c语言中创建子进程运行外部程序

我想通过自己的程序启动一个外部程序（比如aaa.o)，然后在某些条件下关闭这个程序。光用system()不够吧，应该要创建子进程吧？请教关键几步应该怎么写啊？

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main()
{
int pid1, pid2, pid3, pid4, pid5, pid6;

pid1 = getpid();
printf("PID = %d\n", pid1);

pid2 = fork();
if (pid2 == 0)
{
printf("PID = %d, Parent PID = %d\n", getpid(), getppid());
sleep(30);
exit(0);
}

pid3 = fork();
if (pid3 == 0)
{
printf("PID = %d, Parent PID = %d\n", getpid(), getppid());

pid5 = fork();
if (pid5 == 0)
{
printf("PID = %d, Parent PID = %d\n", getpid(), getppid());

pid6 = fork();
if (pid6 == 0)
{
printf("PID = %d, Parent PID = %d\n", getpid(), getppid());
sleep(30);
exit(0);
}

sleep(30);
exit(0);
}

sleep(30);
exit(0);
}

pid4 = fork();
if (pid4 == 0)
{
printf("PID = %d, Parent PID = %d\n", getpid(), getppid());
sleep(30);
exit(0);
}

sleep(30);
return 0;
}
每一个进程都会在退出前 sleep 30秒，从而保证能够用 ps 看到，
编译 gcc testpid.c -o testpid
然后执行，可以看到
PID = 24913
PID = 24914, Parent PID = 24913
PID = 24916, Parent PID = 24913
PID = 24915, Parent PID = 24913
PID = 24917, Parent PID = 24915
PID = 24918, Parent PID = 24917

ps -ef 的结果
24913 24582 0 11:29 pts/19 00:00:00 ./testpid
24914 24913 0 11:29 pts/19 00:00:00 ./testpid
24915 24913 0 11:29 pts/19 00:00:00 ./testpid
24916 24913 0 11:29 pts/19 00:00:00 ./testpid
24917 24915 0 11:29 pts/19 00:00:00 ./testpid
24918 24917 0 11:29 pts/19 00:00:00 ./testpid

#include int main() { int pid; pid=fork(); if(pid==0) printf("i am son,my pid is %d.\n",getpid()); else if(pid>0) { printf("i'm father ，my pid is %d.\n",getpid()); pid=fork(); if(pid==0) printf("i'm daughter process,my pid is %d.\n",getpid()); else if(pid>0) printf("i'm father process,my pid is %d.\n",getpid()); else printf("fork() error.\n"); } else printf("fork() error.\n"); }