Real, Effective, and Saved User ID
The preceding functions dealt with real and effective user IDs and group IDs. There is a third level of identification known as the saved user ID and group ID. Three levels of user ID and group ID can be very confusing. The explanations provided for user ID in the following sections apply equally to group ID.
The Effective User ID
Although the setuid(2) call has not been covered yet, recall that it can change the effective user ID for a process. The effective user ID determines what level of access the current process has. When the effective user ID is zero (root), then the process has unrestricted access, for example.
The Real User ID
The real user ID is what it sounds like. It identifies who you really are. ...
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