C++ || Snippet – How To Use Fork & Execlp For Interprocess Communication
The following is sample code which demonstrates the use of the “fork” and “execlp” function calls on Unix based systems.
The “fork” function call creates a new process by duplicating the calling process; or in more simpler terms, it creates a duplicate process (a child) of the calling (parent) process.
This new process, referred to as the child, is an exact duplicate of the calling process, referred to as the parent.
The “execlp” function call is part of a family of functions which replaces a current running process image with a new process image. That means by using the “execlp” function call, you are basically replacing the entire current running process with a new user defined program.
Though fork and execlp are not required to be used together, they are often used in conjunction with one another as a way of creating a new program running as a child of another process.
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// ============================================================================ // Author: Kenneth Perkins // Date: Aug 19, 2013 // Taken From: http://programmingnotes.org/ // File: ForkExample.cpp // Description: Demonstrate the use of the fork() command for parent and // child process manipulation. // ============================================================================ #include <iostream> #include <cstdlib> #include <sys/wait.h> #include <unistd.h> using namespace std; int main() { // declare variable pid_t pid = -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" << endl; // NOTE: the "execlp" function replaces the current // child process with the terminal "ls" command. // the child process finally dies after the "execlp" // function is complete execlp("/bin/ls", "ls", "-l", NULL); // "ls -l" command cout <<"\nNOTE: This never gets executed - Why?!"; } // the parent process executes here else { cout <<"\nParent is now waiting for child id #"<<pid<<" to complete.." << endl; // use the "wait" function to wait for the child to complete wait(NULL); cout <<"\nThe child process is complete and has terminated!\n"; } cout <<"\nParent 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 #10872 to complete..Starting the child process..
total 1466
-rw-r--r-- 1 admin admin 468 Apr 27 2012 nautilus-computer.desktop
-rwxrwxr-x 1 admin admin 9190 Aug 19 15:17 ForkExample
-rw-rw-r-- 1 admin admin 1640 Aug 19 15:17 ForkExample.cppThe child process is complete and has terminated!
Parent is now exiting...
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