Thread: Program works under C++ but not under C

This program is able to compile and run succesfully under C++, but not C. The program gives a AccessViolation error message on the first line using NULL. Is there some type of headfile I'm missing? I thought iostream.h allowed me to use null.
I'm using Borland C++ builder ver 5 and 6.

Code:

#include <stdio.h>
#include <conio.h>
#include <process.h>
#include <string.h>
#include <stdlib.h>
#include <iostream.h>

int main()
{
   char  starting_time[10], ending_time[10];
   int   starting_hour = 0, ending_hour = 0, starting_minute = 0,
         ending_minute = 0, hour, minute;

   printf("Enter the starting time: ");
   scanf("%c", &starting_time);
   printf("Enter the ending time: ");
   scanf("%c", &ending_time);

   starting_hour = atoi( strtok( starting_time, ":" ) );
   starting_minute = atoi( strtok( NULL, ":" ) );
   ending_hour = atoi( strtok( ending_time, ":" ) );
   ending_minute = atoi( strtok( NULL, ":" ) );

   hour = ending_hour - starting_hour;
   minute = ending_minute - starting_minute;

   if ( hour < 0 )
   {
      hour = hour + 24;
   }
   if ( minute < 0 )
   {
      minute = minute + 60;
      hour = hour - 1;
   }

   printf("Elapsed time is: %d : %d", hour, minute);

   getch();
   return 0;
}