Malloc_tutorial.pdf has many issues about c-how-to

[bradenbest]

Specifically, the adversarial code will make the program crash right here:

as reading from (void *)1 is illegal

free is expected to crash in a situation like this regardless. My point is that the check is unnecessary because the behavior is undefined. This creates a contract between the function and the caller that the caller must avoid doing that sort of thing. In other words, it is 100% valid and expected for free to assume the pointer is valid. Keep it simple. You're not implementing asan or valgrind here. And those tools use much more advanced techniques to detect and report memory errors. Not only do they lay their memory out in an unusual and inefficient way including things like "shadow bytes" and little traps, but they also literally read the binary to get the symbol table so they can tell you where you fucked up. If you're trying to catch memory errors, it is a much better idea to use these mature and battle-tested tools designed for it than to write your own allocator that refuses to end the program even when it's in an invalid state.

It would be one thing to write your own custom free that checks for some obvious error and then kills the program with a helpful error message. But for libc free, that case is already covered by UB.

[bradenbest]

I just realized that I did not provide advice for calloc. So for comparison, here's what my implementation of calloc looks like: