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setuid, seteuid, setgid, setegid — set user and group ID
Standard C Library (libc, -lc)
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-
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
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.
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 is set:
/* ... */
fd = open("/path/to/sensitive/data", O_RDWR);
if (fd == -1)
* Set close-on-exec flag; see fcntl(2) for more information.
if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1)
/* ... */
execve(path, argv, environ);
getgid(2), getuid(2), issetugid(2), setregid(2), setreuid(2)
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
The setuid() and setgid() functions appeared in Version 7 AT&T UNIX.