#include <stdlib.h>
  void free( void* ptr );

The free() function deallocates the space pointed to by ptr, freeing it up for future use. ptr must have been used in a previous call to malloc(), calloc(), or realloc(). An example:

   typedef struct data_type {
     int age;
     char name[20];
   } data;              

   data *willy;
   willy = (data*) malloc( sizeof(*willy) );
   ...
   free( willy );