NAME

       setuid, seteuid, setgid, setegid — set user and group ID

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

       int
       setuid(uid_t uid);

       int
       seteuid(uid_t euid);

       int
       setgid(gid_t gid);

       int
       setegid(gid_t egid);

DESCRIPTION

       The  setuid()  system  call sets the real and effective user IDs and the saved set-user-ID of the current
       process to the specified value.  The setuid() system call is permitted if the specified ID  is  equal  to
       the  real  user  ID  or  the effective user ID of the process, or if the effective user ID is that of the
       super user.

       The setgid() system call sets the real and effective group IDs and the saved set-group-ID of the  current
       process  to  the  specified value.  The setgid() system call is permitted if the specified ID is equal to
       the real group ID or the effective group ID of the process, or if the effective user ID is  that  of  the
       super user.

       The  seteuid() system call (setegid()) sets the effective user ID (group ID) of the current process.  The
       effective user ID may be set to the value of the real user ID or the saved set-user-ID (see intro(2)  and
       execve(2)); in this way, the effective user ID of a set-user-ID executable may be toggled by switching to
       the  real user ID, then re-enabled by reverting to the set-user-ID value.  Similarly, the effective group
       ID may be set to the value of the real group ID or the saved set-group-ID.

RETURN VALUES

       Upon successful completion, the value 0 is returned; otherwise the value -1 is returned  and  the  global
       variable errno is set to indicate the error.

ERRORS

       The system calls will fail if:

       [EPERM]            The  user is not the super user and the ID specified is not the real, effective ID, or
                          saved ID.

SEE ALSO

       getgid(2), getuid(2), issetugid(2), setregid(2), setreuid(2)

STANDARDS

       The  setuid()  and  setgid()  system  calls  are  compliant  with  the  ISO/IEC  9945-1:1990  (“POSIX.1”)
       specification with _POSIX_SAVED_IDS not defined with the permitted extensions from Appendix B.4.2.2.  The
       seteuid()  and  setegid() system calls are extensions based on the POSIX concept of _POSIX_SAVED_IDS, and
       have been proposed for a future revision of the standard.

HISTORY

       The setuid() function appeared in Version 1 AT&T UNIX.  The setgid() function appeared in Version 4  AT&T
       UNIX.

SECURITY CONSIDERATIONS

       Read  and  write  permissions  to files are determined upon a call to open(2).  Once a file descriptor is
       open, dropping privilege does not affect the process's  read/write  permissions,  even  if  the  user  ID
       specified  has  no  read  or  write permissions to the file.  These files normally remain open in any new
       process executed, resulting in a user being able to read or modify potentially sensitive data.

       To prevent these files from remaining open after an exec(3) call, be sure to set the close-on-exec flag:

       void
       pseudocode(void)
       {
               int fd;
               /* ... */

               fd = open("/path/to/sensitive/data", O_RDWR | O_CLOEXEC);
               if (fd == -1)
                       err(1, "open");

               /* ... */
               execve(path, argv, environ);
       }

Debian                                          December 15, 2015                                      SETUID(2)