returns the real user ID of the calling process.
returns the effective user ID of the calling 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 being executed.
These functions are always successful.
In Unix V6 the
(euid << 8) + uid.
Unix V7 introduced separate calls