Subj : Re: Linking without -lpthread doesn't fail??
To   : comp.programming.threads
From : Eric Sosman
Date : Wed Oct 05 2005 02:31 pm



dmarkh@cfl.rr.com wrote On 10/05/05 13:00,:
> When I link a program that has pthread_* calls in it yet I forget to
> link with -lpthread the program links. It does not work and there is no
> indication other that the program not working that I did anything
> wrong. Why is that?

    You didn't mention the operating system you're using.
Some systems provide dummy versions of pthread_* functions
that single-threaded programs can use.  This allows you to
write a function that can be used in either a single- or
multi-threaded program without needing to maintain separate
versions for each.  Your function can call pthread_mutex_lock(),
for example, and "the right thing happens" no matter whether
the function is being used by a single- or multi-threaded
program.  If the program was linked with -lpthread you'll
get the real pthread_mutex_lock() implementation, but if
it was linked without -lpthread you'll get the dummy
version.

    Most of the dummy implementations are no-ops: the dummy
pthread_mutex_lock() will do nothing and just return, claiming
that it has locked the mutex -- the important question is
whether it's now safe to access the protected data, and in a
single-threaded program thread-safety is guaranteed.  The dummy
versions of more complicated functions usually return a failure
indication of some kinds -- for example, pthread_create() will
probably say it was unable to create a new thread.  If your
program checks the returned values of the pthread_* functions,
you may find that many of them report failure if linked without
-lpthread.

    This was the way Solaris worked, up through Solaris 9.
In Solaris 10 there is no longer a distinction between single-
and multi-threaded processes: all processes are potentially
multi-threaded.  A "single-threaded process" is really a multi-
threaded process that happens to have started only one thread
so far; it might perfectly well start another thread in a
moment or two.  This change of viewpoint means that the dummy
implementations are gone, and that you get the real pthread_*
functions even if you don't link with -lpthread.

    That's Solaris; whatever system you're using may act in
a similar way.

-- 
Eric.Sosman@sun.com

.