fork() question

elmafiacs · 01-08-2005, 06:34 AM

Where I can find a good tutorial on the fork() system call, and in process communication? (got them:
http://www-cs.canisius.edu/PL_TUTORI...ED/concurrency
http://www-cs.canisius.edu/PL_TUTORIALS/C/ADVANCED/ipc

Besides that, I want to know how to do this:

- You create a child process to copy a file while doing something different, but you only want to start that process when the user requests to copy a file.

All stuff I've read says something like that I have to start the two process separated and at the beginning of the program, something like:

Code:

int process_id;

main()
{
process_id=fork();

if(process_id==0) {
    break;  /* go on */
else do_other_thing();
}
}

And that's not what I am looking for.

Thanks in advance.

Inunu · 01-08-2005, 10:10 AM

The idea of fork() is to clone the original process code/data to a new one. At the end of each "fork()" call, you get two different processes running up to that exact point in their own execution flow. Unless you want to try something like POSIX threads (see man 3 pthread_create()), this is how fork() works.

Suppose your original process is A. After calling, fork() returns the PID of newly cloned process B to your original one (A). It returns 0 to B, and this new process immediately begins to perform something different. That is why it is so-called fork as the execution path of two processes diverses.

Code:

#include <stdio.h>
main(){
  ....
  if(fork()==0){
    /* The newly created process will run into here */
    FILE *src, *dest;
    char buffer[1024];
    int count;

    if((src=fopen("source_file.txt", "r"))==NULL){
      /* Failed to open source file */
     exit(-1);
    }
    if((dest=fopen("destination_file.txt", "r"))==NULL){
      /* Failed to open destination file */
     exit(-2);
    }
    while((count=fread(buffer, 1, 1024, src))>0)
      fwrite(buffer, 1, count, dest);
    fclose(src);
    fclose(dest);
    exit(0);
  }
  else {
    /* Original process will run into here */
    printf("Begining file copy...\n");

    /* Do your thing */
    ....
  }
}

Alternatively you can use POSIX thread. A new thread will start from a function that you specify and terminate when it exits the function. You can use global or static variables for IPC as they share the same process data space.

elmafiacs · 01-08-2005, 10:25 AM

Thanks.

btmiller · 01-08-2005, 12:42 PM

If you want the second process to wait for something to happen, you can use the sigsuspend system call to block the process until it receives the signal. Before you do that, set up a signal handler (using sigaction) for one of the user-reserved signals (SIGUSR1 or SIGUSR2), and then use sigsuspend on that signal. Then have the parent process send the signal to the child (remember, fork returns the child's PID) when the parent is ready for the child to go. When the child receives the signal, it will wake up and then you can have it do what it needs to do.