Editing Group identifier (section)

== Conventions ==

=== Type ===

Originally, a signed 16-bit integer was used. Since the sign was not necessary – negative numbers do not make valid group IDs – an unsigned integer is now used instead, allowing group IDs between 0 and 65,535. Modern operating systems usually use unsigned 32-bit integers, which allow for group IDs between 0 and 4,294,967,295.

=== Reserved ranges ===

Many Linux systems reserve the GID number range 0 to 99 for statically allocated groups, and either 100−499 or 100−999 for groups dynamically allocated by the system in post-installation scripts. These ranges are often specified in <code>/etc/login.defs</code>, for <code>useradd</code>, <code>groupadd</code> and similar tools.

On FreeBSD, porters who need a GID for their package can pick a free one from the range 50 to 999 and then register this static allocation in [http://svnweb.freebsd.org/ports/head/GIDs?view=co ports/GIDs].<ref>[http://www.freebsd.org/doc/en/books/porters-handbook/ FreeBSD Porter's Handbook], [http://www.freebsd.org/doc/en/books/porters-handbook/users-and-groups.html Section 6.26: Adding Users and Groups]</ref>

=== Special values ===

* 0: The [[superuser]] normally has a GID of zero (0).
* −1: The value <code>(gid_t) -1</code> is reserved by POSIX to identify an omitted argument.<ref>{{Cite web|title=chown|url=https://pubs.opengroup.org/onlinepubs/009695399/functions/chown.html|access-date=2021-12-06|website=pubs.opengroup.org}}</ref>
* 65,534: The Linux kernel defaults to 2<sup>16</sup>−2 = 65,534 (which many Linux distributions map to the group name "nogroup") when a 32-bit GID does not fit into the return value of a 16-bit system call.<ref>{{Cite web|title=Namespaces in operation, part 5: User namespaces [LWN.net]|url=https://lwn.net/Articles/532593/|access-date=2021-12-06|website=lwn.net}}</ref> The value is also returned by [[idmapd]] if a group name in an incoming [[NFSv4]] packet does not match any known group on the system.