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Bug 176

Summary: SDL_stdinc.h uses #if when it should be using #ifdef
Product: SDL Reporter: Max Horn <max>
Component: *don't know*Assignee: Ryan C. Gordon <icculus>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2    
Version: HG 1.2   
Hardware: All   
OS: Other   
Attachments: Patch for SDL_stdinc.h

Description Max Horn 2006-03-22 12:31:14 UTC 
SDL_stdinc.h generates a lot of warnings for me (or rather, errors, since I usually use -Werror). The reason is that it uses #if to check the various HAVE_XYZ flags. But these flags are actually not define to 1 or 0 to describer the presence/absence of a feature -- rather, they are either defined to be 1, or not defined at all!

Hence, the proper way to check for them is to use #ifdef or #if defined().

The attached patch does exactly this.
Comment 1 Max Horn 2006-03-22 12:37:14 UTC 
Created attachment 89 [details]
Patch for SDL_stdinc.h
Comment 2 Sam Lantinga 2006-03-22 12:41:47 UTC 
What compiler do you have that doesn't accept that?

All the compilers I've seen have the following behavior
foo not defined
#if foo -> FALSE
#undef foo
#if foo -> FALSE
#define foo 0
#if foo -> FALSE
#define foo 1
#if foo -> TRUE

For the record, if you look at the autoconf configure tests, they use exactly the same convention:
# Factoring default headers for most tests.
ac_includes_default="\
#include <stdio.h>
#if HAVE_SYS_TYPES_H
# include <sys/types.h>
#endif
#if HAVE_SYS_STAT_H
# include <sys/stat.h>
#endif
#if STDC_HEADERS
# include <stdlib.h>
# include <stddef.h>
#else
...
Comment 3 Max Horn 2006-03-22 13:02:06 UTC 
To clarify: Normaly, GCC (or, to be precise, the preprocessor) will ignore this, and compile the code happily. However, one can specify -Wundef to get a warning about this.

One can probably argue whether to consider this a bug or not; but I think that (a) from a semantic point of view, using "#if FOO" when FOO is not defined is strange, and (b) since it is possible to trigger a warning about this, and a trivial fix exists, it should be corrected.

I can think of two alternative patches, BTW:
1) Simply use #define HAVE_FOO 0, instead of not defining HAVE_FOO at all
2) Change
  #if HAVE_FOO
to
  #if HAVE_FOO+0
which always does the right thing.

But I think I still prefer the attached patch :-).
Comment 4 Sam Lantinga 2006-04-13 09:38:53 UTC 
This patch (and a couple cases you missed) is in CVS.  Thanks!