Unix Processes PIPE - c

Question

I have the following assignment to do :
Ping-pong. Two processes will play the ping-pong game.
The first process will generate a random number between 5000 and 15000 that will be send to the other process.
This process will subtract a random value (between 50 and 1000) and will send the number back,
The chat between the processes will be implemented using pipe channels.
The game ends when the value is below zero.
Each process will print the received value.
So i wrote the following code :
#include <stdio.h>
#include <time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{
int p[2];
int a, n;
pipe(p);
int pid = fork();
if (pid == 0)
{
int r = 0;
srand(time(NULL));
while ( r < 50 || r > 1000)
{
r = rand();
}
if ((a=read(p[0], &n, sizeof(int)))==-1)
perror("Error read pipe:");
printf("Process 2 recived %d\n", a);
a = a - r;
close(p[0]); close(p[1]);
}
else if (pid > 0)
{
srand(time(NULL));
while ( n < 5000 || n > 15000) {
n = rand();
}
while (n > 0)
{
printf("Process 1 recived %d\n", n);
if (write(p[1], &n, sizeof(int))==-1)
perror("Error write pipe:");
wait(0);
}
}
return 0;
}
When it is executed it goes in a infinite loop which prints "Process 1 received 4" and i don't know why.
I created another pipe and now it prints correctly the first received value but from the second process it happens the same thing
Process 1 recived 9083
Process 2 recived 4
Process 1 recived 4
and infinite loop

Answer 1

I give you a corrected program with the explanation in comments:
#include <stdio.h>
#include <time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
int main()
{
int p[2];
int p2[2];
int a, n;
pipe(p);
pipe(p2);
int pid = fork();
if (pid == 0)
{
close(p[1]); // not writing in p, so closing p[1] immediately
close(p2[0]); // not reading in p2, so closing p2[0] immediately
srand(time(NULL));
do {
if ((a = read(p[0], &n, sizeof(int)))==-1)
perror("Error read pipe:");
if (a == 0) // nothing read means all processes have closed p[1]
n = 0;
else
printf("Process 2 recived %d\n", n);
int r = 50 + rand() % 950; // Only need one random number, and need one different each time
n -= r;
if (n > 0)
{
if(write(p2[1], &n, sizeof(int))==-1)
perror("Error write pipe:");
}
} while (n > 0);
close(p[0]); close(p2[1]);
exit(0); // or return(0) - as pointed out by Stian
}
else if (pid > 0)
{
close(p[0]); // not reading in p, so closing p[0] immediately
close(p2[1]); // not writing in p2, so closing p2[1] immediately
srand(time(NULL) + pid); // Adding pid so that processes each use a different seed
n = rand() % 10000 + 5000; // Only need one random number
while (n > 0)
{
if (write(p[1], &n, sizeof(int))==-1)
perror("Error write pipe:");
// wait(0); Wrong use of wait()
if ((a = read(p2[0], &n, sizeof(int)))==-1)
perror("Error read pipe:");
if (a == 0) // nothing read means all processes have closed p2[1]
n = 0;
else
printf("Process 1 recived %d\n", n);
int r = 50 + rand() % 950;
n -= r;
}
close(p[1]); close(p2[0]);
}
wait(NULL); // Better use of wait(). Still not perfect, you need to check return value.
return 0;
}

Answer 2

You need two pipes. One for each direction