1FEATURE_TEST_MACROS(7) Linux Programmer's Manual FEATURE_TEST_MACROS(7)
2
6 feature_test_macros - feature test macros
7
9 #include <features.h>
10
12 Feature test macros allow the programmer to control the definitions
13 that are exposed by system header files when a program is compiled.
14 This can be useful for creating portable applications, by preventing
15 nonstandard definitions from being exposed. Other macros can be used
16 to expose nonstandard definitions that are not exposed by default. The
17 precise effects of each of the feature test macros described below can
18 be ascertained by inspecting the <features.h> header file.
19 In order to be effective, a feature test macro must be defined before
21 including any header files. This can either be done in the compilation
22 command (cc -DMACRO=value) or by defining the macro within the source
23 code before including any headers.
24 Specification of feature test macro requirements in manual pages
26 When a function requires that a feature test macro is defined, the man‐
27 ual page SYNOPSIS typically includes a note of the following form (this
28 example from the chmod(2) manual page):
29 #include <sys/stat.h>
31 int chmod(const char *path, mode_t mode);
33 int fchmod(int fd, mode_t mode);
34 Feature Test Macro Requirements for glibc (see fea‐
36 ture_test_macros(7)):
37 fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
39 The || means that in order to obtain the declaration of fchmod(2) from
41 <sys/stat.h>, either of the following macro definitions must be made
42 before including any header files:
43 #define _BSD_SOURCE
45 #define _XOPEN_SOURCE 500 /* or any value > 500 */
46 Alternatively, equivalent definitions can be included in the compila‐
48 tion command:
49 cc -D_BSD_SOURCE
51 cc -D_XOPEN_SOURCE=500 # Or any value > 500
52 Note that, as described below, some feature test macros are defined by
54 default, so that it may not always be necessary to explicitly specify
55 the feature test macro(s) shown in the SYNOPSIS.
56 In a few cases, manual pages use a shorthand for expressing the feature
58 test macro requirements (this example from readahead(2)):
59 #define _GNU_SOURCE
61 #include <fcntl.h>
62 ssize_t readahead(int fd, off64_t *offset, size_t count);
64 This format is employed in cases where only a single feature test macro
66 can be used to expose the function declaration, and that macro is not
67 defined by default.
68 Feature test macros understood by glibc
70 The following paragraphs explain how feature test macros are handled in
71 Linux glibc 2.x, x > 0.
72 Linux glibc understands the following feature test macros:
74 __STRICT_ANSI__
76 ISO Standard C. This macro is implicitly defined by gcc(1) when
77 invoked with, for example, the -std=c99 or -ansi flag.
78 _POSIX_C_SOURCE
80 Defining this macro causes header files to expose definitions as
81 follows:
82 · The value 1 exposes definitions conforming to POSIX.1-1990
84 and ISO C (1990).
85 · The value 2 or greater additionally exposes definitions for
87 POSIX.2-1992.
88 · The value 199309L or greater additionally exposes definitions
90 for POSIX.1b (real-time extensions).
91 · The value 199506L or greater additionally exposes definitions
93 for POSIX.1c (threads).
94 · (Since glibc 2.3.3) The value 200112L or greater exposes def‐
96 initions corresponding to the POSIX.1-2001 base specification
97 (excluding the XSI extension).
98 · (Since glibc 2.10) The value 200809L or greater exposes defi‐
100 nitions corresponding to the POSIX.1-2008 base specification
101 (excluding the XSI extension).
102 _POSIX_SOURCE
104 Defining this obsolete macro with any value is equivalent to
105 defining _POSIX_C_SOURCE with the value 1.
106 _XOPEN_SOURCE
108 Defining this macro causes header files to expose definitions as
109 follows:
110 · Defining with any value exposes definitions conforming to
112 POSIX.1, POSIX.2, and XPG4.
113 · The value 500 or greater additionally exposes definitions for
115 SUSv2 (UNIX 98).
116 · (Since glibc 2.2) The value 600 or greater additionally
118 exposes definitions for SUSv3 (UNIX 03; i.e., the
119 POSIX.1-2001 base specification plus the XSI extension) and
120 C99 definitions.
121 · (Since glibc 2.10) The value 700 or greater additionally
123 exposes definitions for SUSv4 (i.e., the POSIX.1-2008 base
124 specification plus the XSI extension).
125 _XOPEN_SOURCE_EXTENDED
127 If this macro is defined, and _XOPEN_SOURCE is defined, then
128 expose definitions corresponding to the XPG4v2 (SUSv1) UNIX
129 extensions (UNIX 95). This macro is also implicitly defined if
130 _XOPEN_SOURCE is defined with a value of 500 or more.
131 _ISOC99_SOURCE
133 Exposes C99 extensions to ISO C (1990). This macro is recog‐
134 nized since glibc 2.1.3; earlier glibc 2.1.x versions recognized
135 an equivalent macro named _ISOC9X_SOURCE (because the C99 stan‐
136 dard had not then been finalized). Although the use of the lat‐
137 ter macro is obsolete, glibc continues to recognize it for back‐
138 wards compatibility.
139 _LARGEFILE64_SOURCE
141 Expose definitions for the alternative API specified by the LFS
142 (Large File Summit) as a "transitional extension" to the Single
143 UNIX Specification. (See http://opengroup.org/plat‐
144 form/lfs.html.) The alternative API consists of a set of new
145 objects (i.e., functions and types) whose names are suffixed
146 with "64" (e.g., off64_t versus off_t, lseek64() versus lseek(),
147 etc.). New programs should not employ this interface; instead
148 _FILE_OFFSET_BITS=64 should be employed.
149 _FILE_OFFSET_BITS
151 Defining this macro with the value 64 automatically converts
152 references to 32-bit functions and data types related to file
153 I/O and file system operations into references to their 64-bit
154 counterparts. This is useful for performing I/O on large files
155 (> 2 Gigabytes) on 32-bit systems. (Defining this macro permits
156 correctly written programs to use large files with only a recom‐
157 pilation being required.) 64-bit systems naturally permit file
158 sizes greater than 2 Gigabytes, and on those systems this macro
159 has no effect.
160 _BSD_SOURCE
162 Defining this macro with any value causes header files to expose
163 BSD-derived definitions. Defining this macro also causes BSD
164 definitions to be preferred in some situations where standards
165 conflict, unless one or more of _SVID_SOURCE, _POSIX_SOURCE,
166 _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, or
167 _GNU_SOURCE is defined, in which case BSD definitions are disfa‐
168 vored.
169 _SVID_SOURCE
171 Defining this macro with any value causes header files to expose
172 System V-derived definitions. (SVID == System V Interface Defi‐
173 nition; see standards(7).)
174 _ATFILE_SOURCE (since glibc 2.4)
176 Defining this macro with any value causes header files to expose
177 declarations of a range of functions with the suffix "at"; see
178 openat(2). Since glibc 2.10, this macro is also implicitly
179 defined if _POSIX_C_SOURCE is defined with a value greater than
180 or equal to 200809L.
181 _GNU_SOURCE
183 Defining this macro (with any value) is equivalent to defining
184 _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _LARGEFILE64_SOURCE,
185 _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED, _POSIX_SOURCE,
186 _POSIX_C_SOURCE with the value 200809L (200112L in glibc ver‐
187 sions before 2.10; 199506L in glibc versions before 2.5; 199309L
188 in glibc versions before 2.1) and _XOPEN_SOURCE with the value
189 700 (600 in glibc versions before 2.10; 500 in glibc versions
190 before 2.2). In addition, various GNU-specific extensions are
191 also exposed. Where standards conflict, BSD definitions are
192 disfavored.
193 _REENTRANT
195 Defining this macro exposes definitions of certain reentrant
196 functions. For multithreaded programs, use cc -pthread instead.
197 _THREAD_SAFE
199 Synonym for _REENTRANT, provided for compatibility with some
200 other implementations.
201 _FORTIFY_SOURCE (since glibc 2.3.4)
203 Defining this macro causes some lightweight checks to be per‐
204 formed to detect some buffer overflow errors when employing var‐
205 ious string and memory manipulation functions. Not all buffer
206 overflows are detected, just some common cases. In the current
207 implementation checks are added for calls to memcpy(3), mem‐
208 pcpy(3), memmove(3), memset(3), stpcpy(3), strcpy(3),
209 strncpy(3), strcat(3), strncat(3), sprintf(3), snprintf(3),
210 vsprintf(3), vsnprintf(3), and gets(3). If _FORTIFY_SOURCE is
211 set to 1, with compiler optimization level 1 (gcc -O1) and
212 above, checks that shouldn't change the behavior of conforming
213 programs are performed. With _FORTIFY_SOURCE set to 2 some more
214 checking is added, but some conforming programs might fail.
215 Some of the checks can be performed at compile time, and result
216 in compiler warnings; other checks take place at run time, and
217 result in a run-time error if the check fails. Use of this
218 macro requires compiler support, available with gcc(1) since
219 version 4.0.
220 Default definitions, implicit definitions, and combining definitions
222 If no feature test macros are explicitly defined, then the following
223 feature test macros are defined by default: _BSD_SOURCE, _SVID_SOURCE,
224 _POSIX_SOURCE, and _POSIX_C_SOURCE=200809L (200112L in glibc versions
225 before 2.10; 199506L in glibc versions before 2.4; 199309L in glibc
226 versions before 2.1).
227 If any of __STRICT_ANSI__, _ISOC99_SOURCE, _POSIX_SOURCE,
229 _POSIX_C_SOURCE, _XOPEN_SOURCE, _XOPEN_SOURCE_EXTENDED, _BSD_SOURCE, or
230 _SVID_SOURCE is explicitly defined, then _BSD_SOURCE, and _SVID_SOURCE
231 are not defined by default.
232 If _POSIX_SOURCE and _POSIX_C_SOURCE are not explicitly defined, and
234 either __STRICT_ANSI__ is not defined or _XOPEN_SOURCE is defined with
235 a value of 500 or more, then
236 * _POSIX_SOURCE is defined with the value 1; and
238 * _POSIX_C_SOURCE is defined with one of the following values:
240 · 2, if XOPEN_SOURCE is defined with a value less than 500;
242 · 199506L, if XOPEN_SOURCE is defined with a value greater
244 than or equal to 500 and less than 600; or
245 · (since glibc 2.4) 200112L, if XOPEN_SOURCE is defined with
247 a value greater than or equal to 600 and less than 700.
248 · (Since glibc 2.10) 200809L, if XOPEN_SOURCE is defined with
250 a value greater than or equal to 700.
251 · Older versions of glibc do not know about the values
253 200112L and 200809L for _POSIX_C_SOURCE, and the setting of
254 this macro will depend on the glibc version.
255 · If _XOPEN_SOURCE is undefined, then the setting of
257 _POSIX_C_SOURCE depends on the glibc version: 199506L, in
258 glibc versions before 2.4; 200112L, in glibc 2.4 to 2.9;
259 and 200809L, since glibc 2.10.
260 Multiple macros can be defined; the results are additive.
262
264 POSIX.1 specifies _POSIX_C_SOURCE, _POSIX_SOURCE, and _XOPEN_SOURCE.
265 _XOPEN_SOURCE_EXTENDED was specified by XPG4v2 (aka SUSv1).
266 _FILE_OFFSET_BITS is not specified by any standard, but is employed on
268 some other implementations.
269 _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE, _GNU_SOURCE, _FOR‐
271 TIFY_SOURCE, _REENTRANT, and _THREAD_SAFE are specific to Linux
272 (glibc).
273
275 <features.h> is a Linux/glibc-specific header file. Other systems have
276 an analogous file, but typically with a different name. This header
277 file is automatically included by other header files as required: it is
278 not necessary to explicitly include it in order to employ feature test
279 macros.
280 According to which of the above feature test macros are defined, <fea‐
282 tures.h> internally defines various other macros that are checked by
283 other glibc header files. These macros have names prefixed by two
284 underscores (e.g., __USE_MISC). Programs should never define these
285 macros directly: instead, the appropriate feature test macro(s) from
286 the list above should be employed.
287
289 The program below can be used to explore how the various feature test
290 macros are set depending on the glibc version and what feature test
291 macros are explicitly set. The following shell session, on a system
292 with glibc 2.10, shows some examples of what we would see:
293 $ cc ftm.c
295 $ ./a.out
296 _POSIX_SOURCE defined
297 _POSIX_C_SOURCE defined: 200809L
298 _BSD_SOURCE defined
299 _SVID_SOURCE defined
300 _ATFILE_SOURCE defined
301 $ cc -D_XOPEN_SOURCE=500 ftm.c
302 $ ./a.out
303 _POSIX_SOURCE defined
304 _POSIX_C_SOURCE defined: 199506L
305 _XOPEN_SOURCE defined: 500
306 $ cc -D_GNU_SOURCE ftm.c
307 $ ./a.out
308 _POSIX_SOURCE defined
309 _POSIX_C_SOURCE defined: 200809L
310 _ISOC99_SOURCE defined
311 _XOPEN_SOURCE defined: 700
312 _XOPEN_SOURCE_EXTENDED defined
313 _LARGEFILE64_SOURCE defined
314 _BSD_SOURCE defined
315 _SVID_SOURCE defined
316 _ATFILE_SOURCE defined
317 _GNU_SOURCE defined
318 Program source
320 /* ftm.c */
322 #include <stdio.h>
324 #include <unistd.h>
325 #include <stdlib.h>
326 int
328 main(int argc, char *argv[])
329 {
330 #ifdef _POSIX_SOURCE
331 printf("_POSIX_SOURCE defined\n");
332 #endif
333 #ifdef _POSIX_C_SOURCE
335 printf("_POSIX_C_SOURCE defined: %ldL\n", (long) _POSIX_C_SOURCE);
336 #endif
337 #ifdef _ISOC99_SOURCE
339 printf("_ISOC99_SOURCE defined\n");
340 #endif
341 #ifdef _XOPEN_SOURCE
343 printf("_XOPEN_SOURCE defined: %d\n", _XOPEN_SOURCE);
344 #endif
345 #ifdef _XOPEN_SOURCE_EXTENDED
347 printf("_XOPEN_SOURCE_EXTENDED defined\n");
348 #endif
349 #ifdef _LARGEFILE64_SOURCE
351 printf("_LARGEFILE64_SOURCE defined\n");
352 #endif
353 #ifdef _FILE_OFFSET_BITS
355 printf("_FILE_OFFSET_BITS defined: %d\n", _FILE_OFFSET_BITS);
356 #endif
357 #ifdef _BSD_SOURCE
359 printf("_BSD_SOURCE defined\n");
360 #endif
361 #ifdef _SVID_SOURCE
363 printf("_SVID_SOURCE defined\n");
364 #endif
365 #ifdef _ATFILE_SOURCE
367 printf("_ATFILE_SOURCE defined\n");
368 #endif
369 #ifdef _GNU_SOURCE
371 printf("_GNU_SOURCE defined\n");
372 #endif
373 #ifdef _REENTRANT
375 printf("_REENTRANT defined\n");
376 #endif
377 #ifdef _THREAD_SAFE
379 printf("_THREAD_SAFE defined\n");
380 #endif
381 #ifdef _FORTIFY_SOURCE
383 printf("_FORTIFY_SOURCE defined\n");
384 #endif
385 exit(EXIT_SUCCESS);
387 }
388
390 libc(7), standards(7)
391 The section "Feature Test Macros" under info libc.
393 /usr/include/features.h
395
397 This page is part of release 3.25 of the Linux man-pages project. A
398 description of the project, and information about reporting bugs, can
399 be found at http://www.kernel.org/doc/man-pages/.
400Linux 2009-12-13 FEATURE_TEST_MACROS(7)