06-03-2005
Set GID Bit on Directories
We briefly mentioned that files have a user and group associated with them. Originally, it was just the user and group of whoever created them. But originally, a user could be in only one group at a time. BSD introduced the concept that a user could be in multiple groups simutaneously. So in BSD, which group was used? BSD decided to use the group of the directory that contained the newly created file.
Many modern versions of unix try to have it both ways. A newly created file gets the group of the user unless the directory has the setgid bit. In that case, the newly created file gets the group of the directory.
And there is an exception to that! Changing the owner or group of a file has security concerns. For that reason, some versions of unix will, optionally, prohibit a user other than root from changing the owner of a file. Additionally, a user is prohibited from changing the group of a file unless he is a member of the new group. This restriction will override the setgid bit on a directory if needed.
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LEARN ABOUT NETBSD
setregid
SETREGID(2) BSD System Calls Manual SETREGID(2)
NAME
setregid -- set real and effective group ID's
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
setregid(gid_t rgid, gid_t egid);
DESCRIPTION
This interface is made obsolete by the saved ID functionality in setgid(2) and setegid(2).
The real and effective group ID's of the current process are set according to the arguments. If the real group ID is changed, the saved
group ID is changed to the new value of the effective group ID.
If rgid or egid is -1, the current gid is filled in by the system. Unprivileged users may change the real group ID to the effective group
ID, and may change the effective group ID to the real group ID or the saved group ID; only the super-user may make other changes.
The setregid() function has been used to swap the real and effective group IDs in set-group-ID programs to temporarily relinquish the set-
group-ID value. This purpose is now better served by the use of the setegid() function (see setgid(2)).
When setting the real and effective group IDs to the same value, this function is equivalent to the setgid() function. When setting only the
effective group ID, this function is equivalent to the setegid() function.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.
ERRORS
[EPERM] The current process is not the super-user and a change other than changing the effective group-id to the real group-id was
specified.
SEE ALSO
getgid(2), setegid(2), setgid(2), setuid(2)
HISTORY
The setregid() function call appeared in 4.2BSD. An incompatible version was implemented in 4.4BSD. It was reimplemented in NetBSD 1.2 in a
way compatible with 4.3BSD, SunOS and Linux, but should not be used in new code.
BSD
January 5, 2001 BSD