Switching between foreground and background

Switching between foreground and background

[Silambu Chelvan]

Hi all,
 
I have written some program with signal handler. Is
this possible, if 
one of the signals registered with my program is
raised, my program 
should goto background and should come to foreground
when some other of the 
registered signal is raised. how to do it? 
 
Any function available to switch a process between
foreground and 
background at runtime?
 
with regards,
M. SilambuChelvan


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Re: Switching between foreground and background

[John T. Williams]

I think part of the problem here might be conceptual.

If I understand what you want, you'd like, on a signal, for your program to
go into the background (still running but no longer blocking the terminal
input/output). On another signal you would like the program to come back to
the foreground.

If this is the case, the problem you're facing is that the program being in
the foreground and the background is actually part of the shell, not a
property of the program running.  Most Shells provide a way to switch
programs from the foreground to the background and back.
BASH for example I could type

$>./myprogram
(cntr + Z) //suspends the current program and prings the prompt back
$>bg  (runs the program in the background)

The only way that I know of to force a program to run in the background in
the code is to make it a daemon which is done by forking twice

int main(  ) {
   if(fork()==0)  {
            if( fork() == 0 )  {
                    program code starts here
             }
            exit(0);
    }
    exit(0);
}


----- Original Message ----- 
From: "Silambu Chelvan" <silambu_mdu@yahoo.com>
To: <linux-newbie@vger.kernel.org>
Sent: Monday, September 22, 2003 6:48 AM
Subject: Switching between foreground and background


> Hi all,
>
> I have written some program with signal handler. Is
> this possible, if
> one of the signals registered with my program is
> raised, my program
> should goto background and should come to foreground
> when some other of the
> registered signal is raised. how to do it?
>
> Any function available to switch a process between
> foreground and
> background at runtime?
>
> with regards,
> M. SilambuChelvan
>
>
> __________________________________
> Do you Yahoo!?
> Yahoo! SiteBuilder - Free, easy-to-use web site design software
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> To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
> the body of a message to majordomo@vger.kernel.org
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Re: Switching between foreground and background

[Silambu Chelvan]

Yes, You have understood what I want. Forcing the
program to run in background is not a problem. I need
to get the program back in foreground.

Do you have any idea?

with regards,
M. SilambuChelvan

--- "John T. Williams" <jowillia@vt.edu> wrote:
> I think part of the problem here might be
> conceptual.
> 
> If I understand what you want, you'd like, on a
> signal, for your program to
> go into the background (still running but no longer
> blocking the terminal
> input/output). On another signal you would like the
> program to come back to
> the foreground.
> 
> If this is the case, the problem you're facing is
> that the program being in
> the foreground and the background is actually part
> of the shell, not a
> property of the program running.  Most Shells
> provide a way to switch
> programs from the foreground to the background and
> back.
> BASH for example I could type
> 
> $>./myprogram
> (cntr + Z) //suspends the current program and prings
> the prompt back
> $>bg  (runs the program in the background)
> 
> The only way that I know of to force a program to
> run in the background in
> the code is to make it a daemon which is done by
> forking twice
> 
> int main(  ) {
>    if(fork()==0)  {
>             if( fork() == 0 )  {
>                     program code starts here
>              }
>             exit(0);
>     }
>     exit(0);
> }
> 
> 
> ----- Original Message ----- 
> From: "Silambu Chelvan" <silambu_mdu@yahoo.com>
> To: <linux-newbie@vger.kernel.org>
> Sent: Monday, September 22, 2003 6:48 AM
> Subject: Switching between foreground and background
> 
> 
> > Hi all,
> >
> > I have written some program with signal handler.
> Is
> > this possible, if
> > one of the signals registered with my program is
> > raised, my program
> > should goto background and should come to
> foreground
> > when some other of the
> > registered signal is raised. how to do it?
> >
> > Any function available to switch a process between
> > foreground and
> > background at runtime?
> >
> > with regards,
> > M. SilambuChelvan
> >
> >
> > __________________________________
> > Do you Yahoo!?
> > Yahoo! SiteBuilder - Free, easy-to-use web site
> design software
> > http://sitebuilder.yahoo.com
> > -
> > To unsubscribe from this list: send the line
> "unsubscribe linux-newbie" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at 
> http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at
> http://www.linux-learn.org/faqs
> 


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Re: Switching between foreground and background

[Raghuveer]

John T. Williams wrote:

>I think part of the problem here might be conceptual.
>
>If I understand what you want, you'd like, on a signal, for your program to
>go into the background (still running but no longer blocking the terminal
>input/output). On another signal you would like the program to come back to
>the foreground.
>
>If this is the case, the problem you're facing is that the program being in
>the foreground and the background is actually part of the shell, not a
>property of the program running.  Most Shells provide a way to switch
>programs from the foreground to the background and back.
>BASH for example I could type
>
>$>./myprogram
>(cntr + Z) //suspends the current program and prings the prompt back
>$>bg  (runs the program in the background)
>
>The only way that I know of to force a program to run in the background in
>the code is to make it a daemon which is done by forking twice
>
>int main(  ) {
>   if(fork()==0)  {
>            if( fork() == 0 )  {
>                    program code starts here
>             }
>            exit(0);
>    }
>    exit(0);
>}
>  
>
I don't feel it's required to fork() twice for making it a daemon, once 
is enough. Ya, but should use setpgrp() or setpid() and close stdin, out 
and err after the fork and go to infinite loop.

-Raghu

>
>----- Original Message ----- 
>From: "Silambu Chelvan" <silambu_mdu@yahoo.com>
>To: <linux-newbie@vger.kernel.org>
>Sent: Monday, September 22, 2003 6:48 AM
>Subject: Switching between foreground and background
>
>
>  
>
>>Hi all,
>>
>>I have written some program with signal handler. Is
>>this possible, if
>>one of the signals registered with my program is
>>raised, my program
>>should goto background and should come to foreground
>>when some other of the
>>registered signal is raised. how to do it?
>>
>>Any function available to switch a process between
>>foreground and
>>background at runtime?
>>
>>with regards,
>>M. SilambuChelvan
>>
>>
>>__________________________________
>>Do you Yahoo!?
>>Yahoo! SiteBuilder - Free, easy-to-use web site design software
>>http://sitebuilder.yahoo.com
>>-
>>To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
>>the body of a message to majordomo@vger.kernel.org
>>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>Please read the FAQ at http://www.linux-learn.org/faqs
>>    
>>
>
>-
>To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at http://www.linux-learn.org/faqs
>
>  
>


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Re: Switching between foreground and background

[John T. Williams]

I have to admit that I don't know why, but every resource I've ever read
about causing programs to gain daemon behavior has involved forking 2 times
and then starting the program code.  If anyone here does know why, I'd would
be happy to hear it.



----- Original Message ----- 
From: "Raghuveer" <rvk@gsecone.com>
To: "John T. Williams" <jtwilliams@vt.edu>
Cc: "Silambu Chelvan" <silambu_mdu@yahoo.com>;
<linux-newbie@vger.kernel.org>
Sent: Tuesday, September 23, 2003 7:01 AM
Subject: Re: Switching between foreground and background


> John T. Williams wrote:
>
> >I think part of the problem here might be conceptual.
> >
> >If I understand what you want, you'd like, on a signal, for your program
to
> >go into the background (still running but no longer blocking the terminal
> >input/output). On another signal you would like the program to come back
to
> >the foreground.
> >
> >If this is the case, the problem you're facing is that the program being
in
> >the foreground and the background is actually part of the shell, not a
> >property of the program running.  Most Shells provide a way to switch
> >programs from the foreground to the background and back.
> >BASH for example I could type
> >
> >$>./myprogram
> >(cntr + Z) //suspends the current program and prings the prompt back
> >$>bg  (runs the program in the background)
> >
> >The only way that I know of to force a program to run in the background
in
> >the code is to make it a daemon which is done by forking twice
> >
> >int main(  ) {
> >   if(fork()==0)  {
> >            if( fork() == 0 )  {
> >                    program code starts here
> >             }
> >            exit(0);
> >    }
> >    exit(0);
> >}
> >
> >
> I don't feel it's required to fork() twice for making it a daemon, once
> is enough. Ya, but should use setpgrp() or setpid() and close stdin, out
> and err after the fork and go to infinite loop.
>
> -Raghu
>
> >
> >----- Original Message ----- 
> >From: "Silambu Chelvan" <silambu_mdu@yahoo.com>
> >To: <linux-newbie@vger.kernel.org>
> >Sent: Monday, September 22, 2003 6:48 AM
> >Subject: Switching between foreground and background
> >
> >
> >
> >
> >>Hi all,
> >>
> >>I have written some program with signal handler. Is
> >>this possible, if
> >>one of the signals registered with my program is
> >>raised, my program
> >>should goto background and should come to foreground
> >>when some other of the
> >>registered signal is raised. how to do it?
> >>
> >>Any function available to switch a process between
> >>foreground and
> >>background at runtime?
> >>
> >>with regards,
> >>M. SilambuChelvan
> >>
> >>
> >>__________________________________
> >>Do you Yahoo!?
> >>Yahoo! SiteBuilder - Free, easy-to-use web site design software
> >>http://sitebuilder.yahoo.com
> >>-
> >>To unsubscribe from this list: send the line "unsubscribe linux-newbie"
in
> >>the body of a message to majordomo@vger.kernel.org
> >>More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>Please read the FAQ at http://www.linux-learn.org/faqs
> >>
> >>
> >
> >-
> >To unsubscribe from this list: send the line "unsubscribe linux-newbie"
in
> >the body of a message to majordomo@vger.kernel.org
> >More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >Please read the FAQ at http://www.linux-learn.org/faqs
> >
> >
> >
>
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
> the body of a message to majordomo@vger.kernel.org
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RE: Switching between foreground and background

[Mariano Moreyra]

I don't know if that's the way to gain daemon behavior. If you fork twice,
your process stills attached to the controlling terminal.
I guess you could finish that terminal session without problem. But I think
that a real daemon has to detach him self from the controlling tty.
You can do that, like Raghu said, calling setpgrp and setsid and closing
stdout an err.
There is also a function in unistd that does all of this for you.

int daemon(int nochdir,int noclose); (see "man daemon")

Don't know if exists in all unix or linux distros. But it seems to work.
Saludos.

Manuk


-----Mensaje original-----
De: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org]En nombre de John T.
Williams
Enviado el: Martes, 23 de Septiembre de 2003 13:07
Para: rvk@gsecone.com; John T. Williams
CC: Silambu Chelvan; linux-newbie@vger.kernel.org;
linux-c-programming@vger.kernel.org
Asunto: Re: Switching between foreground and background


I have to admit that I don't know why, but every resource I've ever read
about causing programs to gain daemon behavior has involved forking 2 times
and then starting the program code.  If anyone here does know why, I'd would
be happy to hear it.



----- Original Message -----
From: "Raghuveer" <rvk@gsecone.com>
To: "John T. Williams" <jtwilliams@vt.edu>
Cc: "Silambu Chelvan" <silambu_mdu@yahoo.com>;
<linux-newbie@vger.kernel.org>
Sent: Tuesday, September 23, 2003 7:01 AM
Subject: Re: Switching between foreground and background


> John T. Williams wrote:
>
> >I think part of the problem here might be conceptual.
> >
> >If I understand what you want, you'd like, on a signal, for your program
to
> >go into the background (still running but no longer blocking the terminal
> >input/output). On another signal you would like the program to come back
to
> >the foreground.
> >
> >If this is the case, the problem you're facing is that the program being
in
> >the foreground and the background is actually part of the shell, not a
> >property of the program running.  Most Shells provide a way to switch
> >programs from the foreground to the background and back.
> >BASH for example I could type
> >
> >$>./myprogram
> >(cntr + Z) //suspends the current program and prings the prompt back
> >$>bg  (runs the program in the background)
> >
> >The only way that I know of to force a program to run in the background
in
> >the code is to make it a daemon which is done by forking twice
> >
> >int main(  ) {
> >   if(fork()==0)  {
> >            if( fork() == 0 )  {
> >                    program code starts here
> >             }
> >            exit(0);
> >    }
> >    exit(0);
> >}
> >
> >
> I don't feel it's required to fork() twice for making it a daemon, once
> is enough. Ya, but should use setpgrp() or setpid() and close stdin, out
> and err after the fork and go to infinite loop.
>
> -Raghu
>
> >
> >----- Original Message -----
> >From: "Silambu Chelvan" <silambu_mdu@yahoo.com>
> >To: <linux-newbie@vger.kernel.org>
> >Sent: Monday, September 22, 2003 6:48 AM
> >Subject: Switching between foreground and background
> >
> >
> >
> >
> >>Hi all,
> >>
> >>I have written some program with signal handler. Is
> >>this possible, if
> >>one of the signals registered with my program is
> >>raised, my program
> >>should goto background and should come to foreground
> >>when some other of the
> >>registered signal is raised. how to do it?
> >>
> >>Any function available to switch a process between
> >>foreground and
> >>background at runtime?
> >>
> >>with regards,
> >>M. SilambuChelvan
> >>
> >>
> >>__________________________________
> >>Do you Yahoo!?
> >>Yahoo! SiteBuilder - Free, easy-to-use web site design software
> >>http://sitebuilder.yahoo.com
> >>-
> >>To unsubscribe from this list: send the line "unsubscribe linux-newbie"
in
> >>the body of a message to majordomo@vger.kernel.org
> >>More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>Please read the FAQ at http://www.linux-learn.org/faqs
> >>
> >>
> >
> >-
> >To unsubscribe from this list: send the line "unsubscribe linux-newbie"
in
> >the body of a message to majordomo@vger.kernel.org
> >More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >Please read the FAQ at http://www.linux-learn.org/faqs
> >
> >
> >
>
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
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Re: Switching between foreground and background

[Markus Klotzbuecher]

On Monday 22 September 2003 12:48 pm, Silambu Chelvan wrote:
> Hi all,
>
> I have written some program with signal handler. Is
> this possible, if
> one of the signals registered with my program is
> raised, my program
> should goto background and should come to foreground
> when some other of the
> registered signal is raised. how to do it?
>
> Any function available to switch a process between
> foreground and
> background at runtime?

Hi 

Switching a task between forground and background is nothing else as 
suspending and restarting. What i would try to do is, when you get the signal 
you want to suspend on, call the pause() function (see pause(2) ). When you 
get the signal you want to restart on, just do nothing as this will let the 
pause() function return and continue doing what happend before suspend.

Try something like this:

***************************************
#include <signal.h>
#include <stdio.h>

void suspend()
{
        printf("Suspending...\n");
        pause();
}

void wake()
{
        printf("Woken up.\n");
}

int main()
{

        signal(SIGUSR1, suspend);
        signal(SIGUSR2, wake);

        while(1) {
                /* do whatever */
                printf("doing whatever...\n");
                sleep(1);
        }
}

Cheers

        Markus
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Re: Switching between foreground and background

[Silambu Chelvan]

Hi,

Thanks for your information. 

The method you suggested works fine but I still have a
requirement. The control is still in the process and
could not see the command prompt. The thing is that I
should get the command prompt whenever the process is
put into background so that I can issue some other
command on the prompt.

Any clues??

with regards,
M. SilambuChelvan

--- Markus Klotzbuecher
<markus.klotzbuecher@creamnet.de> wrote:
> On Monday 22 September 2003 12:48 pm, Silambu
> Chelvan wrote:
> > Hi all,
> >
> > I have written some program with signal handler.
> Is
> > this possible, if
> > one of the signals registered with my program is
> > raised, my program
> > should goto background and should come to
> foreground
> > when some other of the
> > registered signal is raised. how to do it?
> >
> > Any function available to switch a process between
> > foreground and
> > background at runtime?
> 
> Hi 
> 
> Switching a task between forground and background is
> nothing else as 
> suspending and restarting. What i would try to do
> is, when you get the signal 
> you want to suspend on, call the pause() function
> (see pause(2) ). When you 
> get the signal you want to restart on, just do
> nothing as this will let the 
> pause() function return and continue doing what
> happend before suspend.
> 
> Try something like this:
> 
> ***************************************
> #include <signal.h>
> #include <stdio.h>
> 
> void suspend()
> {
>         printf("Suspending...\n");
>         pause();
> }
> 
> void wake()
> {
>         printf("Woken up.\n");
> }
> 
> int main()
> {
> 
>         signal(SIGUSR1, suspend);
>         signal(SIGUSR2, wake);
> 
>         while(1) {
>                 /* do whatever */
>                 printf("doing whatever...\n");
>                 sleep(1);
>         }
> }
> 
> Cheers
> 
>         Markus
> -
> To unsubscribe from this list: send the line
> "unsubscribe linux-newbie" in
> the body of a message to majordomo@vger.kernel.org
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> http://vger.kernel.org/majordomo-info.html
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Re: Switching between foreground and background

[Markus Klotzbuecher]

On Monday 22 September 2003 02:38 pm, Silambu Chelvan wrote:
> Hi,
>
> Thanks for your information.
>
> The method you suggested works fine but I still have a
> requirement. The control is still in the process and
> could not see the command prompt. The thing is that I
> should get the command prompt whenever the process is
> put into background so that I can issue some other
> command on the prompt.

Hi

hmm, I see. You will have to actually stop the process to get the command 
prompt, instead of letting it sleep as I suggested earlier. You can do this 
by sending the SIGSTOP signal to it:

#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

void suspend()
{
        printf("Suspending...\n");
        pid_t pid = getpid();
        kill(pid, SIGSTOP); /*send SIGSTOP to itself */
}
...

The problem arises, when you need to start it again. The stopped process needs 
to be sent a SIGCONT signal, which will cause it to continue. Obviously a 
stopped process can't catch any signals and can't start itself again. The 
only way I see is to have a second process catch the signals and send the 
appropriate SIGSTOP and SIGCONT signals to the, lets call it main process.

I hope this helps...

Cheers

	Markus


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