c - When we used malloc without declaring stdlib.h header file compiler returns an int why?

Question

Earlier I asked one question that is if the return of malloc is cast then the error which would be flagged is hidden and I got the answer that is: In (older dialects of) C, you can call a function which hasn't been declared; that implicitly declares a function with a return type of int. So, if you were to call malloc() without including the header, you'd get code that erroneously assumed that it returned int. Without a cast, you'd get a compiler error when you tried to assign it to a pointer. With a cast, the code would compile, potentially giving obscure runtime errors and lengthy debugging sessions.

I understand that without inclusion of <stdlib.h> header file compiler implicitly declares a function with a return type of int.

But I confused in that according to malloc()'s function definition that is defined by creator, it will returns void * whenever we used it, but without <stdlib.h> it returns int. So how does its definition change, is compiler implicitly changing void * to int * type, or there is some other reason?

I know that I couldn't explain my question properly but if anybody understand, please explain me about that.

Answer 1

For C, all undeclared functions are implicitly declared to return int and take an unspecified number of unspecified arguments.

The problem with malloc is that an int and a void * may not be the same size, or even compatible. For example, on modern 64-bit machines pointers are 64-bit types while int is a 32-bit type, so if you don't have a proper declaration of malloc the compiler will drop the high 32 bits of the pointer.