NAME¶

setuid - set user identity

SYNOPSIS¶

#include <sys/types.h>
#include <unistd.h>

int setuid(uid_t uid);

DESCRIPTION¶

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 feature. This allows a set-user-ID (other than root) program to drop all of its user privileges, do some un-privileged work, and then re-engage 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 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 a non-root user, and then regain root privileges afterwards cannot use setuid(). You can accomplish this with the (non-POSIX, BSD) call seteuid(2).

RETURN VALUE¶

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS¶

EAGAIN

The uid does not match the current uid and uid brings process over its RLIMIT_NPROC resource limit.

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.

CONFORMING TO¶

SVr4, POSIX.1-2001. Not quite compatible with the 4.4BSD call, which sets all of the real, saved, and effective user IDs.

NOTES¶

SEE ALSO¶

getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)

COLOPHON¶

This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.