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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 filesystem user ID, normally equal to the effective user ID.
       The setuid() call  also  sets  the  filesystem  user  ID  of  the  calling  process.   See

       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), credentials(7)


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