How to play with pointers in C

When a variable is declared compiler automatically allocates memory for it. This is known as compile time memory allocation or static memory allocation. Memory can be allocated for data variables after the program begins execution. This mechanism is known as runtime memory allocation or dynamic memory allocation.

The memory allocated at compile time is deallocated (i.e., the memory is released so that the same memory can be reused by other data) automatically at the end of the scope of the variable. In the example below the memory allocated for the variable i will be deallocated when the closing brace (}) of the statement block is encountered (considering no compiler optimization).

{
    int i = 0;
}

C provides several functions in stdlib library for dynamic memory allocation. It's easy to both use and misuse these functions. One should write a program following best coding practices when dynamic memory allocation library functions are used.

Memory Allocation With calloc

Given a number of objects to be allocated and size of each object calloc allocates memory. calloc returns a pointer to the first element of the allocated elements. If memory cannot be allocated, calloc returns NULL. If the allocation is successful, calloc initializes all bits to 0.

#define NUMBER_OF_ELEMENTS 100

int32_t *parr = calloc(NUMBER_OF_ELEMENTS, sizeof(int32_t));

if (parr == NULL) /* Memory allocation fails */
{
	printf("Couldn't allocate memory");
}
else  /* Memory allocation successful */
{
	printf("Memory allocation successful");
}

In the example above memory allocation of NUMBER_OF_ELEMENTS 32 bit integers is requested by calloc. If the memory allocation is successful then parr will point to the first element of the allocated memory. Using parr the allocated memory can be used as array. All integers in the array pointed to by parr is initialized to 0.

Memory Allocation With malloc

malloc tries to allocate a given number of bytes and returns a pointer to the first address of the allocated region. If malloc fails then a NULL pointer is returned. malloc doesn't initialize the allocated memory and reading them without initialization invokes undefined behaviour.

The above example of calloc can be implemented as below with malloc:

#define NUMBER_OF_ELEMENTS 100

int32_t *parr = malloc(NUMBER_OF_ELEMENTS * sizeof(int32_t));

if (parr == NULL) /* Memory allocation fails */
{
	printf("Couldn't allocate memory");
}
else  /* Memory allocation successful */
{
	memset(parr, 0, NUMBER_OF_ELEMENTS * sizeof(parr[0]));

	printf("Memory allocation successful");
}

To use the function memset header file <string.h> must be included. Given a pointer and number of bytes, memset initializes all bytes to zero.

Memory Reallocation With realloc

Given a pointer to a previously allocated region and a size, realloc deallocates previous region, allocates memory having the new size and copies old content into the new region upto the lesser of old and new sizes. realloc returns a pointer to the first element of the allocated memory. If new memory allocation fails, old content isn't deallocated, the value of the old content is unchanged and realloc return NULL.

#define NUMBER_OF_ELEMENTS 100

int32_t *parr = calloc(NUMBER_OF_ELEMENTS, sizeof(int32_t));

if (parr == NULL) /* Memory allocation fails */
{
	printf("Couldn't allocate memory");
}
else  /* Memory allocation successful */
{
	printf("Memory allocation successful\n");

	parr = realloc(parr, (NUMBER_OF_ELEMENTS / 2) * sizeof(int32_t));

	if (parr == NULL) /* Memory reallocation fails */
	{
		printf("Memory reallocation fails");
	}
	else /* Memory reallocation successful */
	{
		printf("Memory reallocation successful");
	}
}

In the above example, realloc is used to reduce the dynamic array size to half.

If the first parameter to realloc is NULL, then realloc behaves like malloc.

Deallocation Of Allocated Memory With free

The function free takes a pointer as parameter and deallocates the memory region pointed to by that pointer. The memory region passed to free must be previously allocated with calloc, malloc or realloc. If the pointer is NULL, no action is taken.

Warning: If the pointer passed to free doesn't match to a memory region previously allocated by memory management function or is already passed to free or realloc to deallocate, then the behaviour is undefined.

It's a good practice to set the pointer value to NULL once it is passed to free to avoid accidentally triggered undefined behaviour.

Following example allocates dynamic memory, initializes array elements with random number, reduces the size of the array to half with realloc and output before and after the reallocation proves that realloc kept old values in the array.

Notice that regardless of how much bytes the whole array pointed to by parr is taking, sizeof(parr) equals to the size of a pointer variable in your platform.