When this sample program

#include <stdio.h>
#include <pwd.h>

main()
{
    int rc;
    struct passwd *pw;

    if ((pw = getpwnam("uucp")) == NULL) {
        perror("getpwnam");
        exit(1);
    }

    rc = setuid(pw->pw_uid);
    printf("return code from setuid(<uucp>) is %d\n", rc);
}

is compiled and the ownership and modes are setup like this:

-rwsr-xr-x    1 uucp    sys       74725 Jul 10 15:03 setuid

sys@kestrel(83)>./setuid
return code from setuid(<uucp>) is 0
sys@kestrel(84)>uname -a
kestrel kestrel 3.2.0 V2.1.7 i386

kstailey@owl$ ./setuid 
return code from setuid(<uucp>) is -1
kstailey@owl$ uname -a
NetBSD owl 1.2_ALPHA NetBSD 1.2_ALPHA (OWL) #1: Wed Jun 26 17:14:30 EDT 1996     kstailey@owl:/usr/src/sys/arch/i386/compile/OWL i386

Both of these systems claim to impliment POSIX.  Which return code
from setuid() is the right one and why?