这段code运行时为什么会有segmentation fault？
用的是nachos（一个模拟操作系统的教学软件）
#include "syscall.h"
#include "stdlib.h"
#define OUTPUT_LOOP_LIMIT 500
/*
* Set this up to experiment with different nice values. Negative
* values can cause unexpected execution orders. Figuring out WHY
* is an excellent exercise for the student.
*/
#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);
/* Divorce the test processes from the main process. The main
* thread will create the others and then simply wait for them to
* finish. This step is taken to ensure that the each of the four
* threads is treated as similarly as possible.
*/
/* Commence the forking */
pid = Fork();
if (pid == 0)
{
/*
* Child #1: This will fork the other test processes as well
* as becoming T-A
*/
/* Child #1 Forks a Grandchild #1 */
pid = Fork();
if (pid != 0) {
/*
* Child #1
* Now we fork a second time.
* Grandchild #2 is created
*/
pid = Fork();
if (pid != 0) {
/*
* Child #1: The first Child process makes it here. Now we
* adjust its static priority by calling the NICE system
* call, and then looping to output a series of
* identifying characters.
*/
Nice (A_NICE);
for (a = 0; a < OUTPUT_LOOP_LIMIT; a++) {
Echo ("A", 1);
}
Exit (0);
} else {
/*
* Grandchild #2
* Now we adjust its static priority by calling the
* NICE system call, and then looping to output a series of
* identifying characters.
*/
Nice (B_NICE);
for (b = 0; b < OUTPUT_LOOP_LIMIT; b++) {
Echo ("B", 1);
}
Exit (0);
}
} else {
/*
* Grandchild #1 forks GreatGrandchild #1
*/
pid = Fork();
if (pid != 0) {
/*
* GrandChild #1
* Now we adjust its static priority by calling the
* NICE system call, and then looping to output a series of
* identifying characters.
*/
Nice (C_NICE);
for (c = 0; c < OUTPUT_LOOP_LIMIT; c++) {
Echo ("C", 1);
}
Exit (0);
} else {
/*
* GreatGrandchild #1
* Now we adjust its static priority by calling the
* NICE system call, and then looping to output a series of
* identifying characters.
*/
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 */
}
