Does malloc and new know about each other?

Question

Assume I have 10 kB heap and mix C and C++ code like that.

char* block1 = malloc(5*1024); //allocate 5
char* block2 = new[](4*1024); // allocate 4

Is there C heap and C++ heap or just a single heap common for both? So that "new" knows that the first 5 kb of heap are already allocated?

Answer 1

There may or may not be separate C and C++ heaps. You can't write a conforming C++ program that can tell the difference, so it's entirely up to the implementation.

The standard describes the first step in the default behavior of operator new like this:

Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the C standard library functions malloc or aligned_alloc is unspecified. [new.delete.single]/4.1.

And for malloc itself, the standard says: "[aligned_alloc, calloc, malloc, and realloc] do not attempt to allocate storage by calling ::operator new()" [c.malloc]/3.

So the intention is that it's okay to call malloc from operator new, but it's not required.

In practice, operator new calls malloc.