Subj : Re: why this program can be linked successfully with and without
To   : comp.programming.threads
From : David Butenhof
Date : Thu Jan 06 2005 08:48 pm

Andy Yang wrote:
> Just a simply program. But to my surprise, it can be linked
> successfully with and without -lpthread option:
>    gcc test.c
> or
>    gcc test.c -lpthread
> 
> Both works fine. However, the two linked version show different
> behavior. I am using Fedore Core 3.

Many basic C runtimes (libc, glibc, etc) contain "stubs" for the pthread 
functions that allow you to link and run, but won't actually do 
anything. The mutex operations, for example, often return success 
without doing a thing, while pthread_create() usually fails.

This is principally of use with thread-safe libraries, so they can be 
linked into main programs with or without threads. With threads, they 
"do their threadly things", and without threads they behave normally but 
without the extra overhead of synchronization.

It does mean that you cannot link a library that USES threads into a 
main program that doesn't link libpthread -- you'll still resolve the 
(already loaded) stubs. Several systems have tried to avoid this with 
runtime redirection mechanisms... but they, too, have their share of 
problems and complications. In the end, pretty much everyone has agreed 
that this static "switch" based on the linkage of the main program is 
the best solution.

-- 
Dave Butenhof, David.Butenhof@hp.com
HP Utility Pricing software, POSIX thread consultant
Manageability Solutions Lab (MSL), Hewlett-Packard Company
110 Spit Brook Road, ZK2/3-Q18, Nashua, NH 03062

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