Re: Run program in background.

[Brian Dessent]

Moises Deangelo wrote:

 I did a program in C.
 
 That program needs to run for a long time, because of this it is ideal
 that he works on background.
 
 I do not have been managing do that.
 
  is there some command? Any lib, anything
 
 I thank the help.

Your question is not clear.  Run in the background can mean lots of
things.

If you just want your program to simply run in the background, launch it
with a  at the end of the command from the shell.  However, if it
expects to use stdout, stdin, or stderr, it will stop -- so these must
all be redirected to files or pipes.

This will still leave it attached to the terminal and process group of
the shell, however.  Thus you will not be able to log out of the command
prompt with the background jobs unless you detach them.  To get around
this you can use the nohup and/or setsid commands when launching it.

If you want your program to daemonize itself (rather than relying on the
user to do it when invoking it), then you will have to read some unix
programming books about the steps involved.  For example, Perl's
Proc::Daemon does the following:

1. Fork a child and exit the parent process.

2. Become a session leader (which detaches the program from the
controlling terminal).

3. Fork another child process and exit first child.  This prevents the
potential of acquiring a controlling terminal.

4. Change the current working directory to /.

5. Clear the file creation mask.

6. Close all open file descriptors.

As an alternative, under Cygwin you can run your program as a service. 
In this mode you needn't daemonize from within your program, as the
cygrunsrv program launches your program and waits on it.  Thus cygrunsrv
can run any normal program as a service, without needing special code in
the program.  However, you must be mindful of permissions and ownership
as by default you will be running as the SYSTEM account which does not
always have the same access as user accounts.

Brian

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RE: Run program in background.

[Reid Thompson]

Brian Dessent wrote:
 
 If you just want your program to simply run in the
 background, launch it with a  at the end of the command
 from the shell.  However, if it expects to use stdout, stdin,
 or stderr, it will stop -- so these must all be redirected to files
 or pipes. 

not exactly correct the program does not stop, stdin is not accessible
via the shell, but stdout and stderr are by default, still routed to it.

$ cat back.c
#include stdio.h

int main (void)
{
int ctr = 0;

printf(From stdout\n);
fprintf(stderr, From stderr\n);

for (ctr = 0; ctr  5; ++ctr)
{
printf(%d\n, ctr);
sleep(2);
}

return(0);
}
WS-XP-4960:/home/rthompso 
$ ./a 
[2] 5076
WS-XP-4960:/home/rthompso 
$ From stdout
From stderr
0
1
2
3
4
[2]+  Done./a

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Re: Run program in background.

[Brian Dessent]

Reid Thompson wrote:

  If you just want your program to simply run in the
  background, launch it with a  at the end of the command
  from the shell.  However, if it expects to use stdout, stdin,
  or stderr, it will stop -- so these must all be redirected to files
  or pipes.
 
 not exactly correct the program does not stop, stdin is not accessible
 via the shell, but stdout and stderr are by default, still routed to it.

You're correct.  I guess what I mean to say was that it will stop if it
tries to read stdin, and for the sake of the user at the console stdout
and stderr will need to be redirected to prevent screen pollution.

Brian

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