Is a malloc() needed before a realloc()?

Question

Since I had read realloc will act as malloc if the size pointed is 0, I was using it without malloc(), provided the pointer was static, global, or explicitly set to NULL if automatic.

However, I notice a lot of programmers try to set it or set it to malloc(1). Is it needed?

Answer 1

malloc is not required, you can use realloc only.

malloc(n) is equivalent to realloc(NULL, n).

However, it is often clearer to use malloc instead of special semantics of realloc. It's not a matter of what works, but not confusing people reading the code.

(Edit: removed mention of realloc acting as free, since it's not standard C. See comments.)

Answer 2

From Open Groups' specifications:

If ptr is a null pointer, realloc() shall be equivalent to malloc() for the specified size.

If ptr does not match a pointer returned earlier by calloc(), malloc(), or realloc() or if the space has previously been deallocated by a call to free() or realloc(), the behavior is undefined.