0x0B-malloc_free

0x0B. C — Dynamic Memory Allocation

Automatic Allocation

When variables are declared or when strings within double quotes are used, the program takes care of all the memory allocation.

Dynamic Allocation

The concept of dynamic memory allocation in C language enables the programmer to allocate memory at runtime.

malloc()

The malloc() ("memory allocation") function is used to allocate a certain amount of memory during the execution of a program. It will request a block of memory from the heap.

If the request is granted, the operating system will reserve the requested amount of memory and malloc() will return a pointer to the allocated memory. This memory has read and write permissions.

malloc() does not initialize memory, so each block is initialized with the default garbage value initially.

The return type is a pointer of void which can be cast into pointers of any type.

Syntax:

ptr = (castType*) malloc(size);

where size is the number of bytes to be allocated.

If space is insufficient, allocation fails and the function returns a NULL pointer.

The memory that is allocated with malloc() is not automatically released when the function returns, hence the need to de-allocate the allocated memory.

calloc()

calloc or “contiguous allocation” method in C is used to dynamically allocate the specified number of blocks of memory of the specified type. It is very much similar to malloc() but has two different points and these are:

• It initializes each block with a default value ‘0’.
• It has two parameters or arguments as compare to malloc().

Syntax:

ptr = (cast-type*)calloc(n, element-size);

Where n is the number of elements and element-size is the size of each element. If the space is insufficient, allocation fails and a NULL pointer is returned.

free()

When the amount of memory is not needed anymore, it must be returned to the operating system by calling the function free(). This function de-allocates the memory dynamically allocated using malloc().

Syntax:

free(ptr);

This statement frees the space allocated in the memory pointed by ptr.

realloc()

realloc (or “re-allocation”) method in C is used to dynamically change the memory allocation of a previously allocated memory. If the memory previously allocated with the help of malloc or calloc is insufficient, realloc can be used to dynamically re-allocate memory. Re-allocation of memory maintains the already present value and new blocks will be initialized with the default garbage value.

Syntax:

ptr = realloc(ptr, new_size);

Where ptr is reallocated with new size new_size. If the space is insufficient, allocation fails and a NULL pointer is returned.