setuid(2) setuid(2)
NAME
setuid, seteuid, setgid, setegid - set user and group IDs
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
int setuid(uidt uid);
int seteuid(uidt uid);
int setgid(gidt gid);
int setegid(gidt gid);
DESCRIPTION
The setuid system call sets the real user ID, effective user ID, and
saved user ID of the calling process. The seteuid system call sets
the effective user ID only. The setgid system call sets the real
group ID, effective group ID, and saved group ID of the calling
process. The setegid system call sets the effective group ID only.
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.
setuid
If the effective user ID of the process calling setuid is the
superuser, 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 superuser,
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.
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setuid(2) setuid(2)
seteuid
If the effective user ID of the process calling seteuid is the
superuser, the effective user ID is set to the uid parameter.
If the effective user ID of the calling process is not the superuser,
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 effective user ID of the process calling setgid is the
superuser, 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 superuser,
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.
setegid
If the effective user ID of the process calling setegid is the
superuser, the real group ID is set to the gid parameter.
If the effective user ID of the calling process is not the superuser,
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.
The seteuid and setegid system calls are useful for setting the
effective ID when the effective user ID is the superuser. (setuid and
setgid will also set the real and saved IDs in this case).
setuid, setgid, seteuid and setegid will fail if one or more of the
following is true:
EPERM For setuid, if the effective user ID is not the superuser,
and the uid parameter does not match either the real or
saved user IDs. For setgid, if the effective user ID is
not the superuser, and the gid parameter does not match
either the real or saved group IDs.
EINVAL The uid or gid is out of range.
DIAGNOSTICS
Upon successful completion, a value of 0 is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
SEE ALSO
intro(2), exec(2), getgroups(2), getuid(2), stat(5).
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