setuid(S)

setuid, setgid -- set user and group IDs

Synopsis

   #include <sys/types.h>
   #include <unistd.h>
   
   int setuid(uid_t uid);
   

   int setgid(gid_t gid);

Description

The setuid system call sets the real user ID, effective user ID, and saved user ID of the calling process. The setgid system call sets the real group ID, effective group ID, and saved group ID of the calling process; any supplementary group IDs remain unchanged.

At login time, the real user ID, effective user ID, and saved user ID of the login process are set to the login ID of the user responsible for the creation of the process. The same is true for the real, effective, and saved group IDs; they are set to the group ID of the user responsible for the creation of the process.

When a process calls exec(S) to execute a file (program), the user and/or group identifiers associated with the process can change:

The real user and group IDs are always set to the real user and group IDs of the process calling exec.
The saved user and group IDs of the new process are always set to the effective user and group IDs of the process calling exec.
If the file executed is not a set-user-ID or set-group-ID file, the effective user and group IDs of the new process are set to the effective user and group IDs of the process calling exec.
If the file executed is a set-user-ID file, the effective user ID of the new process is set to the owner ID of the executed file.
If the file executed is a set-group-ID file, the effective group ID of the new process is set to the group ID of the executed file.

The following subsections describe the behavior of setuid and setgid with respect to the three types of user and group IDs.

setuid

If the calling process has the P_SETUID privilege, the real, effective, and saved user IDs are set to the uid parameter.

If the calling process does not have the P_SETUID privilege, but uid is either the real user ID or the saved user ID of the calling process, the effective user ID is set to uid.

setgid

If the calling process has the P_SETUID privilege, the real, effective, and saved group IDs are set to the gid parameter.

If the calling process does not have the P_SETUID privilege, but gid is either the real group ID or the saved group ID of the calling process, the effective group ID is set to gid.

Return values

On success, setuid and setgid return 0. On failure, setuid and setgid return -1 and set errno to identify the error.

Errors

In the following conditions, setuid and setgid fail and set errno to:

EPERM: For setuid, the calling process does not have the P_SETUID privilege and the uid parameter does not match either the real or saved user IDs. For setgid, P_SETUID privilege and the gid parameter does not match either the real or saved group IDs.
EINVAL: The uid or gid is out of range.

References

exec(S), getgroups(S), getuid(S), intro(S), stat(M)

Notices

Considerations for threads programming

This ID number is an attribute of the containing process and is shared by sibling threads.