C++ || Snippet – How To Use Fork & Pipe For Interprocess Communication
The following is sample code which demonstrates the use of the fork, read, and write function calls for use with pipes on Unix based systems.
A pipe is a mechanism for interprocess communication. Data written to a pipe by one process can be read by another process. Creating a pipe is achieved by using the pipe function, which creates both the reading and writing ends of the pipe file descriptor.
In typical use, a parent process creates a pipe just before it forks one or more child processes. The pipe is then used for communication between either the parent or child processes, or between two sibling processes.
A real world example of this kind of communication can be seen in all operating system terminal shells. When you type a command in a shell, it will spawn the executable represented by that command with a call to fork. A pipe is opened to the new child process, and its output is read and printed by the terminal.
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// ============================================================================ // Author: Kenneth Perkins // Date: Aug 19, 2013 // Taken From: http://programmingnotes.org/ // File: ForkPipeExample.cpp // Description: Demonstrate the use of the fork() and pipe() command for // parent and child interprocess communication. // ============================================================================ #include <iostream> #include <cstdlib> #include <cstring> #include <sys/wait.h> #include <unistd.h> using namespace std; // global variables const int BUFFER_SIZE = 256; const int READ_END = 0; const int WRITE_END = 1; int main() { // declare variables int fd[2]; // pipe file descriptor pid_t pid = -1; char writeMsg[BUFFER_SIZE] = "Greetings From Your Parent!"; char readMsg[BUFFER_SIZE]; // create a pipe if(pipe(fd) < 0) { perror("pipe"); exit(1); } cout <<"\nParent is forking a child." << endl; // create a duplicate process of this current program pid = fork(); // exit if something went wrong with the fork if(pid < 0) { perror("fork"); exit(1); } // this code only gets executed by the child process else if(pid == 0) { cout <<"\nStarting the child process..\n\n"; // close the "write" end b/c we arent using it close(fd[WRITE_END]); // get data from the pipe using the read() function read(fd[READ_END], readMsg, sizeof(readMsg)); // close the "read" end close(fd[READ_END]); cout <<"Message from the parent via the pipe: "<<readMsg<<endl; } // the parent process executes here else { cout <<"\nParent is now waiting for child id #"<<pid<<" to complete..\n"; // close the "read" end b/c we arent using it close(fd[READ_END]); // write data on pipe write(fd[WRITE_END], writeMsg, strlen(writeMsg)+1); // close the "write" end close(fd[WRITE_END]); // use the "wait" function to wait for the child to complete wait(NULL); cout <<"\nThe child process is complete and has terminated!\n"; } // NOTE: this message gets displayed twice - Why?! cout <<"\nProgram is now exiting...\n"; return 0; }// http://programmingnotes.org/ |
QUICK NOTES:
The highlighted lines are sections of interest to look out for.
The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.
The following is sample output:
Parent is forking a child.
Parent is now waiting for child id #12776 to complete..Starting the child process..
Message from the parent via the pipe: Greetings From Your Parent!
Program is now exiting...
The child process is complete and has terminated!
Program is now exiting...
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