C Program to Demonstrate fork() and pipe()

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

This article is contributed by Kartik Ahuja. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.