C++ daemon and talking to daemon

[soxs060389]

Hi

I am writing a private media player to get used to C++ and get some actual tings in shape I had ind years ago.

So my initial plan was, to run <dumbplayer> (my media player name) as daemon for an infinite time and send that process info via linux/unix bash commandline

$ dumplayer -playthissong /path/to/song.ext

the deamon itself should be called through just

$ dumplayer

So, the point where I require some help is, how can I make the first process run as an infinite daemon ( without doing an while(1){] loop) and without acquiring 100% cpu, abd still having the possibility to interfere via further commands?

Thx a lot
Sincerely
soxs

Note:
I allready did some research and read some pages about fork(), but the usage didn't come clear how I can later use that process ID again and read with later calls of <dumplayer -option ...> from bash/CLI

[giulix]

Have a look at sockets

[stevea]

First, when creating a daemon, look at "man 3 daemon" library call.

This causes your process (well actually a child copy) to remove itself from the current terminal session and close stdin, stdout ,stderr.

All daemons should be coded to respond to (service) external events. It's entirely up to YOU to arrange for your process to wait on these events without "hot spinning".

Normally a process is scheduled (given cpu time) periodically whenever it's in a "runnable" state. It's your job to make your process not be in a runnable state until the external event occurs.

One common mechanism is the use the "man 2 select" syscall to wait until there is data available.
The "man 2 poll" syscall is also used.
You could setup callback routines (in an X11 environment for example) and then let the main thread sleep forever.
In some cases you can wait for signals with "man 2 rt_sigtimedwait" or "man 2 pause".

All of the above tell the kernel to mark your process to not schedule (not be runnable) until the event has occurred.


[edit]
After scanning your post, perhaps your intention is to have the daemon "dumplayer" wait until you specify a file to play (how?) or alternatively to wait for the player copy to exit. The first sounds like an X11 callback event perhaps. The second ase perhaps you want to wait on a SIGCHLD (death of child pocess) event.

[soxs060389]

There is no gui part planed until furter notice, and thus I'd like to do it this way

dumplayer -play /path/to/song.ext #play this song, and if dumplayer deamon is not running, start it just ahead
dumplayer -start #just start the deamon an play nothing, waiting for further 'dumbplayer -play ... ' operatiorns

[soxs060389]

Originally Posted by giulix

Have a look at sockets

If I use sockets, then I need somehwo to determine if there is allready a processs running, so I can start the deamon process, jsut in case there is none yet. Any proposals?

[giulix]

One option is to use xinetd

[soxs060389]

int StartDaemon() {
    LimitOne limiter; //this creates a lock, so only one process can run at a time
    if (limiter.successfullyLocked()) { //checks wether lock was successfull
        ServerDaemon sound(GLOBAL_DEFAULT_PORT); //run a socket server with some extras 

        if (daemon(0, 0) == -1) {
            cout << "Evil Exit Detected" << endl;
            exit(1);
        }
        cout << "AFTER" << endl; // this never happens, as daemon(0,0) seems to cancel all remaining actions, this is 100% ebil
        sound.run(); //this ash to be run and is a cold spinning (infinite) wait for connects until explicit closeoure
    }
}

Any solution, so I can make it a deamon but still run the remaining actions


why does daemon(1,1) make such a difference to daemon(0,0)
with daemon (1,1) it works just fine, wtf? Are the german manpages wrong or .. well I must be too stupide to red them..

[giulix]

Not sure what you mean, but the daemon man page says that, after fork()ing, the parent exits. With daemon(0, 0) the child redirects standard output to /dev/null, so output to a file instead of using cout

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

int main(int argc, char *argv[]) {

    FILE *f = NULL;

    if((f = fopen("log.txt", "w")) != NULL)

        if(daemon(0, 0) >= 0)

            fprintf(f, "%s\n", "Hello");

        else

            fprintf(f, "%s\n", "Error detaching terminal");
    else

        printf("%s\n", "Cannot open log file");

    return 0;
}

[soxs060389]

Ok, thx, I thought it was supposed as long as the terminal keeeps running t print info, and not do direct forwarding to /dev/null