1- Two types of dynamic memory allocation one is calloc and other is malloc.
There are two differences. First is in the number of arguments. Malloc() takes a single argument (memory required in bytes), while calloc() needs two arguments.
malloc() does not initialize the memory allocated, while calloc() initializes the allocated memory to ZERO.

calloc() allocates a memory area, the length will be the product of its parameters. calloc fills the memory with ZERO's and returns a pointer to first byte. If it fails to locate enough space it returns a NULL pointer.

Syntax: ptr_var=(cast_type *)calloc(no_of_blocks , size_of_each_block); i.e. ptr_var=(type *)calloc(n,s); malloc() allocates a single block of memory of REQUSTED SIZE and returns a pointer to first byte. If it fails to locate requsted amount of memory it returns a null pointer.

ptr_var=(cast_type *)malloc(Size_in_bytes);
The malloc() function take one argument, which is the number of bytes to allocate, while the calloc() function takes two arguments, one being the number of elements, and the other being the number of bytes to allocate for each of those elements. Also, calloc() initializes the allocated space to zeroes, while malloc() does not.

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If a pointer has already been allocated memory using malloc and later this allocated memory needs to be expanded for some reason, the realloc function is used. It will add the consecutive memory spaces to the existing if available, otherwise it will allocate whole new space for the same pointer.