Pipe: How do I ensure if I successfully write into the pipe? - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#define SIZE 50
struct record {
int freq;
char word[SIZE];
};
int main(){
int number_process = 3;
int pipes[number_process][2];
struct record r1;
r1.freq = 10;
strcpy(r1.word, "Cat");
struct record r2;
r2.freq = 20;
strcpy(r2.word, "Elephant");
struct record r3;
r3.freq = 30;
strcpy(r3.word, "Dragon");
struct record records_array[3] = {r1, r2, r3};
for (int i = 0; i < number_process; i++){
if (pipe(pipes[i]) == -1){
perror("pipe");
exit(1);
}
// Create children.
pid_t fork_result = fork();
if (fork_result == -1){
perror("Parent fork");
exit(1);
} else if (fork_result == 0){
if (close(pipes[i][0]) == -1){
perror("Child closes reading port");
exit(1);
}
// Later children is going to close all reading port from pipe that parent creates.
for (int child_no = 0; child_no < i; child_no++) {
if (close(pipes[child_no][0]) == -1) {
perror("close reading ends of previously forked children");
exit(1);
}
}
// Now, I am trying to write each strct record member from the above array into the pipe
// when I run the program, it won't allow me to do so because of bad file descriptor exception.
for (int j = 0; j < number_process; i++){
if (write(pipes[i][1], &(records_array[j]), sizeof(struct record)) == -1){
perror("write from child to pipe");
exit(1);
}
}
// Finishing writing, close the writing end in pipe.
if (close(pipes[i][1]) == -1){
perror("Child closes writing port");
exit(1);
}
// Terminate the process.
exit(0);
} else {
// Parent is closing all the writing ends in pipe.
if (close(pipes[i][1]) == -1){
perror("Parent close writing");
exit(1);
}
}
}
struct record buffer;
for (int i = 0; i < number_process; i++){
// Parent reads from the pipe.
if (read(pipes[i][0], &buffer, sizeof(struct record)) == -1){
perror("parent read");
exit(1);
}
printf("buffer.freq = %d\n", buffer.freq);
printf("buffer.word = %s\n", buffer.word);
}
return 0;
}
I am new to system programming, the following code is some practice I implement to see the pipe functionality. I have a few questions to my code:
1) Is there any system call or library call that will help me to ensure the content that I want to write into the pipe has actually been successfully written into the pipe? In other words, is there any methods available for me to check the content/data I wrote into the pipe?
2) I feel my parent reading part is not implemented correct, when I run this code, my parent reading part reads consecutive 3 same things, although it should be different things each time it read.
3) I have confronted the bad addresses issue when I try to read something from the pipe in my parent process, what is the reason that this error occurs?
Can someone please help me understand this stuff? Really appreciated.

Answer 1

You've got a simple cut-n-paste error. for (int j = 0; j < number_process; i++){
You need to increment j.