Using the setuid bit in Linux

I have this C file:

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    printf("%sn", getlogin());
    printf("%in", getuid());
}

I compile it, set the UID and GID both to root and set the setuid bit, so that it looks like this:

-rwsrwsr-x 1 root    root    8735 Apr  8 19:51 a.out

However when I call $ ./a.out I still get:

user
1000

What am I doing wrong?

Answer

The real user ID is still the user that called the program, but the effective user ID is root. In a setuid program, they are not the same.

To get the effective user ID, call geteuid(). You can also use cuserid() to get the name associated with the effective user ID.