How to start process on Linux OS in C, C++

Question

I have wireless USB Adapter and I use "pstree" command to monitor all processes.
When I plug the USB adapter into my Linux OS I see new process "wpa_supplicant" with "pstree" command.

I use in C/C++ language . I know Linux OS will use "NetworkManager" daemon to monitor network (eth, bluetooth, wifi. etc) but i don't know How to we can start "wpa_supplicant" ? Can i use dbus or systemd?

Thanks Thong LT

Answer 1

Use fork() system call and it will create a child process or if you want to run the executable through C code then use exec() library function and specify the path of the executable.

Here is the code:

#include<unistd.h>
#include<stdio.h>
#include<sys/types.h>
#include<stdlib.h>
#include<sys/wait.h>

int main() {
    pid_t pid,p;
    int i,status;
    printf("Parent process starts with PID = %d it's Parent ID %d\n",(int)getpid(),(int)getppid());
    
    if((pid = fork())==-1) {
        fprintf(stderr,"FORK FAILED\n");
        return -1;
    }
    if(pid == 0 ) {
        printf("Child process starts with PID = %d\n",(int)getpid());
        for(i = 0 ; i < 5; i++) {
            printf("Child prints [%d]\n",i);
            sleep(2);
        }
        _exit(0);
        //exit(0);
    }
    else {
        p  = wait(&status);
        printf("Parent resumes execution, took control from the child %d \n",(int)p);
        //printf("Return status of the child is %d\n",status);
        for(i = 0; i< 5 ; i++) {
            sleep(2);
            printf("Parent prints [%d]\n",i);
            //sleep(2);
        }
        _exit(0);
    }
    return 0;
}

Answer 2

The standard UNIX way is to use fork(2) followed by an exec(3) call (there's a whole family of them—choose whichever suits your needs the best).

Example to illustrate usage:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char **argv)
{
  pid_t     pid;

  printf("before fork\n");   
  if ((pid = fork()) < 0) {

    //It may fail -- super rare
    perror("Fork failed");

  } else if (pid > 0) {
    //If it returns a positive number, you're in the parent process and pid holds the pid of the child

    printf("Mah kid's pid is %d\n", pid);
    printf("Mine's %d\n", getpid());

  } else {
    //If it returns zero, you're in the child process

    //you can do some preparatory work here (e.g., close filedescriptors)

    printf("I'm the child and my pid is %d\n", getpid());

    //exec will replace the process image with that of echo (wherever in the PATH environment variable it is be found (using the execlP version here)
    execlp("echo", "echo", "hello world from echo; I'm now the child because I've replaced the original child because it called an exec function", (char*)NULL);

    printf("This won't run because now we're running the process image of the echo binary. Not this.");
  }

  return EXIT_SUCCESS;
}