结合fork，fifo和execlp？

Question

int main(){
  mkfifo("view",0666);
  int pid = fork();
  if(pid==0){
    close(1);
    int fd = open("view",O_WRONLY);
    dup(fd);
    execlp("cat", "cat", "users", NULL);
    close(fd);
  }
  else{
    wait(NULL);
    int fd = open("view",O_RDONLY);
    char resp[100];
    read(fd,resp,20);
    printf("%s\n",resp);
    close(fd);
  }
}

I have this piece of code but for some reason when I execute it, the procces freezes with no print and with no exit(i have to press CTRL+C). Any idea why? I tried the same thing, but using internal pipe and it worked.

Answer 1

It works perfect if I remove wait(NULL) from the parent process. But I don't really know why.

Answer 2

From the man mkfifo :

Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.

The parent process waits in wait(NULL) for the child finish, but the child waits for someone to open "view" on the other end after the call open("view",O_WRONLY) .

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To have a valid null terminated string you also need to initialize resp by zero char resp[100] = { 0 }; or put zero at the end of read output:

ssize_t size = read(fd,resp,20);
resp[size] = '\0';