RE: PTHREAD_CFLAGS provided by AX_PTHREAD where best to be attached?

[Bob Friesenhahn]

On Fri, 14 Sep 2018, Dudziak Krzysztof wrote:

> Actually question asked initially here does not seem to exist as preprocessing 
> then compilation is one single step. Both ...CFLAGS and ...CPPFLAGS should go 
> to that building stage.
> Eventually the order matters, i.e. their's placement in gcc command line string.
>
> Question asked initially might exist if to replace ax_pthread-generated 
> variables with gcc's -pthread option as question's objective.
> gcc's manual describes it as gcc process driver's option. Latter one translates 
> it and passes it to preprocessor, compiler and linker flags.
> But how to use -pthread in Makefile.am if makefile carries out building in two 
> steps, compilation then linking (with libtool in between
> which possibly is irrelevant here)?
> I believe automake-generated makefile by default work that way.

Libtool attempts to use the C compiler when linking, if at all 
possible.  In this case the C compiler is also supplied with CFLAGS 
content.  If the C compiler is not used for linking, then libtool 
needs to assure that options passed to the linker are compatible with 
it, and avoid passing options which might cause it to fail.

Libtool does remember if the -pthread option (or equivalent) was 
supplied due to storing this information in the ".la" files it writes.

It is not good to only refer to GCC since there are many other 
compilers.  For some compilers (e.g. on IBM AIX), a different compiler 
program needs to be selected in order to support pthreads.

Bob

> krzysiek
>
> -----Original Message-----
> From: Bob Friesenhahn [mailto:address@hidden
> Sent: Thursday, 12. July 2018 20:28
> To: Dudziak Krzysztof <address@hidden>
> Cc: address@hidden
> Subject: Re: PTHREAD_CFLAGS provided by AX_PTHREAD where best to be attached?
>
> On Thu, 12 Jul 2018, Dudziak Krzysztof wrote:
>
> > Hi,
> > For makefiles which build piece of software in two separate steps:
> > compilation then linking, will it be better to attach  PTHREAD_CFLAGS to 
> > higher-level_CPPFLAGS than to higher-level_CFLAGS variable?
> > Autotools along with libtool are used here, user-level variables (CFLAGS, 
> > CPPFLAGS) are not manipulated through whole build chain.
> > I can manipulate variables only at Makefile.am -level.
>
> CPPFLAGS is for the pre-processor while CFLAGS is applied (as well as
> CPPFLAGS) while compiling.  This means that putting the options in CFLAGS does 
> work, although it would be better to deduce which options are for the 
> pre-processor or the compiler and put them in the correct places.
>
> > I learned gcc expects -pthread option which translates among others to
> > some preprocessor define, therefore I wonder if it won't be better to
> > attach PTHREAD_CFLAGS to higher-level_CPPFLAGS rather than to 
> > higher-level_CFLAGS.
>
> It should not work to put -pthread in CPPFLAGS since it is a compiler-driver 
> option and not a pre-processor (e.g. /usr/bin/cpp) option.
>
> Only, -I, -D, and -U type options (see the manual page for 'cpp') should be put 
> in CPPFLAGS.
>
> Bob
> --
> Bob Friesenhahn
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--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/