Program to implement pipes for inter process communication
To create child process we use fork(). fork() returns :
<0 fail to create child (new) process
=0 for the child process
>0 i.e process ID of the child process to the parent process. When >0 parent process will execute.
pipe() is used for passing information from one process to another. pipe() is unidirectional, therefore, for two-way communication between processes, two pipes can be set up, one for each direction.
Program to send a message from parent process to child process.
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
int main()
{
int fd[2],n;
char buffer[100];
pid_t p;
pipe(fd);
p=fork();
if(p>0) //parent
{
close(fd[0]);
printf("Passing value to child\n");
write(fd[1],"hello\n",6);
wait();
}
else // child
{
close(fd[1]);
n=read(fd[0],buffer,100);
write(1,buffer,n);
}
}
// C program to demonstrate use of fork() and pipe()
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<string.h>
#include<sys/wait.h>
int main()
{
// We use two pipes
// First pipe to send input string from the parent
// Second pipe to send a concatenated string from the child
int fd1[2]; // Used to store two ends of first pipe
int fd2[2]; // Used to store two ends of second pipe
char fixed_str[] = "lpuallprogram.in";
char input_str[100];
pid_t p;
if (pipe(fd1)==-1)
{
fprintf(stderr, "Pipe Failed" );
return 1;
}
if (pipe(fd2)==-1)
{
fprintf(stderr, "Pipe Failed" );
return 1;
}
scanf("%s", input_str);
p = fork();
if (p < 0)
{
fprintf(stderr, "fork Failed" );
return 1;
}
// Parent process
else if (p > 0)
{
char concat_str[100];
close(fd1[0]); // Close reading end of first pipe
// Write input string and close writing end of first
// pipe.
write(fd1[1], input_str, strlen(input_str)+1);
close(fd1[1]);
// Wait for the child to send a string
wait(NULL);
close(fd2[1]); // Close writing end of second pipe
// Read string from child, print it and close
// reading end.
read(fd2[0], concat_str, 100);
printf("Concatenated string %s\n", concat_str);
close(fd2[0]);
}
// child process
else
{
close(fd1[1]); // Close writing end of first pipe
// Read a string using first pipe
char concat_str[100];
read(fd1[0], concat_str, 100);
// Concatenate a fixed string with it
int k = strlen(concat_str);
int i;
for (i=0; i<strlen(fixed_str); i++)
concat_str[k++] = fixed_str[i];
concat_str[k] = '\0'; // string ends with '\0'
// Close both reading ends
close(fd1[0]);
close(fd2[0]);
// Write concatenated string and close writing end
write(fd2[1], concat_str, strlen(concat_str)+1);
close(fd2[1]);
exit(0);
}
}
