Communication between parent and child process using pipe in c++ - c++

Question

I wanted to do this problem but I cannot take input message:
Create a program that does the following:
1.Create a parent and a child process
2.The parent process reads a number from keyboard and sends it to the child
3.The child calculates if the given number is prime or not and prints the result on screen
This is my code:
#include <iostream>
#include <unistd.h> // for fork()
#include <string.h> // for strerror()
#include <sys/wait.h>
#include <sys/types.h>
using namespace std;
bool isprime(int number);
int main()
{
int num;
pid_t pid;
int fd[2];
char buffer[100];
pipe(fd);
pid = fork();
//parent process
if (pid > 0)
{
cin>>num;
write(fd[1], &num, sizeof(num));
close(fd[1]);
int status;
//Do not check for errors here
wait(&status);
}
//child process
else if (pid == 0)
{
read(fd[0], buffer, 100);
close(fd[0]);
if (isprime(num))
{
cout<<"number is prime";
}
else
{
cout<<"number is not prime";
}
return EXIT_SUCCESS;
}
else
{
cout << "fork() failed (" << strerror(errno) << ")" << endl;
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
bool isprime(int number)
{
if (number < 2)
return false;
if (number == 2)
return true;
for (int i = 2; (i*i) <= number; i++)
{
// Take the rest of the division
if ((number % i) == 0)
return false;
}
return true;
}
this my result of run

Answer 1

Using a pipe along with fork is not that hard, but you must respect some rules:
each part should close the handle that it does not use. Not doing it is a key for future problems
starting from the fork, changes in one process are not reflected in the other one
Your code should become:
...
//parent process
if (pid > 0)
{
close(fd[0]); // close the part that only the other process will use
cin>>num;
write(fd[1], &num, sizeof(num));
close(fd[1]);
int status;
//Do not check for errors here
wait(&status);
}
//child process
else if (pid == 0)
{
close(fd[1]); // close the part used by the other process
read(fd[0], &num, sizeof(num)); // read into num what the parent has written
close(fd[0]);
...
In real world code, you should check that every read is successfull (both from cin and from the pipe...)