Discussion about C array type semantics

Summary

C array types are weird In this article I’ll explain what I find weird about them, what I’d do differently, and ramble on a few related things. Technically speaking, an array type T[n] (for some n) is distinct from a pointer type T *. A value of type T[n] represents a contiguous sequence of T values in memory, n long. But you can’t actually refer to values of type T[n]. Any expression that would be of that type is immediately converted to a pointer, type T *, namely a pointer to the first element. Since the array indexing operator arr[ix] actually operates on pointers, acting like *(arr + ix), you can basically treat arrays like pointers. An important instance where this doesn’t happen is in sizeof arr, which returns sizeof(T) × n. int arr[3] = {10, 20, 30}; int *arr_ptr = arr; size_t arr_size = sizeof(arr); size_t ptr_size = sizeof(arr_ptr); Additionally, in function signatures, any array type you give to an argument is actually interpreted as a pointer instead. The n denoting the size is completely discarded. That means that, as an exception to the exception, sizeof arr in a function with an argument T arr[n] will not evaluate to sizeof(T) × n. size_t foo(char buf[6]) { return sizeof(buf); } char msg[6] = "!! ??"; size_t msg_size = sizeof(msg); size_t msg_size_in_fn = foo(msg); Note that you can write char buf[static 8] to “enforce” the length, but this just makes it undefined behaviour if you pass a pointer to a shorter array. Similar to restrict, all it does is aid the compiler in optimisation. Instead, you can use a pointer to the array as the argument. Instead of decaying to T *, a pointer to the first element, you can take a reference at the call site to get T (*)[n]. These are effectively the same thing at run-time, but this preserves the length information. It is inconvenient and confusing to write, though. size_t foo(char (*buf)[6]) { return sizeof(*buf); } char msg[6] = "?? !!"; size_t msg_size = sizeof(msg); size_t msg_size_in_fn = foo(&msg); Aside: Func...