\n");
520       #endif
521
522       #ifdef _XOPEN_SOURCE
523           printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
524       #endif
525
526       #ifdef _XOPEN_SOURCE_EXTENDED
527           printf("_XOPEN_SOURCE_EXTENDED defined\n");
528       #endif
529
530       #ifdef _LARGEFILE64_SOURCE
531           printf("_LARGEFILE64_SOURCE defined\n");
532       #endif
533
534       #ifdef _FILE_OFFSET_BITS
535           printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
536       #endif
537
538       #ifdef _BSD_SOURCE
539           printf("_BSD_SOURCE defined\n");
540       #endif
541
542       #ifdef _SVID_SOURCE
543           printf("_SVID_SOURCE defined\n");
544       #endif
545
546       #ifdef _DEFAULT_SOURCE
547           printf("_DEFAULT_SOURCE defined\n");
548       #endif
549
550       #ifdef _ATFILE_SOURCE
551           printf("_ATFILE_SOURCE defined\n");
552       #endif
553
554       #ifdef _GNU_SOURCE
555           printf("_GNU_SOURCE defined\n");
556       #endif
557
558       #ifdef _REENTRANT
559           printf("_REENTRANT defined\n");
560       #endif
561
562       #ifdef _THREAD_SAFE
563           printf("_THREAD_SAFE defined\n");
564       #endif
565
566       #ifdef _FORTIFY_SOURCE
567           printf("_FORTIFY_SOURCE defined\n");
568       #endif
569
570           exit(EXIT_SUCCESS);
571       }
572

SEE ALSO

574       libc(7), standards(7), system_data_types(7)
575
576       The section "Feature Test Macros" under info libc.
577
578       /usr/include/features.h
579

COLOPHON

581       This page is part of release 5.12 of the Linux  man-pages  project.   A
582       description  of  the project, information about reporting bugs, and the
583       latest    version    of    this    page,    can     be     found     at
584       https://www.kernel.org/doc/man-pages/.
585
586
587
588Linux                             2021-03-22            FEATURE_TEST_MACROS(7)
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