an practice problem on C and OS

I am learning OS and using nachos as an instruction tool. Anyway, here is a question. The following code will lead to a "segmentation fault". Specifically, the program runs well until the thread which prints "D" finishes (reaches the upper limit).
Do you have any idea why? Thanks.

Code:

/* nice_free.c 
 *   Test program to run several threads with different priorities.
 *   Each thread uses the NICE system call to adjust its static 
 *   priority by various amounts, to influence execution order. 
 *
 *   This version of nice does not use the console, it uses a
 *   debugging system call, Echo, instead.
 */

#include "syscall.h"
#include "stdlib.h"

#define OUTPUT_LOOP_LIMIT 500
#define A_NICE 0
#define B_NICE 5
#define C_NICE 10
#define D_NICE 15

int
main()
{
    int a,b,c,d;
    int pid;

    Echo ("Starting...\n", 12);

    pid = Fork();

    if (pid == 0)
      {

    pid = Fork(); 

    if (pid != 0) {

      pid = Fork(); 
      if (pid != 0) {

        Nice (A_NICE);
            for (a = 0; a < OUTPUT_LOOP_LIMIT; a++) {
          Echo ("A", 1);
            }
        Exit (0);
      } else {

        Nice (B_NICE);
            for (b = 0; b < OUTPUT_LOOP_LIMIT; b++) {
          Echo ("B", 1);
            }
        Exit (0);
      }
    } else {

      pid = Fork(); 
      if (pid != 0) {

        Nice (C_NICE);
            for (c = 0; c < OUTPUT_LOOP_LIMIT; c++) {
          Echo ("C", 1);
            }
        Exit (0);
      } else {

        Nice (D_NICE);
            for (d = 0; d < OUTPUT_LOOP_LIMIT; d++) {
          Echo ("D", 1);
            }
        Exit (0);
      }
    }
      }
    /*Original Thread*/
    Wait (0);
    Echo ("\nFinished\n", 10);
    Exit(0);        /* and then we're done */
}

Can you post the definitions of all the functions used in your code viz., Fork(), Nice(), Echo(), etc.

check the prototype for Fork. what is it in 'syscall.h'?

Thanks for attention.

Fork() just forks a child process, nothing special.
Echo() works like fprint(). The number indicates the number of characters. There is no problem there.
Nice() is used the adjust the static prioty. A higher value gives higher priority to be excuted.

you should have posted the actual prototypes for those calls. its hard to help when you give vague responses. a google search for Nacho syscall.h shows a different signature for Fork than what you are using. I assume there are different versions so that might not matter.