Hi Doug, Garrett,

On 2026-05-07T22:32:30-0400, Garrett Wollman wrote:

<<On Thu, 7 May 2026 21:34:03 -0400, Douglas McIlroy [off-list ref] said:

quoted

Posix System Interfaces Section 2.2 tells me that I should #define
_POSIX_C_SOURCE before #include <stdlib.h>. That fact is missing from
Linux's man 3 mkstemp.  Arguably the Posix description of mkstemp
should mention it, too.

It is mentioned in this part (see the bottom of the SYNOPSIS):

	 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

	     mkstemp():
		 _XOPEN_SOURCE >= 500
		     || /* glibc >= 2.12: */ _POSIX_C_SOURCE >= 200809L
		     || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE

To be honest, I've never liked that format too much, and think it would
be more readable as this:

		#define _XOPEN_SOURCE  500
		#include <stdlib.h>

		int mkstemp(char *template);

<div class="standards-lawyering">
Conveniently, POSIX.1-2008 removed mkstemp from the XSI option and
made it standard (shaded only CX and not XSI) so in 2008 and newer,
you don't have to define _XOPEN_SOURCE.  You do have to set the
correct _POSIX_C_SOURCE value for the standard your implementation
confirms to,

Hmmm, I should probably give more preference to _POSIX_C_SOURCE in the
manual page.  Thanks!

which currently means that you can only use C17 in
POSIX.1-2024 and cannot use C23 at all.

Well, you could technically ask for a combination of both, by doing:
-D_POSIX_C_SOURCE=200809L -std=c23.  Then it's up to the compiler and
libc to decide what to do with such a petition, but since POSIX usually
only adds ISO C-compatible APIs, it shouldn't be problematic.

This may be the source of the confusion: for ISO C, "the
implementation" is the compiler, header files, and standard library,
so your C23 compiler ships with versions of those that meet the
requirements of the C23 standard.  For POSIX on the other hand, "the
implementation" is the entire operating system, which must support one
and only one specific version of ISO C.  It is up to your operating
system supplier (e.g. Linux distro packager) to build a <stdlib.h>
that meets the requirements of POSIX if they want to claim to be
POSIX-compliant.  (Very likely they don't actually care about formal
compliance.)

An application that uses C23 features (or indeed that is compiled with
anything other than the POSIX.1-2024 `c17` utility) is not a
conforming POSIX application and its behavior is undefined.  (Likewise
POSIX.1-2008 requires compilation with the `c99` utility, since that
standard is aligned to C99.)
</div>

Interestingly, c99(1) is defined by POSIX to only need to accept an
ISO C-conforming program, and thus it doesn't support POSIX interfaces
added to <stdlib.h>.  I'm not sure if this is intentional.

	alx@devuan:~/tmp$ cat test.c 
	#include <stdlib.h>

	int
	main(void)
	{
		mkstemp("foo");
	}
	alx@devuan:~/tmp$ c99 test.c 
	test.c: In function ‘main’:
	test.c:6:9: error: implicit declaration of function ‘mkstemp’ [-Wimplicit-function-declaration]
	    6 |         mkstemp("foo");
	      |         ^~~~~~~

This is with GCC and glibc.  I don't have c17(1) in my system; maybe GCC
has not cared to add it.


Have a lovely day!
Alex

-GAWollman

-- 
<https://www.alejandro-colomar.es>