exit(0) //正常退出当前进程
fork() //之后的语句，子进程和父进程同时运行。但是在子进程中fork返回0，在父进程中fork返回大于1的进程ID。也就是说，你定义的I在父进程和子进程中有不同的值。
if(fork()==0)
通过一个小例子分析fork的工作原理。
#include <stdio.h>#include <unistd.h>
int main()...{
char father[]="father"; char child[]="child"; int nPid,nFatherPid; //asigned and then compared. if((nPid=5)==5)printf("success %d! ",(nPid=3)); nFatherPid=getpid(); printf("father getpid()=%d ",getpid());
if((nPid=fork())==0)...{// child process,in child process fork() return 0 printf("child getpid()= %d nPid=%d ",getpid(),nPid);
} if(nPid==nFatherPid)...{ printf("father getpid()=%d, nFatherPid=%d ",getpid(),nFatherPid); } //father and child are also running. printf("father or child ,getpid()=%d nPid=%d ",getpid(),nPid); //child stoped. if(nPid==0)exit(0); printf("father nPid=%d ",nPid); printf("getpid()=%d ",getpid()); return 1; 
} 
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