引用堆中分配的变量

Question

I recently picked up C but am a little confused about how we reference something allocated in the heap.

For example, assume we did something as follows:

void test(){

    int *a;
    a = (int*)(malloc(sizeof(int)*4));
    a[0] = 1;
    a[1] = 2;
    a[2] = 0;
    a[3] = 0;
}

Now, let's say I am in main, obviously, I cannot do something like a[0]=... outside the function as there is no reference to a , but it is allocated in the heap, so how would I make a reference to it? I am guessing I'd have to return an int* or store the address to a outside the function.

Answer 1

I am guessing I'd have to return an int* or store the address to a outside the function.

Yes, that is exactly how it works.

malloc allocates a block of memory for you and returns a pointer to the first byte of that memory block.

You must make sure to always keep a pointer to the memory block until you call free on it.

This can be done by either returning a pointer to it from the function:

int* test(void){
    int *a;
    a = (int*)(malloc(sizeof(int)*4));
    //... (Also check here that malloc didn't return NULL.)
    return a;
}

or by storing it somewhere else, eg an out-parameter passed to the function or a global variable.

If you leave the function without storing or returning a copy of the pointer anywhere, then you loose the last pointer to the memory block ( a ) and there will be no way to ever reach it again.

This latter situation is known as a memory leak .

store the address to a

It should be the address held by a , not the address of a . a 's value is the address of the allocated memory's first byte. The address of a is the address of the pointer variable itself.