This is about as lame (and no error checking, btw) a sample as I can muster for using a pipe to send int from a parent to a child process, where the child was launched from fork(). It gets more complicated (obviously) for sending and receiving data, but i can't do everything for you. This just forks and waits for an int (actually, the number of bytes that are used by an int) from the child.
Update: Added send+response two-way communication example after this one. See the second code listing for more information.
Hope it helps.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int fd[2];
int val = 0;
// create pipe descriptors
pipe(fd);
// fork() returns 0 for child process, child-pid for parent process.
if (fork() != 0)
{
// parent: writing only, so close read-descriptor.
close(fd[0]);
// send the value on the write-descriptor.
val = 100;
write(fd[1], &val, sizeof(val));
printf("Parent(%d) send value: %d
", getpid(), val);
// close the write descriptor
close(fd[1]);
}
else
{ // child: reading only, so close the write-descriptor
close(fd[1]);
// now read the data (will block)
read(fd[0], &val, sizeof(val));
printf("Child(%d) received value: %d
", getpid(), val);
// close the read-descriptor
close(fd[0]);
}
return 0;
}
Output:
Parent(5943) send value: 100
Child(5945) received value: 100
Update: Expanded to include send+response using two pipe sets
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.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 %d
", pid, val);
// now double it and send it back
val *= 2;
printf("Child(%d): Sending %d back
", 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 child
", 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 %d
", pid, val);
}
// close down remaining descriptors
close(fd[P1_READ]);
close(fd[P1_WRITE]);
// wait for child termination
wait(NULL);
return EXIT_SUCCESS;
}
(compile with, e.g., gcc thisfile.c -o test)
Output
Parent(2794): Sending 42 to child
Child(2797): Received 42
Child(2797): Sending 84 back
Parent(2794): Received 84
