调用fork两次避免僵尸进程
Avoid zombie processes by calling fork twice
/* * Avoid zombie processes by calling fork twice. * APUE-2e 程序清单8-5 */#include <unistd.h>#include <sys/wait.h>#include <stdio.h>#include <stdlib.h>#include "sig_wait_proc.h"int main(){pid_t pid;if( (pid = fork()) < 0 ){printf("fork error.\n");exit(0);}else if(pid == 0)/* first child */{init_wait();if( (pid = fork()) < 0 ){printf("fork error.\n");exit(0);}else if(pid > 0){tell_proc(pid);/* tell child on exiting */exit(0);}/* We're the second child; our parent becomes init as soon as our real parent exits. */wait_proc();/* wait for parent */printf("second child, parent pid = %d\n", getppid());exit(0);}if(waitpid(pid, NULL, 0) != pid)/* wait for first child */{printf("waitpid error.\n");exit(1);}printf("parent, first child pid = %d\n", pid);exit(0);}说明：