]>
Commit | Line | Data |
---|---|---|
28f540f4 RM |
1 | Feature Test Macros |
2 | ------------------- | |
3 | ||
4 | The exact set of features available when you compile a source file | |
5 | is controlled by which "feature test macros" you define. | |
6 | ||
7 | If you compile your programs using `gcc -ansi', you get only the | |
8 | ANSI C library features, unless you explicitly request additional | |
9 | features by defining one or more of the feature macros. *Note GNU CC | |
10 | Command Options: (gcc.info)Invoking GCC, for more information about GCC | |
11 | options. | |
12 | ||
13 | You should define these macros by using `#define' preprocessor | |
14 | directives at the top of your source code files. These directives | |
15 | *must* come before any `#include' of a system header file. It is best | |
16 | to make them the very first thing in the file, preceded only by | |
17 | comments. You could also use the `-D' option to GCC, but it's better | |
18 | if you make the source files indicate their own meaning in a | |
19 | self-contained way. | |
20 | ||
21 | - Macro: _POSIX_SOURCE | |
22 | If you define this macro, then the functionality from the POSIX.1 | |
23 | standard (IEEE Standard 1003.1) is available, as well as all of the | |
24 | ANSI C facilities. | |
25 | ||
26 | - Macro: _POSIX_C_SOURCE | |
27 | If you define this macro with a value of `1', then the | |
28 | functionality from the POSIX.1 standard (IEEE Standard 1003.1) is | |
29 | made available. If you define this macro with a value of `2', | |
30 | then both the functionality from the POSIX.1 standard and the | |
31 | functionality from the POSIX.2 standard (IEEE Standard 1003.2) are | |
32 | made available. This is in addition to the ANSI C facilities. | |
33 | ||
34 | - Macro: _BSD_SOURCE | |
35 | If you define this macro, functionality derived from 4.3 BSD Unix | |
36 | is included as well as the ANSI C, POSIX.1, and POSIX.2 material. | |
37 | ||
38 | Some of the features derived from 4.3 BSD Unix conflict with the | |
39 | corresponding features specified by the POSIX.1 standard. If this | |
40 | macro is defined, the 4.3 BSD definitions take precedence over the | |
41 | POSIX definitions. | |
42 | ||
43 | Due to the nature of some of the conflicts between 4.3 BSD and | |
44 | POSIX.1, you need to use a special "BSD compatibility library" | |
45 | when linking programs compiled for BSD compatibility. This is | |
46 | because some functions must be defined in two different ways, one | |
47 | of them in the normal C library, and one of them in the | |
48 | compatibility library. If your program defines `_BSD_SOURCE', you | |
49 | must give the option `-lbsd-compat' to the compiler or linker when | |
50 | linking the program, to tell it to find functions in this special | |
51 | compatibility library before looking for them in the normal C | |
52 | library. | |
53 | ||
54 | - Macro: _SVID_SOURCE | |
55 | If you define this macro, functionality derived from SVID is | |
56 | included as well as the ANSI C, POSIX.1, and POSIX.2 material. | |
57 | ||
58 | - Macro: _GNU_SOURCE | |
59 | If you define this macro, everything is included: ANSI C, POSIX.1, | |
60 | POSIX.2, BSD, SVID, and GNU extensions. In the cases where POSIX.1 | |
61 | conflicts with BSD, the POSIX definitions take precedence. | |
62 | ||
63 | If you want to get the full effect of `_GNU_SOURCE' but make the | |
64 | BSD definitions take precedence over the POSIX definitions, use | |
65 | this sequence of definitions: | |
66 | ||
67 | #define _GNU_SOURCE | |
68 | #define _BSD_SOURCE | |
69 | #define _SVID_SOURCE | |
70 | ||
71 | Note that if you do this, you must link your program with the BSD | |
72 | compatibility library by passing the `-lbsd-compat' option to the | |
73 | compiler or linker. *Note:* If you forget to do this, you may get | |
74 | very strange errors at run time. | |
75 | ||
76 | We recommend you use `_GNU_SOURCE' in new programs. If you don't | |
77 | specify the `-ansi' option to GCC and don't define any of these macros | |
78 | explicitly, the effect is the same as defining `_GNU_SOURCE'. | |
79 | ||
80 | When you define a feature test macro to request a larger class of | |
81 | features, it is harmless to define in addition a feature test macro for | |
82 | a subset of those features. For example, if you define | |
83 | `_POSIX_C_SOURCE', then defining `_POSIX_SOURCE' as well has no effect. | |
84 | Likewise, if you define `_GNU_SOURCE', then defining either | |
85 | `_POSIX_SOURCE' or `_POSIX_C_SOURCE' or `_SVID_SOURCE' as well has no | |
86 | effect. | |
87 | ||
88 | Note, however, that the features of `_BSD_SOURCE' are not a subset of | |
89 | any of the other feature test macros supported. This is because it | |
90 | defines BSD features that take precedence over the POSIX features that | |
91 | are requested by the other macros. For this reason, defining | |
92 | `_BSD_SOURCE' in addition to the other feature test macros does have an | |
93 | effect: it causes the BSD features to take priority over the conflicting | |
94 | POSIX features. | |
95 |