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 program
7 # Return value of another in main
16608 # Child process PID
After wait # cout<<"After wait"<<endl;

on Terminal.

FYI, Phases after # are comments.