Mark_Andrews at isc.org
Thu Jun 15 14:34:45 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
> 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
> 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-
> 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).
From this I take it the best way to build a threaded library
under FreeBSD is to not specify a threads library when
specifying -shared to gcc.
Hopefully someone will take this and add a page to the
handbook which explains all this.
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: Mark_Andrews at isc.org
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