Re: AX_PTHREAD incorrectly adds -pthread on OSX

Thanks for the explanation!

-Jason

On Fri, Jan 22, 2016, 8:30 PM Daniel Richard G. <[email protected]> wrote:

> Hi Peter,
>
> Spurred by your/this e-mail, I've started working on this macro again to
> finally get a working version pushed out.
>
> On Fri, 2016 Jan  8 19:56+0100, Peter Simons wrote:
> > I'm forwarding this message to Steven and Richard, the macro's
> > authors.
> >
> > Peter
> >
> > Jason Gross <[email protected]> writes:
>
> Hi Jason, sorry for the wait. Lot of loose threads from the holiday!
>
> > > The latest version of AX_PTHREAD
> > > (dd946914edb9da25e2d8c8b2186fab5d8103f3d4, serial 21) does not work
> > > on OS X. It tests for whether or not `clang -o conftest -g -O2
> > > -pthread -Werror conftest.c` succeeds (which it does), when it
> > > actually needs to be checking for `clang -o conftest.o -g -O2
> > > -pthread -Werror - c conftest.c && clang -o conftest -g -O2 -pthread
> > > -Werror conftest.o` (which, I believe, fails). If there's a variant
> > > of AC_LINK_IFELSE that does the compilation separately from the
> > > linking, then the change should be as simple as
>
> AC_LINK_IFELSE() is defined at the Autoconf level. I can't change how it
> works, and there isn't any alternative macro that performs the link
> separately from the compilation. Even if there were, it wouldn't solve
> the problem.
>
> The issue is that [certain older versions of] Clang takes -pthread when
> compiling, but doesn't want the flag when it is only linking. This flies
> in the face of how basically every other C compiler works, where compile-
> time flags are at least silently ignored when linking with the compiler
> frontend, and are at times actually required (e.g. flags indicating 64-
> bit mode) in order for the link to succeed.
>
> That is why Automake always includes $(CFLAGS) when linking with
> $(CC), in addition to $(LDFLAGS). If you want to have a flag that is
> used *only* at compile time, that's actually a bit tricky to pull off
> [with Automake].
>
> What's more, not setting PTHREAD_CFLAGS="-pthread" is actually an
> incorrect result, because then _REENTRANT is not #defined, meaning
> that "errno" and the like are not defined appropriately for multi-
> threaded builds.
>
> Ultimately, it is not possible to build POSIX-threaded software with
> this combination of elements:
>
> 1. A version of Clang that complains about -pthread when only linking
>
> 2. Automake
>
> 3. -pthread
>
> 4. -Werror
>
> One of those has to be taken out. Removing #1 or #2 tends not to be
> practically feasible. #3 is required for correct compilation. So it's #4
> that's got to go. Sometimes, you just can't avoid a warning.
>
> > > plus associated documentation and indentation changes, but I don't
> > > know if anything like AC_COMPILE_AND_LINK_SEPARATELY_IFELSE exists----
> > > does it?
>
> Not in Autoconf proper. There's never been a need for it, and the
> Clang folks have corrected the behavior of the -pthread flag in more
> recent versions.
>
>
> --Daniel
>
>
> --
> Daniel Richard G. || [email protected]
> My ASCII-art .sig got a bad case of Times New Roman.
>