full-disclosure - Re: readdir_r considered harmful

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Message-ID: <a36005b50511051753h47c56d33s662ed38e1a3c65fa@mail.gmail.com>
Date: Sun Nov  6 01:53:54 2005
From: drepper at gmail.com (Ulrich Drepper)
Subject: Re: readdir_r considered harmful

On 11/5/05, Casper.Dik@....com <Casper.Dik@....com> wrote:
> Why not:
>
> 4. Require the readdir() implementation to use state local to dirp.
>
> I've never understood the rationale behind readdir_r;

Then you never really understood the implementation, seems.  Of course
all implementations keep the content of the directory as read with
getdents or so in the DIR descriptor.  But it is usually not the case
that the whole content fits into the buffer allocated.  One could, of
course, resize the buffer to fit the content of the directory read,
even if this means reserving hundreds or thousands of kBs.  But this
is not how most implementations work.

Instead implementations keep work similar to every buffered file I/O
operation.  But this means that buffer content is replaced.  If this
happens and some thread uses readdir() instead of readdir_r(), the
returned string pointer suddenly becomes invalid since it points to
memory which has been replaced.

Next time, before you make such comments, ask Don Cragun to explain
things to you.