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25 .TH FEATURE_TEST_MACROS 7 2009-12-13 "Linux" "Linux Programmer's Manual"
27 feature_test_macros \- feature test macros
30 .B #include <features.h>
33 Feature test macros allow the programmer to control the definitions that
34 are exposed by system header files when a program is compiled.
35 This can be useful for creating portable applications,
36 by preventing nonstandard definitions from being exposed.
37 Other macros can be used to expose nonstandard definitions that
38 are not exposed by default.
39 The precise effects of each of the feature test macros described below
40 can be ascertained by inspecting the
44 In order to be effective, a feature test macro
45 .IR "must be defined before including any header files" .
46 This can either be done in the compilation command
47 .RI ( "cc \-DMACRO=value" )
48 or by defining the macro within the source code before
49 including any headers.
50 .SS Specification of feature test macro requirements in manual pages
51 When a function requires that a feature test macro is defined,
52 the manual page SYNOPSIS typically includes a note of the following form
53 (this example from the
58 .B #include <sys/stat.h>
60 .BI "int chmod(const char *" path ", mode_t " mode );
62 .BI "int fchmod(int " fd ", mode_t " mode );
65 Feature Test Macro Requirements for glibc (see
66 .BR feature_test_macros (7)):
70 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
73 The \fB||\fP means that in order to obtain the declaration of
77 \fIeither\fP of the following macro
78 definitions must be made before including any header files:
83 #define _XOPEN_SOURCE 500 /* or any value > 500 */
87 Alternatively, equivalent definitions can be included in the
93 cc \-D_XOPEN_SOURCE=500 # Or any value > 500
97 Note that, as described below,
98 .BR "some feature test macros are defined by default" ,
99 so that it may not always be necessary to
100 explicitly specify the feature test macro(s) shown in the
103 In a few cases, manual pages use a shorthand for expressing the
104 feature test macro requirements (this example from
109 .B #define _GNU_SOURCE
110 .B #include <fcntl.h>
112 .BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
116 This format is employed in cases where only a single
117 feature test macro can be used to expose the function
118 declaration, and that macro is not defined by default.
119 .SS Feature test macros understood by glibc
120 The following paragraphs explain how feature test macros are handled
121 in Linux glibc 2.\fIx\fP, \fIx\fP > 0.
122 .\" The details in glibc 2.0 are simpler, but combining a
123 .\" a description of them with the details in later glibc versions
124 .\" would make for a complicated description.
126 Linux glibc understands the following feature test macros:
130 This macro is implicitly defined by
132 when invoked with, for example, the
139 Defining this macro causes header files to expose definitions as follows:
142 The value 1 exposes definitions conforming to POSIX.1-1990 and
145 The value 2 or greater additionally exposes
146 definitions for POSIX.2-1992.
148 The value 199309L or greater additionally exposes
149 definitions for POSIX.1b (real-time extensions).
150 .\" 199506L functionality is only available since glibc 2.1
152 The value 199506L or greater additionally exposes
153 definitions for POSIX.1c (threads).
156 The value 200112L or greater exposes definitions corresponding
157 to the POSIX.1-2001 base specification (excluding the XSI extension).
160 The value 200809L or greater exposes definitions corresponding
161 to the POSIX.1-2008 base specification (excluding the XSI extension).
165 Defining this obsolete macro with any value is equivalent to defining
170 Defining this macro causes header files to expose definitions as follows:
173 Defining with any value exposes
174 definitions conforming to POSIX.1, POSIX.2, and XPG4.
176 The value 500 or greater additionally exposes
177 definitions for SUSv2 (UNIX 98).
179 (Since glibc 2.2) The value 600 or greater additionally exposes
180 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
181 plus the XSI extension) and C99 definitions.
183 (Since glibc 2.10) The value 700 or greater additionally exposes
184 definitions for SUSv4 (i.e., the POSIX.1-2008 base specification
185 plus the XSI extension).
188 .B _XOPEN_SOURCE_EXTENDED
189 If this macro is defined, and
191 is defined, then expose definitions corresponding to the XPG4v2
192 (SUSv1) UNIX extensions (UNIX 95).
193 This macro is also implicitly defined if
195 is defined with a value of 500 or more.
198 Exposes C99 extensions to ISO C (1990).
199 This macro is recognized since glibc 2.1.3;
200 earlier glibc 2.1.x versions recognized an equivalent macro named
202 (because the C99 standard had not then been finalized).
203 Although the use of the latter macro is obsolete, glibc continues
204 to recognize it for backwards compatibility.
206 .B _LARGEFILE64_SOURCE
207 Expose definitions for the alternative API specified by the
208 LFS (Large File Summit) as a "transitional extension" to the
209 Single UNIX Specification.
210 (See http://opengroup.org/platform/lfs.html.)
211 The alternative API consists of a set of new objects
212 (i.e., functions and types) whose names are suffixed with "64"
221 New programs should not employ this interface; instead
222 .I _FILE_OFFSET_BITS=64
226 Defining this macro with the value 64
227 automatically converts references to 32-bit functions and data types
228 related to file I/O and file system operations into references to
229 their 64-bit counterparts.
230 This is useful for performing I/O on large files (> 2 Gigabytes)
232 (Defining this macro permits correctly written programs to use
233 large files with only a recompilation being required.)
234 64-bit systems naturally permit file sizes greater than 2 Gigabytes,
235 and on those systems this macro has no effect.
238 Defining this macro with any value causes header files to expose
239 BSD-derived definitions.
240 Defining this macro also causes BSD definitions to be preferred in
241 some situations where standards conflict, unless one or more of
244 .BR _POSIX_C_SOURCE ,
246 .BR _XOPEN_SOURCE_EXTENDED ,
249 is defined, in which case BSD definitions are disfavored.
252 Defining this macro with any value causes header files to expose
253 System V-derived definitions.
254 (SVID == System V Interface Definition; see
257 .BR _ATFILE_SOURCE " (since glibc 2.4)"
258 Defining this macro with any value causes header files to expose
259 declarations of a range of functions with the suffix "at";
262 Since glibc 2.10, this macro is also implicitly defined if
264 is defined with a value greater than or equal to 200809L.
267 Defining this macro (with any value) is equivalent to defining
271 .BR _LARGEFILE64_SOURCE ,
273 .BR _XOPEN_SOURCE_EXTENDED ,
276 with the value 200809L
277 (200112L in glibc versions before 2.10;
278 199506L in glibc versions before 2.5;
279 199309L in glibc versions before 2.1)
283 (600 in glibc versions before 2.10;
284 500 in glibc versions before 2.2).
285 In addition, various GNU-specific extensions are also exposed.
286 Where standards conflict, BSD definitions are disfavored.
289 Defining this macro exposes definitions of certain reentrant functions.
290 For multithreaded programs, use
297 provided for compatibility with some other implementations.
299 .BR _FORTIFY_SOURCE " (since glibc 2.3.4)"
300 .\" For more detail, see:
301 .\" http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
302 .\" [PATCH] Object size checking to prevent (some) buffer overflows
303 .\" * From: Jakub Jelinek <jakub at redhat dot com>
304 .\" * To: gcc-patches at gcc dot gnu dot org
305 .\" * Date: Tue, 21 Sep 2004 04:16:40 -0400
306 Defining this macro causes some lightweight checks to be performed
307 to detect some buffer overflow errors when employing
308 various string and memory manipulation functions.
309 Not all buffer overflows are detected, just some common cases.
310 In the current implementation checks are added for
329 is set to 1, with compiler optimization level 1
331 and above, checks that shouldn't change the behavior of
332 conforming programs are performed.
335 set to 2 some more checking is added, but
336 some conforming programs might fail.
337 Some of the checks can be performed at compile time,
338 and result in compiler warnings;
339 other checks take place at run time,
340 and result in a run-time error if the check fails.
341 Use of this macro requires compiler support, available with
344 .SS Default definitions, implicit definitions, and combining definitions
346 If no feature test macros are explicitly defined,
347 then the following feature test macros are defined by default:
352 .BR _POSIX_C_SOURCE =200809L
353 (200112L in glibc versions before 2.10;
354 199506L in glibc versions before 2.4;
355 199309L in glibc versions before 2.1).
358 .BR __STRICT_ANSI__ ,
361 .BR _POSIX_C_SOURCE ,
363 .BR _XOPEN_SOURCE_EXTENDED ,
367 is explicitly defined, then
371 are not defined by default.
377 are not explicitly defined,
382 is defined with a value of 500 or more, then
386 is defined with the value 1; and
389 is defined with one of the following values:
395 is defined with a value less than 500;
400 is defined with a value greater than or equal to 500 and less than 600;
403 (since glibc 2.4) 200112L,
406 is defined with a value greater than or equal to 600 and less than 700.
412 is defined with a value greater than or equal to 700.
414 Older versions of glibc do not know about the values
415 200112L and 200809L for
416 .BR _POSIX_C_SOURCE ,
417 and the setting of this macro will depend on the glibc version.
421 is undefined, then the setting of
423 depends on the glibc version:
424 199506L, in glibc versions before 2.4;
425 200112L, in glibc 2.4 to 2.9; and
426 200809L, since glibc 2.10.
430 Multiple macros can be defined; the results are additive.
433 .BR _POSIX_C_SOURCE ,
437 .B _XOPEN_SOURCE_EXTENDED
438 was specified by XPG4v2 (aka SUSv1).
441 is not specified by any standard,
442 but is employed on some other implementations.
448 .BR _FORTIFY_SOURCE ,
452 are specific to Linux (glibc).
455 is a Linux/glibc-specific header file.
456 Other systems have an analogous file, but typically with a different name.
457 This header file is automatically included by other header files as
458 required: it is not necessary to explicitly include it in order to
459 employ feature test macros.
461 According to which of the above feature test macros are defined,
463 internally defines various other macros that are checked by
464 other glibc header files.
465 These macros have names prefixed by two underscores (e.g.,
467 Programs should \fInever\fP define these macros directly:
468 instead, the appropriate feature test macro(s) from the
469 list above should be employed.
471 The program below can be used to explore how the various
472 feature test macros are set depending on the glibc version
473 and what feature test macros are explicitly set.
474 The following shell session, on a system with glibc 2.10,
475 shows some examples of what we would see:
481 _POSIX_SOURCE defined
482 _POSIX_C_SOURCE defined: 200809L
485 _ATFILE_SOURCE defined
486 $ \fBcc -D_XOPEN_SOURCE=500 ftm.c\fP
488 _POSIX_SOURCE defined
489 _POSIX_C_SOURCE defined: 199506L
490 _XOPEN_SOURCE defined: 500
491 $ \fBcc -D_GNU_SOURCE ftm.c\fP
493 _POSIX_SOURCE defined
494 _POSIX_C_SOURCE defined: 200809L
495 _ISOC99_SOURCE defined
496 _XOPEN_SOURCE defined: 700
497 _XOPEN_SOURCE_EXTENDED defined
498 _LARGEFILE64_SOURCE defined
501 _ATFILE_SOURCE defined
515 main(int argc, char *argv[])
518 printf("_POSIX_SOURCE defined\\n");
521 #ifdef _POSIX_C_SOURCE
522 printf("_POSIX_C_SOURCE defined: %ldL\\n", (long) _POSIX_C_SOURCE);
525 #ifdef _ISOC99_SOURCE
526 printf("_ISOC99_SOURCE defined\\n");
530 printf("_XOPEN_SOURCE defined: %d\\n", _XOPEN_SOURCE);
533 #ifdef _XOPEN_SOURCE_EXTENDED
534 printf("_XOPEN_SOURCE_EXTENDED defined\\n");
537 #ifdef _LARGEFILE64_SOURCE
538 printf("_LARGEFILE64_SOURCE defined\\n");
541 #ifdef _FILE_OFFSET_BITS
542 printf("_FILE_OFFSET_BITS defined: %d\\n", _FILE_OFFSET_BITS);
546 printf("_BSD_SOURCE defined\\n");
550 printf("_SVID_SOURCE defined\\n");
553 #ifdef _ATFILE_SOURCE
554 printf("_ATFILE_SOURCE defined\\n");
558 printf("_GNU_SOURCE defined\\n");
562 printf("_REENTRANT defined\\n");
566 printf("_THREAD_SAFE defined\\n");
569 #ifdef _FORTIFY_SOURCE
570 printf("_FORTIFY_SOURCE defined\\n");
580 The section "Feature Test Macros" under
582 .\" But beware: the info libc document is out of date (Jul 07, mtk)
584 .I /usr/include/features.h