communicating between two child processes with pipes
33,605
Following is a simple example which certainly can be improved but should get you started. Also follow Link1 and Link2 for more info.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
/* max receiving buffer size; Note: no check or enforcement is made on this value*/
#define BUF_SIZE 256
int main()
{
int pfd1[2];
int pfd2[2];
ssize_t numRead = -1;
/* Note: working under the assumption that the messages
are of equal length*/
const char* messageOne = "Hello from child ONE.\n";
const char* messageTwo = "Hello from child TWO.\n";
const unsigned int commLen = strlen(messageOne) + 1;
char buf[BUF_SIZE];
if (pipe(pfd1) == -1)
{
printf("Error opening pipe 1!\n");
exit(1);
}
if (pipe(pfd2) == -1)
{
printf("Error opening pipe 2!\n");
exit(1);
}
printf("Piped opened with success. Forking ...\n");
// child 1
switch (fork())
{
case -1:
printf("Error forking child 1!\n");
exit(1);
case 0:
printf("\nChild 1 executing...\n");
/* close reading end of first pipe */
if (close(pfd1[0]) == -1)
{
printf("Error closing reading end of pipe 1.\n");
_exit(1);
}
/* close writing end of second pipe */
if (close(pfd2[1]) == -1)
{
printf("Error closing writing end of pipe 2.\n");
_exit(1);
}
/* write to pipe 1 */
if (write(pfd1[1], messageOne, commLen) != commLen)
{
printf("Error writing to pipe 1.\n");
_exit(1);
}
if (close(pfd1[1]) == -1)
{
printf("Error closing writing end of pipe 1.\n");
_exit(1);
}
/* reding from pipe 2 */
numRead = read(pfd2[0], buf, commLen);
if (numRead == -1)
{
printf("Error reading from pipe 2.\n");
_exit(1);
}
if (close(pfd2[0]) == -1)
{
printf("Error closing reding end of pipe 2.\n");
_exit(1);
}
printf("Message received child ONE: %s", buf);
printf("Exiting child 1...\n");
_exit(0);
default:
break;
}
// child 2
switch (fork())
{
case -1:
printf("Error forking child 2!\n");
exit(1);
case 0:
printf("\nChild 2 executing...\n");
/* close reading end of second pipe */
if (close(pfd2[0]) == -1)
{
printf("Error closing reading end of pipe 2.\n");
_exit(1);
}
/* close writing end of first pipe */
if (close(pfd1[1]) == -1)
{
printf("Error closing writing end of pipe 1.\n");
_exit(1);
}
/* read from the first pipe */
if (read(pfd1[0], buf, commLen) == -1)
{
printf("Error reading from pipe 1.\n");
_exit(EXIT_FAILURE);
}
if (close(pfd1[0]) == -1)
{
printf("Error closing reading end of pipe 1.\n");
_exit(EXIT_FAILURE);
}
/* write to the second pipe */
if (write(pfd2[1], messageTwo, commLen) != commLen)
{
printf("Error writing to the pipe.");
_exit(EXIT_FAILURE);
}
if (close(pfd2[1]) == -1)
{
printf("Error closing writing end of pipe 2.");
_exit(EXIT_FAILURE);
}
printf("Message received child TWO: %s", buf);
printf("Exiting child 2...\n");
_exit(EXIT_SUCCESS);
default:
break;
}
printf("Parent closing pipes.\n");
if (close(pfd1[0]) == -1)
{
printf("Error closing reading end of the pipe.\n");
exit(EXIT_FAILURE);
}
if (close(pfd2[1]) == -1)
{
printf("Error closing writing end of the pipe.\n");
exit(EXIT_FAILURE);
}
if (close(pfd2[0]) == -1)
{
printf("Error closing reading end of the pipe.\n");
exit(EXIT_FAILURE);
}
if (close(pfd1[1]) == -1)
{
printf("Error closing writing end of the pipe.\n");
exit(EXIT_FAILURE);
}
printf("Parent waiting for children completion...\n");
if (wait(NULL) == -1)
{
printf("Error waiting.\n");
exit(EXIT_FAILURE);
}
if (wait(NULL) == -1)
{
printf("Error waiting.\n");
exit(EXIT_FAILURE);
}
printf("Parent finishing.\n");
exit(EXIT_SUCCESS);
}
Author by
David
Updated on May 22, 2020Comments
-
David almost 4 years
I'm trying to write code that spawns two child processes that send each other a message over a pipe then terminate. However, when I run the following code only child2 prints it's greeting but child 1 still prints the message it gets from child 2 where child 1 doesn't.
Does anybody know what's wrong with my methodology?
#include <string.h> #include <stdlib.h> #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main(int argc, char** argv) { char chld_1_send[20] = "hello from child 1"; char chld_1_recv[20]; char chld_2_send[20] = "hi from child 2"; char chld_2_recv[20]; int chld_1_outgoing[2]; int chld_2_outgoing[2]; pipe(chld_1_outgoing); pipe(chld_2_outgoing); int chld_1_status, chld_2_status; pid_t chld_1, chld_2; chld_1 = fork(); if(chld_1 == 0) { chld_2 = fork(); } if(chld_1 == 0 && chld_2 == 0) { printf("parent [pid:%d] waiting on both children to finish\n", getpid()); while(wait(&chld_1_status) != chld_1 && wait(&chld_2_status) != chld_2) {} printf("done waiting\n"); } else if(chld_1 != 0 && chld_2 == 0) { printf("this is child 1 [pid:%d] with parent [pid:%d]\n", getpid(), getppid()); write(chld_1_outgoing[1], chld_1_send, strlen(chld_1_send)); while(read(chld_1_outgoing[0], &chld_1_recv, sizeof(chld_2_recv)) < 0) {} printf("child 2 said '%s'\n", chld_1_recv); exit(0); } else if(chld_2 != 0 && chld_1 == 0) { printf("this is child 2 [pid:%d] with parent [pid:%d]\n", getpid(), getppid()); write(chld_2_outgoing[1], chld_2_send, strlen(chld_2_send)); while(read(chld_2_outgoing[0], &chld_2_recv, sizeof(chld_2_recv)) < 0) {} printf("child 1 said '%s'\n", chld_2_recv); exit(0); } printf("both children have terminated successfully\n"); return 0; }
However, running this command prints out this to the terminal and goes into an infinite loop:
$ this is child 2 [pid:15713] with parent [pid:1] child 1 said 'hi from child 2' parent [pid:15714] waiting on both children to finish