#include <sys/types.h>

uid = getuid()
uid_t uid;

euid = geteuid()
uid_t euid;

 DESCRIPTION
 Getuid returns the real user ID of the current process,
 geteuid the effective user ID.

The real user ID identifies the person who is logged in.
The effective user ID
gives the process additional permissions during
execution of \*(lqset-user-ID\*(rq mode processes, which use
getuid to determine the real-user-id of the process that
invoked them.
 "SEE ALSO"
getgid(2), setreuid(2)