setuid() -- System Call (libc)

Set user identifier
#include <unistd.h>
int setuid(id)
int id;

The user identifier  is the number that identifies the  user who ``owns'' a
given file.  The suite of users is defined in file /etc/passwd. When a file
is  executable, the  executing process  inherits  its user  identifier (and
thus, its  user-level permissions)  from the file  where it lives  on disk.
The user identifier comes in three forms:

real This is the identifier of the user who is running the process.

effective
     This is  the user identifier that determines the  access rights of the
     process.   These  rights  are the  same  as  those  of  the real  user
     identifier unless  they have  been altered  by executing a  file whose
     setuid bit is set.

saved effective
     This permits  a process to  step back and  forth between its  real and
     effective  user identifiers.   If you  return  from an  effective user
     identifier to  your real one, the previously effective  id is saved so
     you can revert to it if need be.

The  system  call setuid()  lets  you  alter the  real  and effective  user
identifiers of the calling process to the user identifier uid. The behavior
of setuid() varies depending upon the following:

1. If the effective user identifier is that of the superuser, setuid() sets
   the real, effective, and saved effective user identifiers to uid.

2. If  the real  user identifier  is  the same  as uid,  setuid() sets  the
   effective user identifier to uid.

3. If the saved effective user identifier is same as uid, setuid() sets the
   effective user identifier to uid.

To setuid an existing executable file, use the command chmod.

See Also

chmod,
execution,
getuid(),
libc,
login,
setgid(),
unistd.h
POSIX Standard, §4.2.2

Diagnostics

setuid() returns zero on success, or -1 on failure.

Notes

For more information on the user id, see the Lexicon entry for execution.

© 2012 by Stephen A. Ness