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Re: Turn on compiler warnings by default for AC_PROG_CC, AC_PROG_CXX & A

From: David A. Wheeler
Subject: Re: Turn on compiler warnings by default for AC_PROG_CC, AC_PROG_CXX & AC_PROG_FC
Date: Mon, 10 Feb 2014 17:28:45 -0500 (EST)

On Mon, 10 Feb 2014 17:01:57 -0500, Dale Visser <address@hidden> wrote:

> I've created a new patch (attached) incorporating your suggestions. See my 
> comments inline below.


> Done, with the wording you suggested. On my system (gcc version is 4.8.1), I 
> tested the gcc oddity you mentioned and found that it only applied to flags 
> of the form -Wno-*. E.g., -Wnosuchoption and -Wjunk would still not be added, 
> but -Wno-way would be added and harmlessly ignored.

Okay, much better.

> > ./doc/autoconf.texi:23579: warning: `(' follows defined name 
> > `AC_APPEND_FLAG_IFVALID' instead of whitespace.
> > Can you fix the documentation to remove those warnings?
> Done. For some reason, those warning never showed for me. How fitting... :-)

I tried it out.

Now it works for me, good.

> Thank you for your helpful comments. Let me know what you think of the latest.

Thanks.  I ran a test case (using gcc as the C compiler),
here's what running my configure now looks like:
 checking CFLAGS for maximum warnings... -Wall
 checking for style of include used by make... GNU
 checking dependency style of gcc -std=gnu11... gcc3
 checking if -Wno-such-option can be added to CFLAGS... yes

That looks perfect, it's certainly clear what's going on.

When I run "make" on the resulting Makefile (after "touch"ing my .c file),
I get the following output:
 gcc -std=gnu11 -DHAVE_CONFIG_H -I.     -Wall -Wno-such-option -MT hello.o -MD 
-MP -MF .deps/hello.Tpo -c -o hello.o hello.c
 hello.c:3:22: warning: "/*" within comment [-Wcomment]
 cc1: warning: unrecognized command line option "-Wno-such-option" [enabled by 
 mv -f .deps/hello.Tpo .deps/hello.Po

The first warning is exactly what I expect ("/*" within comment), excellent.
I even added this to
and as expected it turned off "-Wcomment".
This makes it really easy to turn on & off warning flags, while
still making it portable.  Excellent.

I'm a little annoyed by the second warning, though, the one that says:
  cc1: warning: unrecognized command line option "-Wno-such-option" [enabled by 
I presume this happens because there's no error return
given an unrecognized command line option.  That's not a *killer*, but
that would be annoying since it'd repeat in every compile.
Is there a way your "_IFVALID" macro could detect this case
(e.g., by compiling with and without, and noticing if the name of the option
showed up in a line only output by the "it's on" case?)
Or maybe is there some other way to detect that adding a flag would not really 
be valid?
I admit that this can only occur if (1) the compiler is gcc and (2) you use a 
of the form -Wno-* where "*" isn't supported, so maybe this is a case not worth
creating a complex solution.  But if there's an easy refinement it'd be nice to 

--- David A. Wheeler

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