GETUID(2) Linux Programmer's Manual GETUID(2)
getuid, geteuid - get user identity
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.
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
In UNIX V6 the getuid() call returned (euid << 8) + uid. UNIX V7 in-
troduced 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 ge-
teuid32(), supporting 32-bit IDs. The glibc getuid() and geteuid()
wrapper functions transparently deal with the variations across kernel
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
getresuid(2), setreuid(2), setuid(2), credentials(7)
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Linux 2019-03-06 GETUID(2)