Section: Linux Programmer's Manual (2)
setgid - set group identity
int setgid(gid_t gid);
sets the effective group ID of the calling process.
If the calling process is privileged (more precisely: has the
capability in its user namespace),
the real GID and saved set-group-ID are also set.
is implemented like the POSIX version with the
This allows a set-group-ID program that is not set-user-ID-root
to drop all of its group
privileges, do some un-privileged work, and then reengage the original
effective group ID in a secure manner.
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
The group ID specified in
is not valid in this user namespace.
The calling process is not privileged (does not have the
CAP_SETGID capability in its user namespace), and
does not match the real group ID or saved set-group-ID of
the calling process.
POSIX.1-2001, POSIX.1-2008, SVr4.
The original Linux
system call supported only 16-bit group IDs.
Subsequently, Linux 2.4 added
supporting 32-bit IDs.
wrapper function transparently deals with the variation across kernel versions.
C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process
share the same credentials.
The NPTL threading implementation handles the POSIX requirements by
providing wrapper functions for
the various system calls that change process UIDs and GIDs.
These wrapper functions (including the one for
employ a signal-based technique to ensure
that when one thread changes credentials,
all of the other threads in the process also change their credentials.
For details, see
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