Re: [Xen-devel] [PATCH 1/2] libfsimage: replace deprecated readdir_r() with readdir()

[Chris Patterson <cjp256@xxxxxxxxx>]

On Tue, May 31, 2016 at 6:42 AM, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
> On Mon, May 30, 2016 at 3:32 AM, Chris Patterson <cjp256@xxxxxxxxx> wrote:
>> From: Chris Patterson <pattersonc@xxxxxxxxxxxx>
>>
>> Replace the usage of readdir_r() with readdir() to address
>> a compilation error due to the deprecation of readdir_r.
>>
>> glibc has deprecated this for their next release (2.24):
>> https://sourceware.org/bugzilla/show_bug.cgi?id=19056
>>
>> Signed-off-by: Chris Patterson <pattersonc@xxxxxxxxxxxx>
>
> Thanks for the patch, Chris.  A bit more background would have been
> helpful -- I did some searching and found a description[1] which says:
>

Thank you. I should have added these details in the commit message or cover.

> In the current POSIX.1 specification (POSIX.1-2008), readdir(3) is
> not required to be thread-safe.  However, in modern
> implementations (including the glibc implementation), concurrent
> calls to readdir(3) that specify different directory streams are
> thread-safe.  Therefore, the use of readdir_r() is generally
> unnecessary in multithreaded programs.  In cases where multiple
> threads must read from the same directory stream, using readdir(3)
> with external synchronization is still preferable to the use of
> readdir_r(), for the reasons given in the points above.
>
> The use of the specific directory stream is single-threaded, so for
> glibc, it looks like using readdir() will be safe.  But libxl needs to
> be able to build on a number of libc's that are not glibc and still be
> thread-safe.  So we need to either 1) verify that readdir() is thread
> safe on all libc's against which we may compile, or 2) add some

Is there a list of supported libc libraries?  I can look into it and
provide more definitive answers if there are.

> #ifdef-ery to switch to readdir_r() on systems unless we know it's
> thread-safe.
>
> I'm less familiar with best practices for this kind of thing -- Ian,
> do you have an idea?
>
> Thanks again for raising this, Chris.
>
>  -George
>
> [1] http://man7.org/linux/man-pages/man3/readdir_r.3.html
>

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