Recently, I re-tooled my project (cued) to use autotools. I started out
by doing the naive thing when it came to including my project's config.h.
Here is an excerpt from rip.c:
#include "config.h" // HAVE_CDIO_MMC_LL_CMDS_H
#include "unix.h"
#include "util.h"
#define DO_NOT_WANT_PARANOIA_**COMPATIBILITY
#include<cdio/cdio.h>
#include<cdio/mmc.h> // CDIO_MMC_READ_TYPE_ANY
#ifdef HAVE_CDIO_MMC_LL_CMDS_H
#include<cdio/mmc_ll_cmds.h>
#endif
This seemed to work fine. I proceeded to make my project compile under
Linux, Open Indiana (Solaris), and FreeBSD. So far so good. Then, I tried
to port to MacOS, not because I am particularly concerned about supporting
it, but because I figured it was different enough from the other 3
platforms that something might break.
Consequently, when trying to build under MacOS, I was rewarded with this
nastiness:
gcc -DHAVE_CONFIG_H -I. -I../.. -I/opt/local/include
-I/opt/local/include -I/opt/local/include -I/opt/local/include
-I../../lib/cued -std=gnu99 -Wall -Wstrict-aliasing=3 -Wformat=2 -g -O2
-MT rip.o -MD -MP -MF .deps/rip.Tpo -c -o rip.o rip.c
In file included from /opt/local/include/cdio/types.**h:34,
from /opt/local/include/cdio/cdio.**h:35,
from rip.c:25:
/opt/local/include/cdio/cdio_**config.h:306:1: warning: "PACKAGE"
redefined
In file included from rip.c:20:
../../config.h:91:1: warning: this is the location of the previous
definition
In file included from /opt/local/include/cdio/types.**h:34,
from /opt/local/include/cdio/cdio.**h:35,
from rip.c:25:
/opt/local/include/cdio/cdio_**config.h:309:1: warning:
"PACKAGE_BUGREPORT" redefined
Add a half dozen more of these warnings, and you get the gist. Apparently,
there was a conflict between my project's config.h and libcdio's config.h.
Yet, the warning only showed up on MacOS, not any of the other platforms.
The reason it showed up on MacOS is that MacPorts installs header files in
/opt/local rather than /usr/include. GCC treats system headers differently
than other headers. It ignores redefined macros if they are redefined by
system headers (i.e. in /usr/include). This is documented under the GCC
option "-isystem".
So PACKAGE_NAME was defined to be "libcdio" rather than "cued", the name
of my package. VERSION was defined to be "0.83" instead of "1.20". This is
a somewhat insidious problem because I got NO warning on ANY platform other
than MacOS.
Next, I searched this mailing list to see what I could learn. I think I
fixed my problem by switching to the following code (but maybe not, read
on:)
#ifdef HAVE_CONFIG_H
#include "config.h" // HAVE_CDIO_MMC_LL_CMDS_H
#define __CDIO_CONFIG_H__ // avoid conflicts with libcdio
#endif
#include "unix.h"
#include "util.h"
#define DO_NOT_WANT_PARANOIA_**COMPATIBILITY
#include<cdio/cdio.h>
#include<cdio/mmc.h> // CDIO_MMC_READ_TYPE_ANY
#ifdef HAVE_CDIO_MMC_LL_CMDS_H
#include<cdio/mmc_ll_cmds.h>
#endif
I am not totally satisfied with this solution. If every library that I
used included its config.h in the global namespace, I might end up with
something like this (assuming that other projects create config.h include
guards which they generally do not:)
#ifdef HAVE_CONFIG_H
#include "config.h"
#define __CDIO_CONFIG_H__
#define __SNDFILE_CONFIG_H__
#define __CDDB_CONFIG_H__
etc.
MOREOVER, I am not sure that<cdio/types.h> will always do the right thing
when I define __CDIO_CONFIG_H__ before including it. You might argue I
should have used the other proposed solution for these sorts of conflicts
which would look something like this:
#include<cdio/cdio.h>
#include<cdio/cdio_unconfig.h> # remove *all* symbols libcdio defines
#include "config.h"
#include "unix.h"
#include "util.h"
#define DO_NOT_WANT_PARANOIA_**COMPATIBILITY
#include<cdio/mmc.h> // CDIO_MMC_READ_TYPE_ANY
#ifdef HAVE_CDIO_MMC_LL_CMDS_H
#include<cdio/mmc_ll_cmds.h>
#endif
Note that I need to include my "config.h" before<cdio/mmc_ll_cmds.h>, BUT
I am supposed to include the cdio headers before my "config.h". Also I am
not convinced that cdio_unconfig.h won't remove some definitions that are
included in the GCC specs for some platform or in the system header files
for that platform. For example, cdio_unconfig.h undefines "const".
Now the general autotools lore seems to be that you should never include
config.h in a header file, but only in a .c file. This is generally
attributed to the lack of include guard. I think that is missing the
point. Here, I will argue that you should never include config.h in a
header file because of the global namespace pollution and the potential for
silent conflicts that show up as bugs.
One path out of this conflict is to remove code such as this from
<cdio/read.h>:
#ifndef EXTERNAL_LIBCDIO_CONFIG_H
#define EXTERNAL_LIBCDIO_CONFIG_H
/* Need for HAVE_SYS_TYPES_H */
#include<cdio/cdio_config.h>
#endif
#ifdef HAVE_SYS_TYPES_H
/* Some systems need this for off_t and ssize. */
#include<sys/types.h>
#endif
However, things aren't so simple when it comes to<cdio/types.h>, which
seems to have a real need to include its config.h.
Another path out of this conflict would be to create yet another config.h
file, perhaps named cdio_header_config.h that contains ONLY the macros
needed by the cdio headers, such as HAVE_SYS_TYPES_H and HAVE_STDINT_H.
Yet again, /usr/include/cdio/types.h complicates things because there is
so much that it needs.
So, I am stuck, but still thinking about the problem. Thoughts?
Rob