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

       setuid - set user identity

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

       int setuid(uid_t uid);

       setuid()	 sets  the  effective  user ID of the calling process.	If the
       effective UID of the caller is root, the real UID and saved set-user-ID
       are also set.

       Under  Linux,  setuid()	is implemented like the POSIX version with the
       _POSIX_SAVED_IDS feature.  This allows a set-user-ID (other than	 root)
       program 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  set-user-ID-root,	 special  care
       must  be	 taken.	 The setuid() function checks the effective user ID 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 pro‐
       gram to regain root privileges.

       Thus, a set-user-ID-root program wishing to temporarily drop root priv‐
       ileges,	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(2).

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

       EAGAIN The uid does not match the current uid and  uid  brings  process
	      over its RLIMIT_NPROC resource limit.

       EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
	      capability) and uid does not match the real UID  or  saved  set-
	      user-ID of the calling process.

       SVr4,  POSIX.1-2001.   Not quite compatible with the 4.4BSD call, which
       sets all of the real, saved, and effective user IDs.

   Linux Notes
       Linux has the concept of file system user ID,  normally	equal  to  the
       effective user ID.  The setuid() call also sets the file system user ID
       of the calling process.	See setfsuid(2).

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

       getuid(2),  seteuid(2), setfsuid(2), setreuid(2), capabilities(7), cre‐

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2004-05-27			     SETUID(2)

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