setuid(2)                                                            setuid(2)


NAME
     setuid, setgid - set user and group IDs

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

     int setuid(uid_t uid);

     int setgid(gid_t gid);

DESCRIPTION
     The setuid system call sets the real user ID, effective user ID, and
     saved user ID of the calling process.  The setgid system call sets the
     real group ID, effective group ID, and saved group ID of the calling
     process.

     At login time, the real user ID, effective user ID, and saved user ID of
     the login process are set to the login ID of the user responsible for the
     creation of the process.  The same is true for the real, effective, and
     saved group IDs; they are set to the group ID of the user responsible for
     the creation of the process.

     When a process calls exec(2) to execute a file (program), the user and/or
     group identifiers associated with the process can change:

          The real user and group IDs are always set to the real user and
          group IDs of the process calling exec.

          The saved user and group IDs of the new process are always set to
          the effective user and group IDs of the process calling exec.

          If the file executed is not a set-user-ID or set-group-ID file, the
          effective user and group IDs of the new process are set to the
          effective user and group IDs of the process calling exec.

          If the file executed is a set-user-ID file, the effective user ID of
          the new process is set to the owner ID of the executed file.

          If the file executed is a set-group-ID file, the effective group ID
          of the new process is set to the group ID of the executed file.

     The following subsections describe the behavior of setuid and setgid with
     respect to the three types of user and group IDs.

   setuid
     If the calling process has the super-user privilege, the real, effective,
     and saved user IDs are set to the uid parameter.

     If the calling process does not have the super-user privilege, but uid is
     either the real user ID or the saved user ID of the calling process, the
     effective user ID is set to uid.


   setgid
     If the calling process has the super-user privilege, the real, effective,
     and saved group IDs are set to the gid parameter.

     If the calling process does not have the super-user privilege, but gid is
     either the real group ID or the saved group ID of the calling process,
     the effective group ID is set to gid.

     setuid and setgid fail if one or more of the following is true:

     EPERM     For setuid, the calling process does not have the super-user
               privilege and the uid parameter does not match either the real
               or saved user IDs.  For setgid, the calling process does not
               have the super-user privilege and the gid parameter does not
               match either the real or saved group IDs.

     EINVAL    The uid or gid is out of range.

DIAGNOSTICS
     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of -1 is returned and errno is set to indicate the error.

SEE ALSO
     intro(2), exec(2), getgroups(2), getuid(2), stat(5).


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