| .\" Copyright (C), 1994, Graeme W. Wilford. (Wilf.) |
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| .\" Fri Jul 29th 12:56:44 BST 1994 Wilf. <G.Wilford@ee.surrey.ac.uk> |
| .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com> |
| .\" Modified 2002-03-09 by aeb |
| .\" |
| .TH SETGID 2 2019-03-06 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| setgid \- set group identity |
| .SH SYNOPSIS |
| .B #include <sys/types.h> |
| .br |
| .B #include <unistd.h> |
| .PP |
| .BI "int setgid(gid_t " gid ); |
| .SH DESCRIPTION |
| .BR setgid () |
| sets the effective group ID of the calling process. |
| If the calling process is privileged (more precisely: has the |
| .B CAP_SETGID |
| capability in its user namespace), |
| the real GID and saved set-group-ID are also set. |
| .PP |
| Under Linux, |
| .BR setgid () |
| is implemented like the POSIX version with the |
| .B _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. |
| .SH RETURN VALUE |
| On success, zero is returned. |
| On error, \-1 is returned, and |
| .I errno |
| is set appropriately. |
| .SH ERRORS |
| .TP |
| .B EINVAL |
| The group ID specified in |
| .I gid |
| is not valid in this user namespace. |
| .TP |
| .B EPERM |
| The calling process is not privileged (does not have the |
| \fBCAP_SETGID\fP capability in its user namespace), and |
| .I gid |
| does not match the real group ID or saved set-group-ID of |
| the calling process. |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008, SVr4. |
| .SH NOTES |
| The original Linux |
| .BR setgid () |
| system call supported only 16-bit group IDs. |
| Subsequently, Linux 2.4 added |
| .BR setgid32 () |
| supporting 32-bit IDs. |
| The glibc |
| .BR setgid () |
| wrapper function transparently deals with the variation across kernel versions. |
| .\" |
| .SS 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 |
| .BR 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 |
| .BR nptl (7). |
| .SH SEE ALSO |
| .BR getgid (2), |
| .BR setegid (2), |
| .BR setregid (2), |
| .BR capabilities (7), |
| .BR credentials (7), |
| .BR user_namespaces (7) |
