Question

Can you help me explain how fork, getpid(), cout work in this code?

I got a quiz today which asked me how many times cout and fork() is executed.

#include <iostream>
#include <unistd.h>
using namespace std;
int main ( int argc, char *argv [] )
{
int pid;
cout << getpid()<<endl;
pid = fork();

if (pid == 0)
{
cout << getpid() <<endl;
fork();

cout << getpid()<<endl;
fork();

cout << getpid()<<endl;
}
return 0;
}

Answer 1

getpid() : returns the process ID of the calling process. This is often used by routines that generate unique temporary filenames.

Return type: getpid() returns the process ID of the current process. It never throws any error therefore is always successful.

Fork system call use for creates a new process, which is called child process, which runs concurrently with process (which process called system call fork) and this process is called parent process. After a new child process created, both processes will execute the next instruction following the fork() system call. A child process uses the same pc(program counter), same CPU registers, same open files which use in the parent process.

It takes no parameters and returns an integer value. Below are different values returned by fork().

in program if (pid == 0) this line checks that is the process is parent or a child , if pid==0 that means its child and if its nonzero then its parent , as many as time fork is called it will create its sub child under the parent process.

Negative Value: creation of a child process was unsuccessful.

Zero: Returned to the newly created child process.

Positive value: Returned to parent or caller. The value contains process ID of newly created child process.