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Full Discussion: Puzzled with user ID.
Top Forums Programming Puzzled with user ID. Post 38372 by Perderabo on Tuesday 15th of July 2003 09:32:57 AM
Old 07-15-2003
Remember that I said that I was over-simplifying things a bit.

If you are not running a suid program all 3 uids should be the same. And ideally your real uid will never change.

But all 3 can be affected by various system calls. And each version of unix is a little different in this area.

Posix has attempted to tame this situation. The sequence of events that I described is the Posix concept of how this should work. If a version of unix doesn't support the sequence that I described, it is not posix compliant.

But, for example, BSD has a system call that will swap the real and effective uids. HP-UX actually allows you to fiddle with all three uids. Older versions of BSD did not even have a saved-uid. It's this variety that makes things confusing.
 

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getuid(2)							   System Calls 							 getuid(2)

NAME
getuid, geteuid, getgid, getegid - get real user, effective user, real group, and effective group IDs SYNOPSIS
#include <sys/types.h> #include <unistd.h> uid_t getuid(void); uid_t geteuid(void); gid_t getgid(void); gid_t getegid(void); DESCRIPTION
The getuid() function returns the real user ID of the calling process. The real user ID identifies the person who is logged in. The geteuid() function returns the effective user ID of the calling process. The effective user ID gives the process various permissions during execution of "set-user-ID" mode processes which use getuid() to determine the real user ID of the process that invoked them. The getgid() function returns the real group ID of the calling process. The getegid() function returns the effective group ID of the calling process. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |Async-Signal-Safe | +-----------------------------+-----------------------------+ SEE ALSO
Intro(2), setuid(2), attributes(5), standards(5) SunOS 5.11 28 Dec 1996 getuid(2)
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