On 2018-08-22 13:07, Joe Perches wrote: > On Wed, 2018-08-22 at 13:00 +0200, Rasmus Villemoes wrote: >> This wraps strchr and friends in macros that ensure the return value has >> type const char* if the passed-in string (which the return value points >> into) also has type const char*. The (s)+0 thing is to force a const >> char[] (e.g. a string literal) to decay to a const char* for the >> __same_type comparison. > [] >> diff --git a/include/linux/string.h b/include/linux/string.h > [] >> +#define strchr(s, c) ( \ >> + __builtin_choose_expr(__same_type((s) + 0, const char *), \ >> + (const char *)strchr(s, c), \ >> + strchr(s, c))) >> +#endif > [] >> diff --git a/lib/string.c b/lib/string.c > [] >> @@ -367,7 +367,7 @@ EXPORT_SYMBOL(strncmp); >> * @s: The string to be searched >> * @c: The character to search for >> */ >> -char *strchr(const char *s, int c) >> +char *(strchr)(const char *s, int c) > > I've tried to use this macro/function wrapping > a few times before, but it seems that it's fairly > unusual in the kernel. I believe there may not > be any current uses of that style. > > A comment explaining the form might be useful. >
True. I dislike the more conventional #undef strchr approach, because that would mean any other function in string.c that calls strchr and happens to be defined after strchr() would not be 'instrumented'. It's more a principle than a practical thing because, well, none of the instrumented functions happen to be used by other functions in string.c. Rasmus
