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       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
       program to regain root privileges.

       Thus, a set-user-ID-root  program  wishing  to  temporarily  drop  root
       privileges,  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 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.

       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),


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