Re: mkstemp(3)

[Garrett Wollman <wollman@bimajority.org>]

<<On Thu, 7 May 2026 21:34:03 -0400, Douglas McIlroy <douglas.mcilroy@dartmouth.edu> said:

> 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.

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

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>

-GAWollman