Casting malloc to a primitive type

Question

How is this allowed in C?

int p= (int) malloc (sizeof(int));

I only get a warning when I compile in gcc.

warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

A void pointer cannot be casted to a primitive type right?

*((int*)(x))//is allowed - assume x is of type void*

But how is direct cast to primitive also allowed?

yes. It's allowed so that you can cast a pointer to [`intptr_t` or `uintptr_t`](http://stackoverflow.com/q/6326338/995714). — phuclv, Dec 02 '16 at 06:27
@LưuVĩnhPhúc: The ability to cast pointers to integer types predates the existence of `intptr_t` and `uintptr_t`. — Keith Thompson, Dec 02 '16 at 06:47
C allows a lot of things that might be poor practice. The language depends on programmers to know what they're doing. — Keith Thompson, Dec 02 '16 at 06:47

score 4 · Accepted Answer · edited May 23 '17 at 11:45

This is not forbidden, but implementation defined behaviour. Quoting C11, chapter §6.3.2.3

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.

That is why compiler emits the warning.

That said, quoting chapter 7.20.1.4, Integer types capable of holding object pointers, we have intptr_t and uintptr_t.

The following type designates a signed integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:

intptr_t

The following type designates an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:

uintptr_t

Having said that, just for sake of completeness's sake, let me add that, there is fair amount of reasons on why not to cast the return value of malloc() and family in C..

Casting malloc to a primitive type

1 Answers1