5 ms·
Better then #define ZFREE(p) do { free(p); p = NULL; } while(0)
by nn3 6y ago
Better then
#define ZFREE(p) do { free(p); p = NULL; } while(0)
- okl 6y agoYou can even do #define free(p) do { free(p); p = NULL; } while(0) And when you want to call the original free: (free)(p); The preprocessor will not substitute this occurence of free.
- devenblake 6y agoC n00b here - why the `do {} while(0)`? Couldn't you just do something like `#define free(p) { free(p); p = NULL; }`?
- deaddodo 6y agoSemi-experienced C user here, I believe the anonymous block is perfectly adequate here. No idea why they are wrapping it in a single instance do loop, unless they’re unaware of block scoping or I’m unaware of some UB here.
- btrask 6y agodo {} while(0) is a common idiom for macros in C, because it consumes the trailing semicolon, which a bare {} block doesn't do. if(x) MACRO(); else something(); expands to if(x) { ... }; // Error! else something();
- btrask 6y agoHere's the actual macro I (sometimes) use: #define FREE(ptrptr) do { \ __typeof__(ptrptr) const __x = (ptrptr); \ free(*__x); *__x = NULL; \ } while(0) There might be a better way of doing it though. Also, __typeof__() obviously isn't standard C. Edit to add: I've honestly been moving away from using a macro and just putting both statements on one line like in the OP. For something so simple, using a macro seems like overkill.
- Thorrez 6y agoWhat's the benefit of that over nn3's version?
- dromtrund 6y agoIt'll only evaluate the pointer once. It's possible to make this a function though, that might be preferable
- btrask 6y agoI tried making it a plain function at one point but ran into some weirdness around using void * * with certain arguments (const buffers?). You don't want to accept plain void * because it's too easy to pass a pointer instead of a pointer to a pointer. Using a macro is (ironically) more type safe. Maybe someone else could figure out how to do it properly, since I'd definitely prefer a function.
- Thorrez 6y agoGood point. But it seems like it would require usage like this: int* p = malloc(sizeof(int)); FREE(&p); What if we instead define the macro like this: #define FREE(ptr) do { \ __typeof__(ptr)* const __x = &(ptr); \ free(*__x); *__x = NULL; \ } while(0) Then make usage slightly shorter, as well as more similar to free(): int* p = malloc(sizeof(int)); FREE(p);
- btrask 6y agoTaking a pointer-to-pointer is intentional to make it clear that the pointer will be modified. That's actually the most important difference from nn3's version IMHO.