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(libc.info)Setting User ID

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Setting the User ID
===================

   This section describes the functions for altering the user ID (real
and/or effective) of a process.  To use these facilities, you must
include the header files `sys/types.h' and `unistd.h'.

 - Function: int seteuid (uid_t NEWEUID)
     This function sets the effective user ID of a process to NEWUID,
     provided that the process is allowed to change its effective user
     ID.  A privileged process (effective user ID zero) can change its
     effective user ID to any legal value.  An unprivileged process
     with a file user ID can change its effective user ID to its real
     user ID or to its file user ID.  Otherwise, a process may not
     change its effective user ID at all.

     The `seteuid' function returns a value of `0' to indicate
     successful completion, and a value of `-1' to indicate an error.
     The following `errno' error conditions are defined for this
     function:

    `EINVAL'
          The value of the NEWUID argument is invalid.

    `EPERM'
          The process may not change to the specified ID.

     Older systems (those without the `_POSIX_SAVED_IDS' feature) do not
     have this function.

 - Function: int setuid (uid_t NEWUID)
     If the calling process is privileged, this function sets both the
     real and effective user ID of the process to NEWUID.  It also
     deletes the file user ID of the process, if any.  NEWUID may be any
     legal value.  (Once this has been done, there is no way to recover
     the old effective user ID.)

     If the process is not privileged, and the system supports the
     `_POSIX_SAVED_IDS' feature, then this function behaves like
     `seteuid'.

     The return values and error conditions are the same as for
     `seteuid'.

 - Function: int setreuid (uid_t RUID, uid_t EUID)
     This function sets the real user ID of the process to RUID and the
     effective user ID to EUID.  If RUID is `-1', it means not to
     change the real user ID; likewise if EUID is `-1', it means not to
     change the effective user ID.

     The `setreuid' function exists for compatibility with 4.3 BSD Unix,
     which does not support file IDs.  You can use this function to
     swap the effective and real user IDs of the process.  (Privileged
     processes are not limited to this particular usage.)  If file IDs
     are supported, you should use that feature instead of this
     function.  Note: Enable/Disable Setuid.

     The return value is `0' on success and `-1' on failure.  The
     following `errno' error conditions are defined for this function:

    `EPERM'
          The process does not have the appropriate privileges; you do
          not have permission to change to the specified ID.