04-30-2009
"one of my lab assignments is to explain the difference between Unix and Linux"
That's not a very fair question for an intro level class.
The really short answer is "Many millions of lines of code".
The not so short answer would be that Unix is an OS, copyright owned by Bell Labs while Linux is a kernel, and depends on a crapload of utilities from the GNU project to put the whole OS together. Linux tries to be POSIX compliant, I'm not sure Unix is entirely POSIX compliant either.
The longer answers would explain the differences in the kernel architectures, schedulers, process tables, device handling, etc. Feelings about how those would work in the "perfect system" are the types of feelings that start flamewars.
Good luck with your class!
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LEARN ABOUT X11R4
setgid32
SETGID(2) Linux Programmer's Manual SETGID(2)
NAME
setgid - set group identity
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
int setgid(gid_t gid);
DESCRIPTION
setgid() sets the effective group ID of the calling process. If the calling process is privileged (has the CAP_SETGID capability in its
user namespace), the real GID and saved set-group-ID are also set.
Under Linux, setgid() is implemented like the POSIX version with the _POSIX_SAVED_IDS feature. 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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EINVAL The group ID specified in gid is not valid in this user namespace.
EPERM The calling process is not privileged (does not have the CAP_SETGID capability), and gid does not match the real group ID or saved
set-group-ID of the calling process.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
NOTES
The original Linux setgid() system call supported only 16-bit group IDs. Subsequently, Linux 2.4 added setgid32() supporting 32-bit IDs.
The glibc setgid() 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 setgid()) 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
nptl(7).
SEE ALSO
getgid(2), setegid(2), setregid(2), capabilities(7), credentials(7), user_namespaces(7)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the
latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SETGID(2)