The setuid() function sets the real user ID, effective user ID, and saved user ID of the calling process. The setgid() function sets the real group ID, effective group ID, and saved group ID of the calling process. The setegid() and seteuid() functions set the effective group and user IDs respectively for the calling process. See intro(2) for more information on real, effective, and saved user and group IDs.

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 one of the exec family of functions (see exec(2)) to execute a file (program), the user and/or group identifiers associated with the process can change. If the file executed is a set-user-ID file, the effective and saved user IDs of the process are set to the owner of the file executed. If the file executed is a set-group-ID file, the effective and saved group IDs of the process are set to the group of the file executed. If the file executed is not a set-user-ID or set-group-ID file, the effective user ID, saved user ID, effective group ID, and saved group ID are not changed.

If the effective user ID of the process calling setuid() is the super-user, the real, effective, and saved user IDs are set to the uid argument.

If the effective user ID of the calling process is not the super-user, 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.

If the effective user ID of the process calling setgid() is the super-user, the real, effective, and saved group IDs are set to the gid argument.

If the effective user ID of the calling process is not the super-user, 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.

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The setuid() and setgid() functions will fail if:

EINVAL

The value of uid or gid is out of range.

EPERM

For setuid() and seteuid() the effective user of the calling process is not super-user, and the uid argument does not match either the real or saved user IDs. For setgid() and setegid() the effective user of the calling process is not the super-user, and the gid argument does not match either the real or saved group IDs.

See attributes(5) for descriptions of the following attributes:

intro(2), exec(2), getgroups(2), getuid(2), stat(3HEAD), attributes(5)