The Size of an Array in C

Recently I saw a piece of code that looked a little something like this:

void zero_array(char array[])

{

memset(arr, 0, sizeof(array));

}

I have to say, I was surprised by this. How could a program know the size of an arbitrary array? I expected sizeof to always be determined at compile time, and furthermore I wouldn't have thought that the size of an array could be determined at runtime anyway. These aren't forth-style arrays, where the length is stored before the elements- you only have a pointer with no additional information.

As it turns out, C99 added semantics to sizeof such that, given an array as an argument, the size can be determined at runtime in the right circumstances. This is because a chunk of allocated memory has some (implementation specific) information associated with it by the memory manager.

This of course means that this will only work on pointers returned from the memory manager (or of a static size), which can't be expressed by the type and is entirely implicit in the structure of the program. I'm not sure I would use this very often, especially if I didn't have strict control over the origin of a pointer, but its interesting and possibly useful, and I wanted to record it here for future reference.