waiting for 2 child processes - c

Question

I'm trying to write a C program in which the main process creates two children: Ping and Pong. Ping prints “ping” followed by a number, and Pong prints “pong” followed by a number, the output must be as the sample run in Figure 1:"
Here is what I tried to do:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
void main(){
//initializing pipes
int td[2];
int td2[2];
pipe(td);
pipe(td2);
int pid=fork();
if(pid){ //in parent process
int pid2=fork();
if(pid2){// still in parent process
//1st time
int number1;
printf("Enter a number: ");
scanf("%d",&number1);
write(td[1],&number1,sizeof(number1));
printf("<>");
write(td2[1],&number1,sizeof(number1));
printf("--");
//2nd time
int number2;
printf("Enter a number: ");
scanf("%d",&number2);
write(td[1],&number2,sizeof(number2));
printf("<>");
write(td2[1],&number2,sizeof(number2));
printf("--");
}
else{// in pong process
int number;
read(td2[0],&number,sizeof(number));
printf("pong%d \n",number);
}
}
else{ //in ping process
int number;
read(td[0],&number,sizeof(number));
printf("ping%d \n",number);
}
}//main end
explanation: the problem that I faced here, is that the pong gets printed before ping and the parent process doesn't wait for its children to end ( and some output gets printed after root/desktop etc..)
Another problem that I solved, I had read method in the parent process, it fixed my problem because I know that read forces the program to wait until something is written to the pipe, but in this case, we have "write" in the parent process, so the parent is not waiting
I also tried to implementing wait(NULL) but it doesn't seem to work.
any suggestion would be much appreciated

Answer 1

It appears that you (your instructor) are over-complicating.
So, you want your main() to do
int main(void) {
int n;
//set up pipes and forks
printf("Enter a number"); scanf("%d", &n);
// make child1 output " ping<n>\n"
puts(" <>");
// make child2 output " pong<n>\n"
printf(" --\nEnter a second number"); scanf("%d", &n);
// make child1 output " ping<n>\n"
puts(" <>");
// make child2 output " pong<n>\n"
puts(" -- THE END --");
// close pipes
return 0;
}
So, except for "ping" vs "pong" (and using different pipes) the children are absolutely identical. Maybe we can send the string with the number and keep the number of functions down? No ... what about setting the string when creating the process? This sounds better
// set up pipes and forks
int pipes1[2], pipes2[2];
pipe(pipes1);
if (fork() == 0) /* child #1 */ child("ping", pipes1);
close(pipes1[0]); // the read end of the pipe belongs to the child
pipe(pipes2);
if (fork() == 0) /* child #2 */ child("pong", pipes2);
close(pipes2[0]); // we are not using the read end of the pipe
Now... how do we make a child (children are waiting at their read() call) work? Well, easy! We write on our end of the pipe
scanf("%d", &n); // get value from user
write(pipes1[1], &n, sizeof n); // automatically unblock child1
write(pipes2[1], &n, sizeof n); // automatically unblock child2
Repeat these statements for the 2nd user input.
Don't forget to close the write ends of the pipes
close(pipes1[1]);
close(pipes2[1]);
And that's the main() function. What about the function for the children? The child(char *sign, int pipes[2])?
void child(char *sign, int pipes[2]) {
close(pipes[1]); // close write end
int i;
for (;;) {
if (read(pipes[0], &i, sizeof i)) {
printf(" %s%d\n", sign, i); // print and go back to waiting at the read()
} else {
break; // exit the loop when read fails
}
}
close(pipes[0]); // no more reading
exit(EXIT_SUCCESS); // not going back to main()
}