Re: forking processes and writing to log file

Joe Van D. wrote:

On 9/26/06, Joe Van D. [email protected] wrote:

Hello,

So, on linux, I’m forking a new Ruby process. I redirect stdout and
stderr into a file and then I execute another program via ‘system’.

I should probably mention that the other program that’s being executed
is rsh, which executes a command on a different machine.

You either did not redirect stdout/stderr to a file properly, or
something you execute tries to output directly to the controlling
terminal (/dev/tty). You can disassosiate your spawned process from a
controlling terminal, just to be sure (not with ‘system’ though):

logfile = “my_log_file.log”
pid = fork do
File.open(logfile, ‘w’) do |_io|
$stdout.reopen(_io)
$stderr.reopen(_io)
end

Process.setsess # In this scenario disassociates the controlling
# terminal until another tty device is open (will
# become new controlling tty).

exec “the_command_line_used_to_be_launched_by_system”
end

Process.waitpid pid

===
Gennady.

On 9/26/06, Gennady B. [email protected] wrote:

You either did not redirect stdout/stderr to a file properly, or

Process.setsess # In this scenario disassociates the controlling
# terminal until another tty device is open (will
# become new controlling tty).

exec “the_command_line_used_to_be_launched_by_system”
end

Process.waitpid pid

I assume you mean Process.setsid?

I have this function called at the beginning of the fork:

Redirect stdout and stderr to a file and flush the buffers on a write

def redirect_all_output_to_file file
log = File.new(file, “w+”)
STDOUT.reopen log; STDERR.reopen log
STDOUT.sync = true
end

When I add Process.setsid to the end of that function, when I
background the ruby process, stuff keeps writing to the log file, but
my controlling terminal gets all messed up. The only keys on my
keyboard that work are ‘e’, ‘s’, the space bar, and the backspace key,
and they only work half the time. The terminal works normally once
the ruby process is killed.

On 9/26/06, Joe Van D. [email protected] wrote:

end
I assume you mean Process.setsid?
background the ruby process, stuff keeps writing to the log file, but
my controlling terminal gets all messed up. The only keys on my
keyboard that work are ‘e’, ‘s’, the space bar, and the backspace key,
and they only work half the time. The terminal works normally once
the ruby process is killed.

Ah, this appears to be an rsh issue, not a Ruby one. (can’t use ssh
here because people are too lazy to set up ssh keys or something)

$ rsh machine some_command &
That seems to need to wait for input, even if some_command doesn’t
read from stdin.

$ ssh machine some_command &
works as expected.

My workaround is to create a file with half a dozen blank lines and do
$ rsh machine some_command < file_with_blank_lines &

But that seems icky.

On 9/26/06, Joe Van D. [email protected] wrote:

is rsh, which executes a command on a different machine.
$stderr.reopen(_io)

When I add Process.setsid to the end of that function, when I
That seems to need to wait for input, even if some_command doesn’t
read from stdin.

$ ssh machine some_command &
works as expected.

My workaround is to create a file with half a dozen blank lines and do
$ rsh machine some_command < file_with_blank_lines &

But that seems icky.

Sorry for all the non-ruby related noise, but the problem is fixed.
From the BUGS section in the rsh man page:

 If you are using csh(1) and put a rsh in the background without 

redirect-
ing its input away from the terminal, it will block even if no
reads are
posted by the remote command. If no input is desired you should
redirect
the input of rsh to /dev/null using the -n option.

Joe

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