c99中不赞成使用getresuid吗？

Question

I'm trying to compile a function containing a call to getresuid . However it generates the following warning:

setuid.c:8:3: warning: implicit declaration of function 'getresuid' is invalid
in C99 [-Wimplicit-function-declaration]
getresuid(&ruid, &euid, &suid);
^

Undefined symbols for architecture x86_64:
"_getresuid", referenced from:
_main in setuid-ba46f8.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1

Why is there a linker error saying symbol not found for architecture x86_64? How do you get it to successfully link?

If it helps, my original code was:

#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main() {
    uid_t euid, ruid, suid;
    getresuid(&ruid, &euid, &suid);
    printf("EUID: %d, RUID: %d, SUID: %d\n", euid, ruid, suid);
    return 0;
}

I'm trying to compile on Mac OS X 10.10.2 (Yosemite).

Answer 1

You need to define before all the headers

#define _GNU_SOURCE

as getresuid is a GNU extension function.

Answer 2

The direct answer is "No; getresuid() is simply not part of C99 — nor of POSIX 2008 (2013)". So to use it, you have to use a platform-specific extension, and it is not available on all platforms.

Incidentally, you don't need #include <sys/types.h> in your code.

Your code compiles OK for me on Ubuntu 14.04. However, it fails to compile on Mac OS X 10.10.3 (Yosemite), which is derived from BSD rather than Linux. The getresuid() function is simply not available — not implemented — on the platform.

You'll need to use getuid() and geteuid() for the 'r' (real UID) and 'e' (effective UID) information; I don't think there's a sensible way to get the 's' (saved UID) information. Most of the time, the three UID values are the same. Only a program that expects to be run with the 'setuid' bit set needs to worry about these, and such programs might want to worry about the saved UID. However, you're probably limited to inspecting the effective UID when the program starts; that's the saved UID.