getuid, geteuid — get user identity


#include <unistd.h>
#include <sys/types.h>

uid_t getuid(void);
uid_t geteuid(void);


getuid() returns the real user ID of the calling process.

geteuid() returns the effective user ID of the calling process.


These functions are always successful.

Conforming to

POSIX.1-2001, POSIX.1-2008, 4.3BSD.



In UNIX V6 the getuid() call returned (euid << 8) + uid. UNIX V7 introduced separate calls getuid() and geteuid().

The original Linux getuid() and geteuid() system calls supported only 16-bit user IDs. Subsequently, Linux 2.4 added getuid32() and geteuid32(), supporting 32-bit IDs. The glibc getuid() and geteuid() wrapper functions transparently deal with the variations across kernel versions.

On Alpha, instead of a pair of getuid() and geteuid() system calls, a single getxuid() system call is provided, which returns a pair of real and effective UIDs. The glibc getuid() and geteuid() wrapper functions transparently deal with this. See syscall(2) for details regarding register mapping.

See Also

getresuid(2), setreuid(2), setuid(2), credentials(7)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By

credentials(7), gawk(1), getlogin(3), getresuid(2), lockfile-progs(1), mksh(1), oping(8), pam_close_session(3), pam_open_session(3), procenv(1), ps(1), seteuid(2), setpgid(2), setresuid(2), setreuid(2), setuid(2), signal-safety(7), strace(1), syscalls(2), user_namespaces(7).

The man pages geteuid(2), geteuid32(2) and getuid32(2) are aliases of getuid(2).

2019-03-06 Linux Programmer's Manual