Manual Pages  — SETUID

The seteuid() system call (setegid()) sets the effective user ID (group ID) of the current process. The effective user ID may be set to the value of the real user ID or the saved set-user-ID (see intro(2) and execve(2)); in this way, the effective user ID of a set-user-ID executable may be toggled by switching to the real user ID, then re-enabled by reverting to the set-user-ID value. Similarly, the effective group ID may be set to the value of the real group ID or the saved set-group-ID.

To prevent these files from remaining open after an exec(3) call, be sure to set the close-on-exec flag:

void
pseudocode(void)
{
        int fd;
        /* ... */

        fd = open("/path/to/sensitive/data", O_RDWR | O_CLOEXEC);
        if (fd == -1)
                err(1, "open");

        /* ... */
        execve(path, argv, environ);
}