Advanced Programming in the UNIX® Environment

ptg10805159

258 Process Control Chapter 8

seteuidandsetegidFunctions

POSIX.1 includes the two functionsseteuid and setegid.These functions are

similar tosetuidandsetgid,but only the effective user ID or effective group ID is

changed.

#include <unistd.h>

int seteuid(uid_tuid);

int setegid(gid_tgid);

Both return: 0 if OK,−1 on error

An unprivileged user can set its effective user ID to either its real user ID or its saved

set-user-ID. Foraprivileged user,only the effective user ID is set touid.(This behavior

differs from that of thesetuidfunction, which changes all three user IDs.)

Figure8.19 summarizes all the functions that we’ve described herethat modify the

three user IDs.

real

user ID

effective

user ID

saved

set-user-ID

superuser

setreuid(ruid,euid)

superuser

setuid(uid)

superuser

seteuid(uid)

euid

euid uid uid uid

ruid

uid

unprivileged

setreuid

unprivileged

setreuid

unprivileged

setuidorseteuid

unprivileged

setuidorseteuid

execof

set-user-ID

Figure 8.19Summary of all the functions that set the various user IDs

Group IDs

Everything that we’ve said so far in this section also applies in a similar fashion to

group IDs. The supplementary group IDs arenot affected bysetgid,setregid,or

setegid.