Notes

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// some macros to make the code more understandable
// regarding which pipe to use to a read/write operation
//
// Parent: reads from P1_READ, writes on P1_WRITE
// Child: reads from P2_READ, writes on P2_WRITE
#define P1_READ 0
#define P2_WRITE 1
#define P2_READ 2
#define P1_WRITE 3

// the total number of pipe *pairs* we need
#define NUM_PIPES 2

int main(int argc, char *argv[])
{
int fd[2*NUM_PIPES];
int val = 0, len, i;
pid_t pid;

// create all the descriptor pairs we need
for (i=0; i<NUM_PIPES; ++i)
{
if (pipe(fd+(i*2)) < 0)
{
perror("Failed to allocate pipes");
exit(EXIT_FAILURE);
}
}

// fork() returns 0 for child process, child-pid for parent process.
if ((pid = fork()) < 0)
{
perror("Failed to fork process");
return EXIT_FAILURE;
}

// if the pid is zero, this is the child process
if (pid == 0)
{
// Child. Start by closing descriptors we
// don't need in this process
close(fd[P1_READ]);
close(fd[P1_WRITE]);

// used for output
pid = getpid();

// wait for parent to send us a value
len = read(fd[P2_READ], &val, sizeof(val));
if (len < 0)
{
perror("Child: Failed to read data from pipe");
exit(EXIT_FAILURE);
}
else if (len == 0)
{
// not an error, but certainly unexpected
fprintf(stderr, "Child: Read EOF from pipe");
}
else
{
// report what we received
printf("Child(%d): Received %dn", pid, val);

// now double it and send it back
val *= 2;

printf("Child(%d): Sending %d backn", pid, val);
if (write(fd[P2_WRITE], &val, sizeof(val)) < 0)
{
perror("Child: Failed to write response value");
exit(EXIT_FAILURE);
}
}

// finished. close remaining descriptors.
close(fd[P2_READ]);
close(fd[P2_WRITE]);

return EXIT_SUCCESS;
}

// Parent. close unneeded descriptors
close(fd[P2_READ]);
close(fd[P2_WRITE]);

// used for output
pid = getpid();

// send a value to the child
val = 42;
printf("Parent(%d): Sending %d to childn", pid, val);
if (write(fd[P1_WRITE], &val, sizeof(val)) != sizeof(val))
{
perror("Parent: Failed to send value to child ");
exit(EXIT_FAILURE);
}

// now wait for a response
len = read(fd[P1_READ], &val, sizeof(val));
if (len < 0)
{
perror("Parent: failed to read value from pipe");
exit(EXIT_FAILURE);
}
else if (len == 0)
{
// not an error, but certainly unexpected
fprintf(stderr, "Parent(%d): Read EOF from pipe", pid);
}
else
{
// report what we received
printf("Parent(%d): Received %dn", pid, val);
}

// close down remaining descriptors
close(fd[P1_READ]);
close(fd[P1_WRITE]);

// wait for child termination
wait(NULL);

return EXIT_SUCCESS;
}