Dynamic Memory

Currently, we can only declare arrays to be of a constant size (like 10). This is not always convenient – sometimes we want to make the size based on some user input. If we want to allocate a dynamic amount of space, we need to use C’s dynamic memory functions. Each of these functions is in <stdlib.h>.

malloc

This function allocates a contiguous block of memory with the specifies size (number of bytes). It returns a void pointer to the block of memory. (This pointer will be automatically cast to the correct type when you store it.)

Here is the prototype:

void* malloc(int numBytes);

For example, we could allocate an array like this:

int nums1[5];

Or we could do the same thing using malloc. If we use malloc, we need to specify the number of bytes to reserve. We want 5 ints, and each int takes up sizeof(int) bytes. So, the total needed is 5*sizeof(int):

//The result of malloc is automatically cast to an int*
int* nums2 = malloc(5*sizeof(int));

Now, we can treat nums2 just like an array. For instance, if we wanted to initialize all elements in nums2 to 0:

int i;
for (i = 0; i < 5; i++) {
	nums2[i] = 0; 	//The compiler converts this to *(nums2+i) = 0
}

Allocating arrays with malloc has several key difference from standard array allocation:

malloc can handle a variable for the desired size; a standard array cannot
The result of malloc is a pointer; the result of a standard array allocation is a constant pointer
malloc memory is allocated on the heap. If there is not enough space to do the allocation, malloc will return NULL. An array is allocated on the program stack – if there is not enough space, the program simply won’t compile.

calloc

The calloc function is very similar to malloc. The only difference is that when arrays are allocated using calloc, all elements are automatically initialized to 0.

Here is the prototype:

void* calloc(int numElems, int sizePerElem);

The prototype of calloc is also a little different than the one for malloc. It takes two arguments – the number of elements you want in the array, and the number of bytes needed for each elements. Like malloc, calloc returns a void pointer to the contiguous block of memory it allocated. This pointer will be automatically cast to the appropriate type when you store it.

Here’s how to create an array of 10 ints, all initialized to 0:

int* nums = calloc(10, sizeof(int));

Now you can use nums just like an array. For example:

nums[0] = 4;

Like malloc, calloc will return NULL if there is not enough space to do the allocation. In both cases, it’s a good idea to check if the pointer is NULL before you use it.

For example:

int* nums = calloc(10, sizeof(int));
if (nums == NULL) {
	printf("Not enough space.\n");
}
else {
	//Use nums as usual
}

realloc

The realloc function allows you to easily expand and shrink the space allocated for an array.

Here is the prototype:

void* realloc(void* origPtr, int newSize);

This function takes your original pointer and the desired new size in bytes. It looks for a contiguous block of memory with the desired size. If it can find one, it copies the contents of the old array into the new block of memory. Then it releases the space needed for the old array, and returns a void pointer to the new block of memory.

The realloc function doesn’t always behave as you intend. Here are the possible return values of realloc:

NULL (if not enough space is found)
The original pointer (if there is enough space at that location)
A new pointer to a different spot in memory

Suppose we allocate the nums array like this:

int* nums = malloc(10*sizeof(int));

Now we decide that we want nums to hold 15 elements instead of 10. Here’s what we might try:

nums = realloc(nums, 15*sizeof(int));

Suppose that realloc could not find enough space to grant the request – so it returns NULL. This means that we assign NULL to our original nums pointer. Now nums does not reference the original array – in fact, nothing does. The original array is stuck in memory with no way to get at it – this is called a memory leak.

To fix this problem, assign a temporary pointer to the result of realloc. Then, if it’s not NULL, reassign the original pointer. This keeps you from losing your array:

int *temp = realloc(nums, 15*sizeof(int));
if (temp != NULL) {
	nums = temp;
}

free

In Java and C#, any memory that you’re no longer using will be cleaned up by the garbage collector. C has no garbage collector, so you are in charge of releasing memory that you’re done with. If you never release any allocated memory, you will eventually run out of space.

The C function that releases dynamic memory is called free. Here is the prototype:

void free(void* pointer);

Note that even though free takes a void pointer, it can take any type of pointer that has been dynamically allocated.

Here’s an example of using free:

int* nums = malloc(10*sizeof(int));
int i;
for (i = 0; i < 10; i++) {
	nums[i] = i;
}

//done using nums
free(nums);