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How to run a program as daemon ?

Posted on 2002-03-07
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Last Modified: 2013-12-27
I have an application which should run as daemon when user specifies -d option.
I am doing the following to make it daemon.

1)Fork
2)If successful, exit parent
3)make child as session leader
4)cd /
5)umask(0)

My application runs fine when it is not running as daemon(when -d optionnot specified)
When I specify -d option, when I debug, I can see that parent exits. then I cant track what happens.

Please help
kgreddy
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Question by:kgreddy
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Expert Comment

by:ahoffmann
ID: 6848499
you need a debugger which follows the forked process, or use a debugger which can attach to a running process
See man gdb for example.
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Expert Comment

by:Nisus091197
ID: 6851037
Under Solaris I like to use the truss command.  It is like the strace command under linux.

e.g.

truss date

Regards, Nisus
http://www.omnimodo.com
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by:ahoffmann
ID: 6851290
 truss -p ...
is the solution then, but keep in mind that it only traces system calls
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Nisus091197 earned 50 total points
ID: 6852036
truss -f -p pid-of-process

The -f will follow child processes.

That should do the trick.

Regards, Nisus
http://www.omnimodo.com
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by:ecw
ID: 6855068
Yep, as nisus says, truss -f

But ...

A daemon process should not itself be a session or process group leader.  It is usual for the things to be competely disassociated from the any controlling tty (hence the session leader step), however the first tty device opened by a session or process group leader which hasn't already got a ctty, will become the controlling tty for that session or pgrp.  This is resolved by using ONOCTTY flag, or traditionally, before ONOCTTY,

  parent process forks
   first child setpgrp/setsid
   first child closes all open fd's
   first child forks
     second childs runs as daemon
   first child exits
  parent exits

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Expert Comment

by:Nisus091197
ID: 6915667
Hi kgreddy,

Has my suggestion been helpful?  Anything more we can help with?
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by:Moondancer
ID: 6940829
Sorry Asker chose not to respond.  I finalized this today.
EE Moderator
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by:Nisus091197
ID: 6940946
very festive.

Have EE stopped giving away Tshirts?  I asked for my guru one ages ago?
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by:Moondancer
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T-shirt program went away when the site was sold.  The new owners are looking at new incentive programs.  Check the "What's New" for an update now and again.  The current focus is a site-wide rewrite to fix many of the inherited roadblocks/problems.  When new incentive programs are added, they will be announced in the What's New category.
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