Open In App

C Program to Demonstrate fork() and pipe()

Improve
Improve
Like Article
Like
Save
Share
Report

fork() is used to create a child process. This child process is a copy of the original(parent) process. It is the primary method of process creation on Unix-like operating systems.( See this article for reference).

Syntax:

fork(); 
// It does not take any parameter, it returns 
// integer values. It may return negative, 
// positive or zero integer values.

pipe(): It is used for inter-process communication in Linux. It is a system function. (See this article for reference)

Syntax:

int pipe(int pipefd[2]);

C program to demonstrate fork() and pipe():

Write Linux C program to create two processes P1 and P2. P1 takes a string and passes it to P2. P2 concatenates the received string with another string without using string function and sends it back to P1 for printing.

Examples: 

Other string is: forgeeks.org

Input  : www.geeks
Output : www.geeksforgeeks.org
        
Input :  www.practice.geeks
Output : practice.geeksforgeeks.org

Explanation:  

  • To create child process we use fork(). fork() returns : 
    • <0 fail to create child (new) process
    • =0 for 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.

Example: 

int fd[2];
pipe(fd);
fd[0]; //-> for using read end
fd[1]; //-> for using write end

Inside Parent Process : We firstly close the reading end of first pipe (fd1[0]) then write the string though writing end of the pipe (fd1[1]). Now parent will wait until child process is finished. After the child process, parent will close the writing end of second pipe(fd2[1]) and read the string through reading end of pipe (fd2[0]). 

Inside Child Process : Child reads the first string sent by parent process by closing the writing end of pipe (fd1[1]) and after reading concatenate both string and passes the string to parent process via fd2 pipe and will exit. 

Input

www.geeks

C




// C program to demonstrate use of fork() and pipe()
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
 
int main()
{
    // We use two pipes
    // First pipe to send input string from parent
    // Second pipe to send concatenated string from 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[] = "forgeeks.org";
    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 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);
    }
}


Output:

Concatenated string www.geeksforgeeks.org



Last Updated : 03 Jan, 2022
Like Article
Save Article
Previous
Next
Share your thoughts in the comments
Similar Reads