gcc -pthread

Daniel Eischen deischen at freebsd.org
Thu Jun 15 13:48:48 UTC 2006


On Thu, 15 Jun 2006, Mark Andrews wrote:

>
> 	Is there any good reason why "gcc -pthread" links in
> 	-lpthead except when -shared is specified?

Because one may want to build applications to use different
threading libraries.  Application A may work better with libthr,
while application B prefers libpthread.  Both applications may
also rely on libfoo which needs a threading library in order
to be MT-safe.  If libfoo records a dependency on libpthread,
and application A uses libthr, then that won't work (crashy
crashy).

I think we took the approach a long time ago that there
were basically 2 reasons for a library to require threading:

   1) To be MT-safe when used in the presence of a threaded
      application; and

   2) To create threads behind the scenes in support of an
      application (i.e., a hypothetical libaio).

In the case of 1, the library can use #pragma weak on
pthread_create and detect whether or not a threads
library is present and only use locking when pthread_create
!= NULL (libc provides stubs for all the pthread functions
necessary for locking).  In the case of 2, applications
should know what libraries need threads and be required
to supply -lpthread (or -lthr, or -pthread) when they are
built.

This may or may not change in the future because all
the UNIX world is Linux :(  (And for possible problems
when using symbol versioning.)

> 	Also the gcc man page does not match reality.
>
>       -pthread
>              Link a user-threaded process against libc_r instead of libc.
>
> 	man pthread mentions these libraries but provided no discussion
> 	on if or when you should include them on the command line.  Nor
> 	does it mention "-pthread".
>
> LIBRARY
>     POSIX Threads Library (libpthread, -lpthread)
>     1:1 Threading Library (libthr, -lthr)
>     Reentrant C Library (libc_r, -lc_r)
>
> 	The hacker handbook seems to indicate that you shouldn't need
> 	to use -lpthread or -lc_r.
>
> 	http://www.freebsd.org/doc/en_US.ISO8859-1/books/porters-handbook/dads-pthread.html

That's the porters handbook and they prefer to use PTHREAD_LIBS so
one can override the default threading library when building the
port.  PTHREAD_LIBS defaults to -pthread because -pthread does
the right thing on different archs where the default threading
library varies (x86, amd64, ia64 use libpthread, sparc64 uses
libthr, etc).

-- 
DE


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