[OS] pipes

/* pipe2.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int main(int argc, char* argv[])
{
    int pipefds[2];
      pid_t pid;
    char buf[30];
    //create pipe
    if(pipe(pipefds) == -1){
        perror("pipe");
        exit(EXIT_FAILURE);
    }
      memset(buf,0,30);
      pid = fork();
    if (pid > 0) {
      printf(" PARENT write in pipe\n");
    //parent close the read end
          close(pipefds[0]);
    //parent write in the pipe write end                 
      write(pipefds[1], "CSCI3150", 9);
    //after finishing writing, parent close the write end
          close(pipefds[1]);
    //parent wait for child                
      wait(NULL);                         
    }else {
      //child close the write end  
      close(pipefds[1]);   //-----line *
      //child read from the pipe read end until the pipe is empty   
      while(read(pipefds[0], buf, 1)==1)   
        printf("CHILD read from pipe -- %s\n", buf);
      //after finishing reading, child close the read end
      close(pipefds[0]);
      printf("CHILD: EXITING!");
      exit(EXIT_SUCCESS);
    }
    return 0;
}

if “—–line *” commented out, We can see that the child process doesn’t exit. What’s going on? Note that the child process still has pipefds[1] open. The system will assume that a write could occur while any process has the write end open, even if the only such process is the one that is currently trying to read from the pipe, and the system will not report EOF, therefore. Thus child is waiting to read from pipefds[0], and the operating system doesn’t know that no process will be writing to pipdfds[1]. So the child process blocks until data arrives to be read(A3)