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Re: [patch #8081] AX_PTHREAD.m4 not setting PTHREAD_LIBS to -pthread for

From: Al Pacifico
Subject: Re: [patch #8081] AX_PTHREAD.m4 not setting PTHREAD_LIBS to -pthread for gcc 4.7.2
Date: Wed, 19 Jun 2013 23:03:56 -0700

Thank you for the excellent explanation.

On Wed, Jun 19, 2013 at 11:49 AM, Daniel Richard G. <address@hidden> wrote:
On Wed, 2013 Jun 19 11:00-0700, Al Pacifico wrote:
> Forgive me if I am incorrect, but when I experimented with this,
> PTHREAD_LIBS seemed not to get set to anything at all by the macro
> compiling my code.

That can be a valid result. If -pthread is needed in both the
compilation and linking steps, then the result should be


and not


because you are supposed to use PTHREAD_CFLAGS in both steps anyway.
Hence the comment

My now contains 

    [Define if you have POSIX threads libraries and header files.])

and seems to work as I had expected.

#   NOTE: You are assumed to not only compile your program with these flags,
#   but also link it with them as well. e.g. you should link with

> The macro documentation says it "sets the PTHREAD_LIBS output variable
> to the threads library and linker flags, and the PTHREAD_CFLAGS output
> variable to any special C compiler flags that are needed."

The key point, at least for me, was this paragraph:
Compiler flags and linker flags are not mutually-exclusive sets, not
least because linking is typically done via the compiler frontend (cc)
and not by invoking the linker (ld) directly. Any flags that you can use
in the compile step (e.g. -O2, -DFOO, -I/tmp/include) will generally be
accepted in the linking step, even if it's not applicable then. (The
reverse may not be true, e.g. -lfoo.)

Given that, it's a lot less error-prone to use PTHREAD_CFLAGS (and other
CFLAGS variables) when linking, rather than duplicating the applicable
flags into PTHREAD_LIBS/LDFLAGS/etc. variables and not using any CFLAGS
variables then.

Obviously I am not the only one who failed to understand this despite reading the macro's documentation. Might I suggest inclusion of an example in the macro's documentation?


Daniel Richard G. || address@hidden
My ASCII-art .sig got a bad case of Times New Roman.

Al Pacifico
Seattle, WA

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