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| author | Michael Kerrisk <mtk.manpages@gmail.com> | 2019-01-10 13:01:03 +1300 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2019-01-10 13:01:03 +1300 |
| commit | 1be4da28c54b62bd012495a3bdaa86ef0ea8dc13 (patch) | |
| tree | 7c6454003aff99f192743dca2570944aa00da1a8 | |
| parent | c3a4afcae04a87ffa0c704ded9d0ce76137f323d (diff) | |
| download | man-pages-1be4da28c54b62bd012495a3bdaa86ef0ea8dc13.tar.gz | |
feature_test_macros.7: Add more detail on why FTMs must be defined before including any header
Reported-by: Andreas Westfeld <andreas.westfeld@htw-dresden.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
| -rw-r--r-- | man7/feature_test_macros.7 | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7 index e8d575ac2f..87630f4b99 100644 --- a/man7/feature_test_macros.7 +++ b/man7/feature_test_macros.7 @@ -36,6 +36,23 @@ This can be done either in the compilation command .RI ( "cc \-DMACRO=value" ) or by defining the macro within the source code before including any headers. +The requirement that the macro must be defined before including any +header file exists because header files may freely include one another. +Thus, for example, in the following lines, defining the +.B _GNU_SOURCE +macro may have no effect because the header +.I <abc.h> +itself includes +.I <xyz.h> +(POSIX explicitly allows this): +.PP +.in +4n +.EX +#include <abc.h> +#define _GNU_SOURCE +#include <xys.h> +.EE +.in .PP Some feature test macros are useful for creating portable applications, by preventing nonstandard definitions from being exposed. |