man/man2/setreuid.2

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 SETREUID 2 "Mar 22, 2004"
 NAME
setreuid - set real and effective user IDs
 SYNOPSIS

#include <unistd.h>

int setreuid(uid_t ruid, uid_t euid);


 DESCRIPTION

The setreuid() function is used to set the real and effective user IDs of
the calling process. If ruid is -1, the real user ID is not changed;
if euid is -1, the effective user ID is not changed. The real and
effective user IDs may be set to different values in the same call.

If the {PRIV_PROC_SETID} privilege is asserted in the effective set of
the calling process, the real user ID and the effective user ID can be
set to any legal value.

If the {PRIV_PROC_SETID} privilege is not asserted in the effective set
of the calling process, either the real user ID can be set to the effective
user ID, or the effective user ID can either be set to the saved set-user ID
from execve() (seeexec(2)) or the real user ID.

In either case, if the real user ID is being changed (that is, if ruid is
not -1), or the effective user ID is being changed to a value not
equal to the real user ID, the saved set-user ID is set equal to the new
effective user ID.

All privileges are required to change to uid 0.
 RETURN VALUES

Upon successful completion, 0 is returned. Otherwise, -1 is returned,
errno is set to indicate the error, and neither of the user IDs will be
changed.
 ERRORS

The setreuid() function will fail if:
EINVAL

The value of ruid or euid is less than 0 or greater than
UID_MAX (defined in <limits.h>).


EPERM

The {PRIV_PROC_SETID} privilege is not asserted in the effective set of the
calling processes and a change was specified other than changing the real user
ID to the effective user ID, or changing the effective user
ID to the real user ID or the saved set-user ID. See
privileges(5) for additional restrictions which apply when changing to
UID 0.


 USAGE

If a set-user-ID process sets its effective user ID to its real
user ID, it can still set its effective user ID back to the saved
set-user ID.
 ATTRIBUTES

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


ATTRIBUTE TYPE ATTRIBUTE VALUE
Interface Stability Standard


 SEE ALSO

exec(2), getuid(2), setregid(2), setuid(2),
attributes(5), privileges(5), standards(5)