getpwent_r(3) needs gr->gr_mem to be freed, but that's not documented

[Alejandro Colomar]

Hi Mark,

On Sun, Sep 28, 2025 at 02:54:59PM -0700, Mark Harris wrote:
> Alejandro Colomar wrote:
> >
> > On Sun, Sep 28, 2025 at 10:26:33PM +0200, Alejandro Colomar wrote:
> > > Hi!
> > >
> > > Another thing I realized about getpwent_r(3) et al. is that gr->gr_mem
> >
> > Actually, getgrent_r(3).
> >
> > > is allocated by the function, but is expected to be freed by the caller.
> > >
> > > I didn't find this documented, neither in the manual page nor in the
> > > glibc manual.  Is it documented but I missed it?  Or did I misunderstand
> > > something?  Or is it just that nobody documented it?
> 
> All memory referenced by the structure, including gr_mem, comes out of
> the buffer supplied to the function.  POSIX doesn't have getgrent_r,
> but it has getgrgid_r, and says:

Hmmm, that's good.  Thanks!  At least, the user doesn't need to free(3)
anything weird.

So, a good estimate of the size to be allocated prior to the
sgetgrent_r() call should be:

	size = strlen(s) + 1 + strchrcnt(s, ',') + 2;

That would be wasting a little bit if there are any commas outside of
the fourth ':'-delimited field, but it should work.

BTW, where's this exactly in the glibc source code?  It's a bit hard to
follow.


Have a lovely day!
Alex

>     Storage referenced by the group structure is allocated from the
> memory provided with the buffer parameter, which is bufsize bytes in
> size.

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
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