free in c

the C library function void free(void *ptr) 
  deallocates the memory previously allocated by a call to calloc, malloc, or realloc.

//malloc or "memory allocation" reserves a part of the memory

pointer = (cast-type*) malloc(byte-size)
  
//Example
ptr = (int*) malloc(100 * sizeof(int));

Since the size of int is 4 bytes, this statement will allocate 400 bytes
of memory. And, the pointer ptr holds the address of the first byte in
the allocated memory.