SETUID(2) Linux Programmer's Manual SETUID(2)
setuid - set user identity
int setuid(uid_t uid);
setuid() 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.
Under Linux, setuid() is implemented like the POSIX version with the _POSIX_SAVED_IDS fea-
ture. This allows a set-user-ID (other than root) program to drop all of its user privi-
leges, do some un-privileged work, and then reengage the original effective user ID in a
If the user is root or the program is set-user-ID-root, special care must be taken. The
setuid() function checks the effective user ID of the caller and if it is the superuser,
all process-related user ID's are set to uid. After this has occurred, it is impossible
for the program to regain root privileges.
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
setuid(). You can accomplish this with seteuid(2).
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EAGAIN The uid does not match the current uid and uid brings process over its RLIMIT_NPROC
EPERM The user is not privileged (Linux: does not have the CAP_SETUID capability) and uid
does not match the real UID or saved set-user-ID of the calling process.
SVr4, POSIX.1-2001. 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 file system user ID, normally equal to the effective user ID.
The setuid() call also sets the file system user ID of the calling process. See setf-
If uid is different from the old effective UID, the process will be forbidden from leaving
The original Linux setuid() system call supported only 16-bit user IDs. Subsequently,
Linux 2.4 added setuid32() supporting 32-bit IDs. The glibc setuid() wrapper function
transparently deals with the variation across kernel versions.
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)
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Linux 2010-11-22 SETUID(2)