setuid(2)

NAME

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() 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. 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 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.

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

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 parameter.

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 parameter.

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.

RETURN VALUES

Upon successful completion, a value of 0 is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

setuid() and setgid() fail if one or more of the following is true:

EINVAL The uid or gid is out of range.

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

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