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Use feature test macro for syscall prototype

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In order to cover the largest number of glibc and musl libc versions,
withouth warnings, the decision here is to use `_GNU_SOURCE`,
even if it enables a larger than necessary feature set.

A feature macro is needed, because otherwise the `check_c_source_compiles`
for `HAVE_SYSCALL_GETRANDOM` fails in cases when for example
the default compiler flags include `-std=c99`:

````
src.c:6:13: error: implicit declaration of function ‘syscall’ [-Wimplicit-function-declaration]
    6 |             syscall(SYS_getrandom, NULL, 0, 0);
      |             ^~~~~~~
````
But this check should pass, as `SYS_getrandom` is available,
only the declaration of `syscall` in `unistd.h` is conditional behind a macro.

The exact minimal public macros, for enabling this are in `features.h`, and
are version dependent.

According to [5.04](
https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/Archive/man-pages-5.04.tar.gz)
and older versions of the `man 2 syscall` page,
the recommended feature test macro is `_GNU_SOURCE`.
Later on in [5.05](
https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/Archive/man-pages-5.05.tar.gz)
this statement has changed, to provide a smaller minimal feature set.
Namely up to `glibc 2.18`  is `_BSD_SOURCE || _SVID_SOURCE`,
but after that the `_DEFAULT_SOURCE` is recommended,
and `_BSD_SOURCE || _SVID_SOURCE` is deprecated, and emits warning in later versions.
Regardless of that the `_GNU_SOURCE` is still fully supported
in every version and is suitable for our purposes.

The musl libc doesn't use `_SVID_SOURCE` at all, but `_BSD_SOURCE` always works,
plus in some newer versions `_DEFAULT_SOURCE` also sets `_BSD_SOURCE`,
but `_GNU_SOURCE` covers the largest set of versions and is unlikely
to be deprecated in the future.

Further info about feature test macros:

In glibc:
https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html

In musl libc under the `Feature Test Macros Supported by musl` section:
https://musl.libc.org/doc/1.1.24/manual.html

Signed-off-by: Ferenc Géczi <ferenc.gm@gmail.com>
This commit is contained in:
Ferenc Géczi 2024-04-27 00:00:01 +00:00
parent 9134d0d6e0
commit 73627c7456
2 changed files with 2 additions and 0 deletions
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1
expat/ConfigureChecks.cmake
View file

@ -60,6 +60,7 @@ if(NOT HAVE_OFF_T)
endif()
check_c_source_compiles("
#define _GNU_SOURCE
#include <stdlib.h> /* for NULL */
#include <unistd.h> /* for syscall */
#include <sys/syscall.h> /* for SYS_getrandom */

1
expat/configure.ac
View file

@ -274,6 +274,7 @@ AS_HELP_STRING([--without-sys-getrandom],
AS_IF([test "x$with_sys_getrandom" != xno],
[AC_MSG_CHECKING([for syscall SYS_getrandom (Linux 3.17+)])
AC_LINK_IFELSE([AC_LANG_SOURCE([
#define _GNU_SOURCE
#include <stdlib.h> /* for NULL */
#include <unistd.h> /* for syscall */
#include <sys/syscall.h> /* for SYS_getrandom */