setgid() -- System Call (libc)

Set group id and user id
#include <unistd.h>
int setgid(id) int id;

The  group identifier  is the  number that identifies  the user  group that
``owns'' a given file.  File  /etc/group establishes the set of groups that
your COHERENT  system recognizes.   (For details on  how this file  is laid
out,  see the  Lexicon entry  for group).  When a  file is  executable, the
executing process inherits  its group identifier (and thus, its group-level
permissions) from the  file in which it resides on  disk.  For example, the
program troff  resides in file /bin/troff. This file  is ``owned'' by group
bin; thus, when you execute troff, its group-level permissions are those of
group bin.

The group identifier comes in three forms:

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

effective
     This is the group identifier  that determines the access rights of the
     process.   These  rights are  the  same  as those  of  the real  group
     identifier unless  they have  been altered  by executing a  file whose
     setgid bit is  set.  For example, the program troff  does not have the
     setgid bit set; thus, when you execute troff, the group permissions of
     the troff process  remain those of your group, not  those of the group
     bin. On the other hand, the program /usr/lib/uucp/uucico does have the
     setgid bit set; thus, when  you invoke uucico, the uucico process uses
     the permissions of uucico's group (that is, of group uucp), instead of
     your group.

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

The  system  call  setgid() lets  you  set  the  real  and effective  group
identifiers  of  the  calling process  to  the  group  identifier gid.  The
behavior of setgid() varies depending upon the following:

1. If setgid() is invoked by a user whose effective user identifier is that
   of  the superuser  root, setgid()  sets the  real, effective,  and saved
   effective group identifiers to gid.

2. If setgid() is invoked by a user whose real group identifier is the same
   as gid, setgid() sets the effective group identifier to gid.

3. If setgid() is invoked by  a user whose saved effective group identifier
   is same as gid, setgid() sets the effective group identifier to gid.

If all goes  well, setgid() returns zero.  If a  problem arises, it returns
-1.

See Also

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

© 2012 by Stephen A. Ness