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Linux Programmer's Manual (2)
setuid - set user identity
int setuid(uid_t uid);
sets the effective user ID of the calling process.
If the effective UID of the caller is root,
the real UID and saved set-user-ID are also set.
is implemented like the POSIX version with the
This allows a set-user-ID (other than root) program to drop all of its user
privileges, do some un-privileged work, and then reengage the original
effective user ID in a secure manner.
If the user is root or the program is set-user-ID-root, special care must be
function checks the effective user ID of the caller and if it is
the superuser, all process-related user ID's are set to
After this has occurred, it is impossible for the program to regain root
Thus, a set-user-ID-root program wishing to temporarily drop root
privileges, assume the identity of an unprivileged user, and then regain
root privileges afterward cannot use
You can accomplish this with
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
does not match the current uid and
brings process over its
The user is not privileged (Linux: does not have the
does not match the real UID or saved set-user-ID of the calling process.
Not quite compatible with the 4.4BSD call, which
sets all of the real, saved, and effective user IDs.
Linux has the concept of the filesystem user ID, normally equal to the
effective user ID.
call also sets the filesystem user ID of the calling process.
is different from the old effective UID, the process will
be forbidden from leaving core dumps.
The original Linux
system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added
supporting 32-bit IDs.
wrapper function transparently deals with the variation across kernel versions.
This page is part of release 3.66 of the Linux
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Time: 21:42:58 GMT, July 12, 2014