Why is max groups set so low (16)?

Bruce Evans bde at zeta.org.au
Tue Nov 25 18:37:21 PST 2003

On Tue, 25 Nov 2003, David O'Brien wrote:

> After spending 3 hours debugging Samba problems I found the cause of the
> problems is that I am >16 groups and our NGROUPS_MAX is set to 16.  My
> question for this group, is why is NGROUPS_MAX set so low?  Is it because
> NFS_MAXGRPS is 16 as specified in some NFS standard?

It is a compile-time constant, so it can't be changed from its historical
value without breaking binary compatibility.  POSIX specifies only that it
be no smaller than _POSIX_NGROUP_MAX (8) if it exists.

The binary compatibility problems seem to be small.  libc doesn't have any
references at all to NGROUPS_MAX except in man pages, but that is partly
because it mostly misspells NGROUPS_MAX as NGROUPS.  getgroups(2) and
setgroups(2) are limited by whatever the kernel wants, not by their API,
although their documentation says that there is a compile-time limit
(getgroups.2 spells NGROUPS_MAX correctly, but setgroups.2 spells it


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