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

Output:

Concatenated string www.geeksforgeeks.org