Forking and executing process

Summary

This passage tells you how to fork a child process and execute another command in child process on Linux.

Though we are using C library on Linux to do such stuff, however I would like to use C++ to handle outputs.

Libraries

• unistd.h
• iostream (For C++)
• stdio.h (For C)

Code

#include<iostream>#include<unistd.h>using std::cout;using std::endl;int main(int argc, char* argv[]){    pid_t fpid, sp;    int retValue;    if((fpid = fork()) < 0){        cout<<"Error"<<endl;    }else if(fpid == 0){        cout<<"Child process PID: "<<getpid()<<endl;        if(argc > 1){            execl(argv[1], NULL);        }    }else{        cout<<"Main process:"<<endl;    }    cout<<"Before wait"<<endl;    sp = wait(&retValue);    cout<<WEXITSTATUS(retValue)<<endl;    cout<<sp<<endl;    cout<<"After wait"<<endl;    return 0;}

Let’s say we have a program that prints Test and return 7 at the end of execution (return 7; in main).

Explanations

• if((fpid = fork()) < 0): fork() returns -1 when it cannot start a process.
• else if(fpid == 0): fork() returns 0 when it’s in child process.
• else{cout<<"Main process:"<<endl;}: fork() returns PID when it’s main process.
• cout<<"Son process PID: "<<getpid()<<endl;: Prints child process PID.
• sp = wait(&retValue);: wait returns waited PID while &retValue passed pointer to retValue to wait(), and wait() will put process ended information into refValue.
• cout<<WEXITSTATUS(retValue)<<endl;: WEXITSTATUS(retValue) gets return value in main() of the process.

Results

Run the code with YOUR_PROGRAM_NAME ANOTHER_PROGRAM while YOUR_PROGRAM_NAME is the compilation of codes above and ANOTHER_PROGRAM is another program that you want to execute in our program.

This code will print

Main process: # cout<<"Main process:"<<endl;Before wait # cout<<"Before wait"<<endl;Son process PID: 16608 # cout<<"Son process PID: "<<getpid()<<endl;Test # Output of another program7 # Return value of another in main16608 # Child process PIDAfter wait # cout<<"After wait"<<endl;

on Terminal.

FYI, Phases after # are comments.