malloc

    #include <stdlib.h>
    void *malloc(size_t size);

The malloc() function shall allocate unused space for an object whose size in bytes is specified by size and whose value is unspecified.

The order and contiguity of storage allocated by successive calls to malloc() is unspecified.
The pointer returned if the allocation succeeds shall be suitably aligned so that it may be assigned to a pointer
to any type of object and then used to access such an object in the space allocated (until the space is explicitly freed or reallocated).
Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points
to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer shall be returned.
If the size of the space requested is 0, the behavior is implementation-defined: the value returned shall be either a null pointer or a unique pointer.

RETURN VALUE

Upon successful completion with size not equal to 0, malloc() shall return a pointer to the allocated space.
If size is 0, either a null pointer or a unique pointer that can be successfully passed to free() shall be returned.
Otherwise, it shall return a null pointer and set errno to indicate the error.

/* 
 * malloc example code
 * http://code-reference.com/c/stdlib.h/malloc
 */
#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <stdlib.h>
 
int testit ( int *x )
{
     if (x != NULL) {
         printf("allocation of memory was successfully.\n");
     }
     else {
          printf("allocation of memory failed.\n");
     }
return 0;
}
 
int main ( void )
{
  int *buffer, i;
 
  printf("how many int to malloc ?: ");
  scanf("%i", &i);
 
  printf("set buffer to %i * %i = %d bytes\n",i, sizeof(int), i*sizeof(int));
 
  buffer = (int*) malloc (i*sizeof(int)); // i * size of int... eg. 100 * 4byte
  testit(buffer); 
 
  printf("try to free the buffer\n");
  free (buffer);
  return 0;
}

  Output allocation successfully:
  user@host:~$  ./malloc 
  how many int to malloc ?: 999999
  set buffer to 999999 * 4 = 3999996 bytes
  allocation of memory was successfully.
  try to free the buffer

  Output allocation failed:
  user@host:~$  .//malloc 
  how many int to malloc ?: 1234567890123
  set buffer to 2147483647 * 4 = -4 bytes
  allocation of memory failed.
  try to free the buffer