[Buildroot] [PATCH 2/2] fwup: link in pthreads for static builds

[Frank Hunleth]

Hi Thomas,

On Thu, Aug 11, 2016 at 6:06 PM, Thomas Petazzoni
<thomas.petazzoni at free-electrons.com> wrote:
> Hello,
>
> On Wed, 10 Aug 2016 18:35:46 -0400, Frank Hunleth wrote:
>> Both libconfuse (via libintl) and libarchive have references to pthreads
>> but do not specify pthreads in their pkg-config descriptions. This
>> change adds pthreads to the fwup linker options so that the link step
>> succeeds.
>
> I've finally taken some time to look into this, and I believe your fix
> is not the right fix, and the bug is in fact in uClibc (and I have
> already posted a mail on the uClibc mailing list about this).
>
> The fact that libintl and libarchive do *not* link with libpthread is
> intentional and actually a feature. As explained in my mail to the
> uClibc mailing list [1], the idea is that those libraries don't need
> threads, they only need to be thread-safe if threads are used by the
> application using them.
>
> So the libc part of the C library contains a stub implementation of
> those functions, while the libpthread part of the C library contains
> the real implementation.
>
> So, libintl and libarchive are only linked against libc.so. If you make
> a regular, non-threaded application, linked against libintl and/or
> libarchive, then those libraries will use the stub version of the
> pthread functions. This is correct and actually good: your application
> is not multi-threaded, so why suffer the cost of acquiring/releasing a
> mutex?
>
> If on the other hand, your application is multi-threaded, it will link
> against libpthread. In this case, the pthread_mutex_*() calls from
> libarchive or libintl will use the real implementation from libpthread,
> and those libraries will have a thread-safe behavior.
>
> The problem is that this doesn't work with static linking with uClibc.
> I believe it's a uClibc bug, but I'm not entirely sure either.
>
> So, if fwup links fine dynamically (i.e without linking against
> pthread), there should be no reason to link it with pthread for static
> linking.

Thank you so much for investigating this further and posting to the
uclibc mailing list. I much prefer the behaviour you describe since
fwup does not use pthreads. I'll monitor the responses to your post.

Thanks!
Frank

>
> [1] http://mailman.uclibc-ng.org/pipermail/devel/2016-August/001132.html
>
> Thomas
> --
> Thomas Petazzoni, CTO, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com