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

CONFORMING TO

430       POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.
431
432       _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1), but is  not
433       present  in SUSv2 and later.  _FILE_OFFSET_BITS is not specified by any
434       standard, but is employed on some other implementations.
435
436       _BSD_SOURCE,     _SVID_SOURCE,     _DEFAULT_SOURCE,     _ATFILE_SOURCE,
437       _GNU_SOURCE, _FORTIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are specific
438       to Linux (glibc).
439

NOTES

441       <features.h> is a Linux/glibc-specific header file.  Other systems have
442       an  analogous  file,  but typically with a different name.  This header
443       file is automatically included by other header files as required: it is
444       not  necessary to explicitly include it in order to employ feature test
445       macros.
446
447       According to which of the above feature test macros are defined,  <fea‐
448       tures.h>  internally  defines  various other macros that are checked by
449       other glibc header files.  These macros  have  names  prefixed  by  two
450       underscores  (e.g.,  __USE_MISC).   Programs  should never define these
451       macros directly: instead, the appropriate feature  test  macro(s)  from
452       the list above should be employed.
453

EXAMPLE

455       The  program  below can be used to explore how the various feature test
456       macros are set depending on the glibc version  and  what  feature  test
457       macros  are  explicitly  set.  The following shell session, on a system
458       with glibc 2.10, shows some examples of what we would see:
459
460           $ cc ftm.c
461           $ ./a.out
462           _POSIX_SOURCE defined
463           _POSIX_C_SOURCE defined: 200809L
464           _BSD_SOURCE defined
465           _SVID_SOURCE defined
466           _ATFILE_SOURCE defined
467           $ cc -D_XOPEN_SOURCE=500 ftm.c
468           $ ./a.out
469           _POSIX_SOURCE defined
470           _POSIX_C_SOURCE defined: 199506L
471           _XOPEN_SOURCE defined: 500
472           $ cc -D_GNU_SOURCE ftm.c
473           $ ./a.out
474           _POSIX_SOURCE defined
475           _POSIX_C_SOURCE defined: 200809L
476           _ISOC99_SOURCE defined
477           _XOPEN_SOURCE defined: 700
478           _XOPEN_SOURCE_EXTENDED defined
479           _LARGEFILE64_SOURCE defined
480           _BSD_SOURCE defined
481           _SVID_SOURCE defined
482           _ATFILE_SOURCE defined
483           _GNU_SOURCE defined
484
485   Program source
486
487       /* ftm.c */
488
489       #include <stdio.h>
490       #include <unistd.h>
491       #include <stdlib.h>
492
493       int
494       main(int argc, char *argv[])
495       {
496       #ifdef _POSIX_SOURCE
497           printf("_POSIX_SOURCE defined\n");
498       #endif
499
500       #ifdef _POSIX_C_SOURCE
501           printf("_POSIX_C_SOURCE defined: %ldL\n", (long) _POSIX_C_SOURCE);
502       #endif
503
504       #ifdef _ISOC99_SOURCE
505           printf("_ISOC99_SOURCE defined\n");
506       #endif
507
508       #ifdef _ISOC11_SOURCE
509           printf("_ISOC11_SOURCE defined\n");
510       #endif
511
512       #ifdef _XOPEN_SOURCE
513           printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
514       #endif
515
516       #ifdef _XOPEN_SOURCE_EXTENDED
517           printf("_XOPEN_SOURCE_EXTENDED defined\n");
518       #endif
519
520       #ifdef _LARGEFILE64_SOURCE
521           printf("_LARGEFILE64_SOURCE defined\n");
522       #endif
523
524       #ifdef _FILE_OFFSET_BITS
525           printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
526       #endif
527
528       #ifdef _BSD_SOURCE
529           printf("_BSD_SOURCE defined\n");
530       #endif
531
532       #ifdef _SVID_SOURCE
533           printf("_SVID_SOURCE defined\n");
534       #endif
535
536       #ifdef _DEFAULT_SOURCE
537           printf("_DEFAULT_SOURCE defined\n");
538       #endif
539
540       #ifdef _ATFILE_SOURCE
541           printf("_ATFILE_SOURCE defined\n");
542       #endif
543
544       #ifdef _GNU_SOURCE
545           printf("_GNU_SOURCE defined\n");
546       #endif
547
548       #ifdef _REENTRANT
549           printf("_REENTRANT defined\n");
550       #endif
551
552       #ifdef _THREAD_SAFE
553           printf("_THREAD_SAFE defined\n");
554       #endif
555
556       #ifdef _FORTIFY_SOURCE
557           printf("_FORTIFY_SOURCE defined\n");
558       #endif
559
560           exit(EXIT_SUCCESS);
561       }
562

SEE ALSO

564       libc(7), standards(7)
565
566       The section "Feature Test Macros" under info libc.
567
568       /usr/include/features.h
569

COLOPHON

571       This page is part of release 4.16 of the Linux  man-pages  project.   A
572       description  of  the project, information about reporting bugs, and the
573       latest    version    of    this    page,    can     be     found     at
574       https://www.kernel.org/doc/man-pages/.
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578Linux                             2017-09-15            FEATURE_TEST_MACROS(7)
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