Subj : Re: redhat linux standard C library does not have pthread_mutex_trylock ???
To   : comp.programming.threads
From : Paul Pluzhnikov
Date : Mon Feb 07 2005 08:11 am

"David Schwartz" <davids@webmaster.com> writes:

> > - Is this a problem or a feathure of Linux?

I think the reason e.g. pthread_mutex_lock() stub is defined in
libc.so.6, while pthread_mutex_trylock() isn't, is that the former
is needed by glibc itself for thread-safe operation.

> You may 
> even be able to create a 'weakly-linked' pthread_mutex_trylock function such 
> that the real one, if present, will take over automatically.

This is the best approach, and is trivially done:

  extern int pthread_mutex_trylock(pthread_mutex_t *) __attribute__((weak));
  ...
    if (pthread_mutex_trylock) {
       pthread_mutex_trylock(&mtx); // call it only if linked
                                    // against libpthread
    }

> > - What's the overhead of linking a single thread app with -lpthread
> 
>     There is no overhead on Linux, as I understand it.

Oh there *is* overhead. 

It's not likely to be very significant in a properly-coded
application, but an app that spins in a tight loop doing just
pthread_mutex_lock()/unlock(), shows a factor of 10 slowdown when
linked against libpthread on RedHat-6.2 (glibc-2.13) and a factor
of 3 on RHEL-3u4 (glibc-2.3.2-95.30).

Cheers,
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