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getuid, geteuid - get user identity
getuid() returns the real user ID of the current process.
geteuid() returns the effective user ID of the current process.
When a normal program is executed, the effective and real user ID of
the process are set to the ID of the user executing the file. When a
set ID program is executed the real user ID is set to the calling user
and the effective user ID corresponds to the set ID bit on the file
These functions are always successful.
In Unix V6 the getuid() call returned (euid << 8) + uid. Unix V7
introduced separate calls getuid() and geteuid().