gcc -pthread vs gcc -lpthread

[Dima Pasechnik]

I wonder how we deal with this issue in packages that have hand-written Makefiles;

The issue is that some versions of gcc/g++ (if used as a linker) need -pthread option to be specified, apart from linking

with -lpthread.

In particular this has to be dealt on https://trac.sagemath.org/ticket/23353

Thanks,

Dima

[Jeroen Demeyer]

On 2018-04-20 13:48, Dima Pasechnik wrote:
> The issue is that some versions of gcc/g++ (if used as a linker) need
> -pthread option to be specified

That's a true statement but why do you call it an "issue"?

The GCC documentation says

-pthread
Adds support for multithreading with the pthreads library.
This option sets flags for both the preprocessor and linker.

[François Bissey]

-pthread like -openmp should always be the right option to use. gcc/g++ and other
are compiler drivers (the real compilers are cc1/cc1++) and those switches tells
the driver to set everything right for compiling and linking with these features.

That’s the theory at least. Do you have an example where -pthread at linking
time doesn’t do its job? The -lpthread could be just someone not understanding
the feature, after all, there is a libpthread.

François

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[Dima Pasechnik]

On Friday, April 20, 2018 at 1:00:56 PM UTC+1, François Bissey wrote:

-pthread like -openmp should always be the right option to use. gcc/g++ and other
are compiler drivers (the real compilers are cc1/cc1++) and those switches tells  
the driver to set everything right for compiling and linking with these features.

That’s the theory at least. Do you have an example where -pthread at linking
time doesn’t do its job? The -lpthread could be just someone not understanding
the feature, after all, there is a libpthread.

clang -pthread  does not do what gcc -pthread  does, see

https://stackoverflow.com/a/19382746/557937

This means that with clang you need -lpthread

whereas with gcc you need -pthread

It's probably OK to have them both, no?

[François Bissey]

> On 21/04/2018, at 02:43, Dima Pasechnik <dim...@gmail.com> wrote:
>
> clang -pthread does not do what gcc -pthread does, see
> https://stackoverflow.com/a/19382746/557937
>
> This means that with clang you need -lpthread
> whereas with gcc you need -pthread
>
> It's probably OK to have them both, no?
>

That should be a clang bug. I see there is a bit of stuff on google
about these troubles (different between linux and OS X as well).
I f clang really cannot behave I think it should be
$CC -pthread -lpthread
in that order. But that’s still horrible in a number of scenario.
autotools is good at hiding that stuff from you.

François