Re: Smart --with-package=DIR (for absentminded users)

[Benoit SIGOURE]

On Jul 5, 2007, at 1:29 PM, Thomas Lavergne wrote:

> Hi all,

Hello Thomas,

>    Sorry if this is already in the archive. I looked through it and  
> was not lucky. You must also know that I am quite new to autoconf  
> but... I need to learn fast!
>
>    I want to implement the --with-package=DIR option to my  
> project's configure script. The --with-package[=yes|no] thing is  
> straightforward using AC_ARG_WITH(). I have however some questions  
> regarding others' experience wth the =DIR part.
>
> 1. What should I do if the user is so dumb to actually give a DIR  
> which does not exist? IMHO, I should issue an AC_MSG_ERROR and  
> terminate the configuration: we cannot configure taking into  
> account the user's input. What do you think of that and is there a  
> consensus on how to handle such a situation.

AC_MSG_ERROR is what I'd go for.

> 2. How can I detect that /path/to/foo helps in any way for  
> accessing foo.h and libfoo.a? Specifically, if library foo is  
> available in the system directories (e.g. /usr/include and /usr/ 
> lib), then providing --with-foo=yes and --with-foo=/wrong/path/to/ 
> foo will have the same effect. For foo is accessible to the  
> compiler and linker even if the options -I and -L point to 'wrong'  
> directories, i.e directories not containing foo.h and libfoo.a. Is  
> there a (portable) way of detecting that? In my case, the user  
> might have a custom-made version of foo (with the same file names)  
> and want to use it. I agree he should not have given a wrong path  
> to foo in the first place but users are users and I want to avoid  
> that the software is configured and compiled with the 'system- 
> version' of the library when the user (thought he) especially  
> demanded to use another version.
> Any (portable or not) idea?

The autoconf way of dealing with this is to test the user's input.   
If you know that libfoo *must* come with a header, say, foo.h, you  
can test whether this header is usable with AC_CHECK_HEADERS [1] for  
instance.  Then it is generally a good idea to test that the header  
"works".  Say that you know for sure that the header must contain a  
#define LIBFOO_VERSION, you should try to compile (or only  
preprocess) a sample program that does something like:
#include <foo.h>
#ifndef LIBFOO_VERSION
# error foo.h did not define LIBFOO_VERSION
#endif

See for instance AC_PREPROC_IFELSE [2]

Similarily, you should test that you can link with libfoo and try to  
compile (with AC_COMPILE_IFELSE [3] for instance) a small main that  
uses one of the symbols of libfoo.

During these tests, you will have to add flags in CPPFLAGS (-I...)  
and LDFLAGS (-L... -l...).  Be careful, you should not overwrite  
existing flags (that were set by previous tests or most probably by  
the user).  Also you should try to avoid to change these flags since  
they're reserved for the user.  Do something like:

old_CPPFLAGS=$CPPFLAGS
CPPFLAGS="$CPPFLAGS $flags_deduced_from_user_input"
AC_PREPROC_IFELSE(...your test here...)
CPPFLAGS=$old_CPPFLAGS
LIBFOO_CPPFLAGS=$flags_deduced_from_user_input
AC_SUBST([LIBFOO_CPPFLAGS])

And then use $(LIBFOO_CPPFLAGS) in your Makefile.am:
target_CPPFLAGS = $(LIBFOO_CPPFLAGS)
or
AM_CPPFLAGS = $(LIBFOO_CPPFLAGS)

(once again here do not use CPPFLAGS directly, they're reserved for  
the user)

I hope this helps.
Cheers,


[1] http://www.gnu.org/software/autoconf/manual/html_node/Generic- 
Headers.html
[2] http://www.gnu.org/software/autoconf/manual/html_node/Running-the- 
Preprocessor.html
[3] http://www.gnu.org/software/autoconf/manual/html_node/Running-the- 
Compiler.html

--
Benoit Sigoure aka Tsuna
EPITA Research and Development Laboratory

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