Re: Process.fork weirdness
- From: Joel VanderWerf <vjoel@xxxxxxxxxxxxxxxxx>
- Date: Mon, 13 Jul 2009 13:24:54 -0500
Joel VanderWerf wrote:
Gary Wright wrote:
You need to flush the output stream before forking so that the child
process doesn't get any buffered data. Otherwise, every time the child
process exits, it will flush the duplicate data out to the
'output.txt' file.
Huh, I never knew that. That's dangerous if the parent process is
multithreaded, isn't it? Because even if the forking thread flushes all
open files, other threads can keep filling buffers before the fork happens.
In process.c, rb_f_fork() flushes stdout and stderr before forking, so
at least these two buffers are protected from other ruby threads. It
seems like there should be an option to flush everything.
Maybe the best policy is to do any one of the following:
1. Avoid buffered output (puts, write), and instead use unbuffered calls
(syswrite, send) or set io.sync=true.
2. Don't mix threads and fork, and flush everything before forking.
3. Create children only as
fork {do_stuff; exit!}
fork {do_stuff; exec ...}
system "..."
and so on. (And be careful that the do_stuff doesn't flush any buffered
IO that belongs to the parent.)
Most libraries aren't going to follow #1, and #2 seems impractical to
enforce.
Actually, I guess #3 is why I haven't seen this problem before... it's
often a good idea to use exit! in fork to avoid duplicating at_exit()
handlers anyway. I wish there were a method that the child could call
before anything else to _clear_ (not flush) all buffers and also to
clear the at_exit handlers.
--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407
.
- References:
- Process.fork weirdness
- From: Janus Bor
- Re: Process.fork weirdness
- From: Gary Wright
- Re: Process.fork weirdness
- From: Joel VanderWerf
- Process.fork weirdness
- Prev by Date: Re: calculating an array of dates
- Next by Date: unsubscribe
- Previous by thread: Re: Process.fork weirdness
- Next by thread: Re: Process.fork weirdness
- Index(es):
Relevant Pages
|
