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feature_test_macros.7: Improve discussion of _DEFAULT_SOURCE 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-03-10 10:05:24 +01:00
parent 496bdb5642
commit dfb1232ca8
1 changed files with 34 additions and 11 deletions
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45
man7/feature_test_macros.7
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@ -22,7 +22,7 @@
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH FEATURE_TEST_MACROS 7 2014-01-06 "Linux" "Linux Programmer's Manual"
.TH FEATURE_TEST_MACROS 7 2014-03-10 "Linux" "Linux Programmer's Manual"
.SH NAME
feature_test_macros \- feature test macros
.SH SYNOPSIS
@ -282,17 +282,24 @@ System V-derived definitions.
.BR standards (7).)
.TP
.BR _DEFAULT_SOURCE " (since glibc 2.19)"
Defining this macro provides an effect similar to
the feature test macros that are defined by default; that is:
cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
This macro can be defined to ensure that the "default"
definitions are provided even when the defaults would otherwise
be disabled,
as happens when individual macros are explicitly defined,
or the compiler is invoked in one of its "standard" modes (e.g.,
.IR "cc\ \-std=c99" ).
Defining
.B _DEFAULT_SOURCE
without defining other individual macros
or invoking the compiler in one of its "standard" modes has no effect.
The "default" definitions comprise those required by POSIX.1-2008
as well as various definitions derived from BSD and System V.
On glibc 2.19 and earlier, these defaults were approximately equivalent
to explicitly defining the following:
cc \-D_BSD_SOURCE \-D_SVID_SOURCE \-D_POSIX_C_SOURCE=200809
.TP
.BR _ATFILE_SOURCE " (since glibc 2.4)"
Defining this macro with any value causes header files to expose
@ -305,8 +312,6 @@ is defined with a value greater than or equal to 200809L.
.TP
.B _GNU_SOURCE
Defining this macro (with any value) is equivalent to defining
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _ATFILE_SOURCE ,
.BR _LARGEFILE64_SOURCE ,
.BR _ISOC99_SOURCE ,
@ -323,7 +328,17 @@ with the value 700
(600 in glibc versions before 2.10;
500 in glibc versions before 2.2).
In addition, various GNU-specific extensions are also exposed.
Where standards conflict, BSD definitions are disfavored.
Since glibc 2.19, defining
.BR _GNU_SOURCE
also has the effect of defining
.BR _DEFAULT_SOURCE .
In glibc versions before 2.20, defining
.BR _GNU_SOURCE
also had the effect of defining
.BR _BSD_SOURCE
and
.BR _SVID_SOURCE .
.TP
.B _REENTRANT
Defining this macro exposes definitions of certain reentrant functions.
@ -387,6 +402,8 @@ If no feature test macros are explicitly defined,
then the following feature test macros are defined by default:
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _DEFAULT_SOURCE
(since glibc 2.19),
.BR _POSIX_SOURCE ,
and
.BR _POSIX_C_SOURCE =200809L
@ -405,9 +422,10 @@ If any of
or
.B _SVID_SOURCE
is explicitly defined, then
.BR _BSD_SOURCE
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
and
.B _SVID_SOURCE
.BR _DEFAULT_SOURCE
are not defined by default.
If
@ -483,6 +501,7 @@ but is employed on some other implementations.
.BR _BSD_SOURCE ,
.BR _SVID_SOURCE ,
.BR _DEFAULT_SOURCE ,
.BR _ATFILE_SOURCE ,
.BR _GNU_SOURCE ,
.BR _FORTIFY_SOURCE ,
@ -596,6 +615,10 @@ main(int argc, char *argv[])
printf("_SVID_SOURCE defined\\n");
#endif
#ifdef _DEFAULT_SOURCE
printf("_DEFAULT_SOURCE defined\\n");
#endif
#ifdef _ATFILE_SOURCE
printf("_ATFILE_SOURCE defined\\n");
#endif