Question

The program can read all the data from the pipe. However, the program just stop. It cannot continue to process. I think it stop in the child process.

//I created two pipe before fork (fd[0] and fd[1]).
//child process
if(pid == 0){
    close(fd[a][1]);
    buf[6];
    int i;
    while ((i = read(fd[a][0], buf, 6)) > 0) {
        printf("%s", buf);     
    }
    close(fd[a][0]);
exit(0); 
}

//parent process
write(fd[a][1], "12", 2);
write(fd[a][1], "14", 2);
write(fd[a][1], "15", 2);
write(fd[b][1], "12", 2);
write(fd[b][1], "14", 2);
write(fd[b][1], "15", 2);
printf("done!\n");
close(fd[0][1]);
close(fd[1][1]);
wait(NULL);
printf("Really done!!!\n");
...                      // The program cannot run after wait(NULL);

--output--
121415done

Answer 1

You are issuing read in a while loop, effectively blocking yourself until something goes wrong. If you need to read 6 characters, this is how you would write the child process:

char buf[7];
int left = 6;
int so_far = 0;   // instead of so_far, (6 - left) could also be used
while (left > 0)
     int num_read = read(fd[0][0], buf + so_far, left);
     left -= num_read;
     so_far += num_read;
}
buf[so_far] = '\0';
printf("%s\n", buf);