setuid man page on ElementaryOS

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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 reengage 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 an unprivileged user, and then regain
       root privileges afterward cannot use setuid().  You can accomplish this
       with 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 has the concept of the filesystem user ID, normally equal to  the
       effective  user ID.  The setuid() call also sets the filesystem 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.

       The original Linux setuid() system call supported only 16-bit user IDs.
       Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
       glibc  setuid() wrapper function transparently deals with the variation
       across kernel versions.

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

       This  page  is  part of release 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2010-11-22			     SETUID(2)

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