POSIX specifies the fmemopen() function shall fail with EINVAL when the size argument specifies a buffer size of zero. When compiled with the top of trunk, the program below aborts indicating that glibc fails to conform to this requirement.

#include <assert.h>
#include <errno.h>
#include <stdio.h>

int main (void)
{
    char c = 0;
    FILE *f = fmemopen (&c, 0, "w");
    printf ("%p, errno = %m\n", f);
    assert (f == 0 && errno == EINVAL);
    return 0;
}
0x100002f0040, errno = Success
a.out: /build/tmp/t.c:10: main: Assertion `f == 0 && (*__errno_location ()) == 22' failed.