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  From: Greg Lehey <grog@lemis.com>
  To  : Daryl Tester <dt@picknowl.com.au>
  Date: Sun, 30 Apr 2000 14:57:44 +0930

Passing file descriptors (was: Protecting files against power failure)

On Saturday, 29 April 2000 at 20:34:28 +0930, Daryl Tester wrote:
> David Newall wrote:
>
> [I originally wrote:]
>>> (although, of course, this shouldn't affect the shell's file
>>> descriptor - care to hazard a guess why?).
> [But later retracted after being enlightened with a hammer.]
>
>> File descriptors can be shared amongst different processes.  Consider these
>> two command sequences:
>>
>>   bash$ some-process < some-file & some-process < some-file
>>   bash$ (some-process & some-process) < some-file
>
> File descriptors can be _inherited_ by child processes, as both your
> above examples show.  They can't be passed to any arbitrary process.

I don't know about Linux, but UNIX can definitely pass file
descriptors.  Rich Stevens goes into it in great detail in "Advanced
Programming in the UNIX environment", pages 479-490, but he doesn't
mention Linux.  System V uses the I_SENDFD ioctl, whereas BSD uses an
obscure feature of sendmsg(2).

Greg
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