getuid(), geteuid() - get user identification
#include <unistd.h>
#include <sys/types.h>
uid_t getuid (void)
uid_t geteuid (void)
The getuid(2) function returns the real user ID of the calling process. The geteuid(2) function returns the effective user ID of the calling process.
The real user ID is that of the user who has invoked the program.
The effective user ID is the user ID of the process, if it has the set-user-ID mode set. Since the effective user ID gives the process additional permissions during execution, getuid(2) is used to determine the real user ID of the calling process.
The getuid(2) and geteuid(2) functions are always successful, and no return value is reserved to indicate an error.
getgid(2)