Question

linux C编程问题，子进程往pipe里写，子进程再fork出来的子进程为什么读不了，求大神解答。

#include<sys/types.h>
#include<unistd.h>
#include<cstring>
#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
int main(){
int fd[2];
if(pipe(fd)!=0){
std::cout<<"pipe failed"<<std::endl;
exit(0);
}
pid_t pid=fork();
if(pid<0){
std::cout<<"fork failed"<<std::endl;
exit(0);
}else if(pid > 0){
exit(0);
}else if(pid == 0){
close(fd[0]);
fd[0]=-1;
int n_w = write(fd[1],"XXXXXX",10);
std::cout<<"child1 write :"<<n_w<<"long words"<<std::endl;
pid_t pid1=fork();
if(pid1<0){
std::cout<<"fork1 failed"<<std::endl;
exit(0);
}else if(pid1 > 0){
exit(0);
}else{
std::cout<<"chiled2 process"<<std::endl;
close(fd[1]);
fd[1]=-1;
char buf[1024]={0};
int n_r = read(fd[0],&buf,10);
std::cout<<buf<<":"<<n_r<<std::endl;
if(n_r == -1){
std::cout<<"errno:"<<errno<<std::endl;
}
std::cout<<strlen(buf)<<std::endl;
}
}
return 0;
}
本人菜鸟一枚，想学习下pipe，试验了父进程写，孙子进程读；子进程1写，子进程2读都没问题……
写了个子进程写，孙子进程读，读不出来，求大神解答，程序执行结果如下：
child1 write :10long words
chiled2 process
:-1
errno:9
0

Answer 1

22 行在子进程开始把读文件描述符关了, 孙子当然读不出来. 改了就行了:

#include<sys/types.h>
#include<unistd.h>
#include<cstring>
#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<errno.h>
int main(){
 int fd[2];
 if(pipe(fd)!=0){
  std::cout<<"pipe failed"<<std::endl;
  exit(0);
 }
 pid_t pid=fork();
 if(pid<0){
  std::cout<<"fork failed"<<std::endl;
  exit(0);
 }else if(pid > 0){
        exit(0);
 }else if(pid == 0){
  /** 孙进程还要读,这里不能关掉 
  close(fd[0]);
  fd[0]=-1;
  */
  int n_w = write(fd[1],"XXXXXX",10);
  std::cout<<"child1 write :"<<n_w<<"long words"<<std::endl;
  pid_t pid1=fork();
  if(pid1<0){
   std::cout<<"fork1 failed"<<std::endl;
   exit(0);
  }else if(pid1 > 0){
        exit(0);
  }else{
   std::cout<<"chiled2 process"<<std::endl;
   close(fd[1]);
   fd[1]=-1;
   char buf[1024]={0};
   int n_r = read(fd[0],&buf,10);
      std::cout<<buf<<":"<<n_r<<std::endl;
      if(n_r == -1){
        std::cout<<"errno:"<<errno<<std::endl;
       }
   std::cout<<strlen(buf)<<std::endl;
  }
}
return 0;
}

执行结果:

child1 write :10long words
chiled2 process
XXXXXX:10
6