Ralf Wildenhues <Ralf.Wildenhues@xxxxxx> writes: > * gnoop@xxxxxxx wrote on Wed, Jun 22, 2005 at 10:17:02PM CEST: >> For example, on linux the scandir prototype is: >> >> int scandir(const char *dir, struct dirent ***namelist, >> int(*select)(const struct dirent *), >> int(*compar)(const struct dirent **, const struct dirent **)); >> >> and on darwin it's: >> >> int >> scandir(const char *dirname, struct dirent ***namelist, >> int (*select)(struct dirent *), >> int (*compar)(const void *, const void *)); >> >> I.e. the const is missing from darwin's 3rd parameter. > > Can't you just ignore this difference in your program? If you pass a > select function which eats a const argument, you are within ANSI C > bounds in either case. Sorry, that won't work in general. (Putting my language-lawyer hat on.) On darwin, you'd be passing (int (*) (struct dirent *)) to (int (*) (struct dirent const *)), and C89 and C99 both say that two function types are compatible only if their arguments are compatible, and that (struct dirent const *) is not compatible with (struct dirent *). There is an exception for pointers; e.g., you are allowed to assign a (T *) value to a (T const *) variable. But this exception does not apply to multiple levels (e.g., you can't assign (T **) to (T const **). In particular, it does not apply when assigning a (T1 (*) (T2 const *)) value to a (T1 (*) (T2 *)) variable. Arguably this is a botch. There are some cases that are safe that C does not allow. Perhaps the best known is assigning a (T **) value to a (T const * const *) variable. (I think C++ fixes this latter botch.) But the botch is part of Standard C and we have to work around it in the portable code that we write. (Language-lawyer hat off.) _______________________________________________ Autoconf mailing list Autoconf@xxxxxxx http://lists.gnu.org/mailman/listinfo/autoconf
