gnulib/doc/posix-functions/va_arg.texi

@node va_arg
@section @code{va_arg}
@findex va_arg

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9699919799/functions/va_arg.html}

Gnulib module: ---

Portability problems fixed by Gnulib:
@itemize
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
The second argument of @code{va_arg} must be a type that is invariant under
the ``default argument promotions'' (ISO C 99 6.5.2.2 paragraph 6).  This
means that the following are not valid here:
@table @asis
@item @samp{float}
Use @samp{double} instead.
@item @samp{bool}
Use @samp{int} instead.
@item Integer types smaller than @samp{int}.
Use @samp{int} or @samp{unsigned int} instead.
@end table

This is a portability problem because you don't know the width of some
abstract types like @code{uid_t}, @code{gid_t}, @code{mode_t}.  So, instead of
@smallexample
mode = va_arg (ap, mode_t);
@end smallexample
you have to write
@smallexample
mode = (sizeof (mode_t) < sizeof (int)
        ? va_arg (ap, int)
        : va_arg (ap, mode_t));
@end smallexample
@end itemize