setuid

SETUID(2)		  Linux Programmer’s Manual		    SETUID(2)



NAME
       setuid - set user identity

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       int setuid(uid_t uid);

DESCRIPTION
       setuid  sets  the  effective  user  ID of the current process.  If the
       effective userid of the caller is root, the real and saved  user	 ID’s
       are also set.

       Under  Linux,  setuid  is  implemented like the POSIX version with the
       _POSIX_SAVED_IDS feature.  This allows a setuid (other than root) pro-
       gram  to	 drop all of its user privileges, do some un-privileged work,
       and then re-engage the original effective user ID in a secure  manner.

       If  the	user is root or the program is setuid root, special care must
       be taken. The setuid function checks the effective uid of  the  caller
       and  if	it is the superuser, all process related user ID’s are set to
       uid.  After this has occurred, it is impossible	for  the  program  to
       regain root privileges.

       Thus,  a	 setuid-root  program wishing to temporarily drop root privi-
       leges, assume the identity of a non-root user, and  then	 regain	 root
       privileges afterwards cannot use setuid.	 You can accomplish this with
       the (non-POSIX, BSD) call seteuid.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno  is
       set appropriately.

ERRORS
       EPERM  The user is not the super-user, and uid does not match the real
	      or saved user ID of the calling process.

       EAGAIN The uid does not match the current uid and uid  brings  process
	      over it’s NPROC rlimit.

CONFORMING TO
       SVr4, SVID, POSIX.1.  Not quite compatible with the 4.4BSD call, which
       sets all of the real, saved, and effective user IDs.   SVr4  documents
       an additional EINVAL error condition.

LINUX-SPECIFIC REMARKS
       Linux  has  the	concept	 of filesystem user ID, normally equal to the
       effective user ID.  The setuid call also sets the filesystem  user  ID
       of the current process.	See setfsuid(2).

       If  uid	is  different from the old effective uid, the process will be
       forbidden from leaving core dumps.

SEE ALSO
       getuid(2), setreuid(2), seteuid(2), setfsuid(2)



Linux 1.1.36			  1994-07-29			    SETUID(2)