On Mon, 22 Sep 2014, Andrew MacLeod wrote: > Josephs solution was to identify these and instead put a default definition in > default.h ... then change all the uses to #if instead.. ie, > #if BLAH > > This way we can ensure that the definition has been seen and it will be a > compile error if not.
No, my suggestion was that whenever possible we should change preprocessor conditionals - #ifdef or #if - into C conditionals - "if (MACRO)". Changing from #ifdef to #if does nothing to make a missing tm.h include produce an error - the undefined macro simply quietly gets treated as 0 in preprocessor conditionals. To get an error from #if in such cases, you'd need to build GCC with -Wundef (together with existing -Werror), and I'd guess there are plenty of places that are not -Wundef clean at present. Now, I think moves of defaults to defaults.h are generally a good idea, and that moving from defined/undefined to true/false semantics are also a good idea - even if the way the macro is used means you can't take the further step of converting from #if to if (). They don't solve the problem of making a missing tm.h include immediately visible, but they *do* potentially help with future automatic refactoring to convert target macros into hooks. Obviously such moves do require checking the definitions and uses of the macros in question; you need to make sure you catch all places that use #ifdef / #if defined etc. on the macro (and make sure they have the same default). And if you're changing the semantics of the macro from defined / undefined to true / false, you need to watch out for any existing definitions with an empty expansion, or an expansion to 0, etc. -- Joseph S. Myers jos...@codesourcery.com
