Why does adding to a pointer with += work, but pointer + 1 doesn't?

Question

I am allocating memory for an array, but I am moving where the pointer points forward a little. Accessing the elements works fine. It started to produce a problem with freeing the allocated memory though. Malloc complains that the pointer being freed was never allocated. The problem can reproduced with this simplified code:

int *pointer = malloc(sizeof(int)) + 1;
free(pointer - 1);

I started experimenting, and found this slight variation of the code to work.

int *pointer = malloc(sizeof(int));
pointer += 1;
free(pointer - 1);

What is the += doing different than just adding 1 to the pointer malloc returns in one line?

Answer 1

The return type of malloc is void *, which, according to the standard, cannot be used for pointer arithmetic.

There is a GNU extension, however, that treats sizeof(void) as 1 and that is why your first fragment compiles. Later on, pointer - 1 subtracts sizeof(int) and not 1, hence the pointer mismatch.

The actual offsets applied when doing pointer arithmetic always depend on the type of the pointer. Since pointer is an int *, the actual offsets will be multiplied by sizeof(int). If sizeof(void) is 1 or if it's a char * pointer (sizeof(char) == 1), no multiplication takes place.

Answer 2

Pointer arithmetic can be done using either of + or +=. It has nothing to do with the error produced.

malloc returns void * type. C standard says that:

C11: 6.2.5 Types (p19):

The void type comprises an empty set of values; it is an incomplete type that cannot be completed.

6.5.6 Additive operators (p2):

For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

void * is pointer to incomplete type. Therefore, no addition will be performed with void * type. That's why

int *pointer = malloc(sizeof(int)) + 1;  

is wrong and will lead to run time error on executing free(pointer - 1);.