SystemStackError: stack level too deep

Ruby
1

Now I am writing a server code to handle each connection by forking a
new process. Here is the code:

@tcpServer = TCPServer.new(host, port)

while true
if tcp = @tcpServer.accept
fork do
// handle all request in this connection
end
tcp && tcp.close
end
end

@tcpServer.close_read
@tcpServer.close_write

I also set up trap to handle CLD signal:
trap(“CLD”) {
pid_info_array = Process.waitall
}

After a period of time, I got the error:

SystemStackError: stack level too deep

The position is at the line of Process.waitall.

I have tried to use Process.wait, but this problem still exist.

Any ideas. Thanks.

2

On 29.02.2008 22:15, Oliver P. wrote:

tcp && tcp.close

After a period of time, I got the error:

SystemStackError: stack level too deep

The position is at the line of Process.waitall.

I have tried to use Process.wait, but this problem still exist.

Any ideas. Thanks.

You are doing an unhealthy mix: Process.waitall and wait block to wait
for an event but SIGCLD is supposed to be called when that event
happens.

So either, use Process.waitall in main or change the trap code to do
something like manipulating a child process counter so you can detect
when the last is gone.

But at the heart of your problem is a logic error: you cannot have an
endless loop and wait for all children to exit at the same time. Either
you decide in the main loop that now no more children are to be created
OR you just have a main loop that runs forever - but then there is no
point in waiting for all childs to terminate.

Kind regards

robert

3

Robert K. wrote:

On 29.02.2008 22:15, Oliver P. wrote:

tcp && tcp.close

After a period of time, I got the error:

SystemStackError: stack level too deep

The position is at the line of Process.waitall.

I have tried to use Process.wait, but this problem still exist.

Any ideas. Thanks.

But at the heart of your problem is a logic error: you cannot have an
endless loop and wait for all children to exit at the same time. Either
you decide in the main loop that now no more children are to be created
OR you just have a main loop that runs forever - but then there is no
point in waiting for all childs to terminate.

Kind regards

robert
Hi Robert:

Thanks for your replay.

You are doing an unhealthy mix: Process.waitall and wait block to wait
for an event but SIGCLD is supposed to be called when that event
happens.

So either, use Process.waitall in main or change the trap code to do
something like manipulating a child process counter so you can detect
when the last is gone.

Sorry. I am not sure what do you mean. In my code, it trap the CLD
signal to run wait method to clean the zombie sub-process, which is same
as the sample code in book ‘Programming ruby’.

When a child process dies, a signal, SIGCHLD (or SIGCLD) is sent to its
parent process. So when I run Process.wait, I always should get the
response at once. And this code can work. The only problem is sometimes
SystemStackError was raised.

Can you please use sample code to show me where is the problem?

Thanks.

4

2008/3/3, Oliver P. [email protected]:

Robert K. wrote:

as the sample code in book ‘Programming ruby’.
There is no trace of Process.waitall in the signal handler sample code.

When a child process dies, a signal, SIGCHLD (or SIGCLD) is sent to its
parent process. So when I run Process.wait, I always should get the
response at once. And this code can work. The only problem is sometimes
SystemStackError was raised.

Can you please use sample code to show me where is the problem?

The problem is in waitall. The sample code otherwise works ok. Try
this

trap “CLD” do
puts “#{Process.wait} exited”

alternative

p Process.wait2
end

5.times do |i|
pid = fork do
puts “proc #{i}”
end and puts “#{pid} created”
end

puts “all children created”

sleep 10

Here’s another approach

require ‘pp’

5.times do |i|
pid = fork do
puts “proc #{i}”
end and puts “#{pid} created”
end

puts “all children created”

pp Process.waitall

Kind regards

robert

5

Robert K. wrote:

2008/3/3, Oliver P. [email protected]:

Robert K. wrote:

as the sample code in book ‘Programming ruby’.
There is no trace of Process.waitall in the signal handler sample code.

When a child process dies, a signal, SIGCHLD (or SIGCLD) is sent to its
parent process. So when I run Process.wait, I always should get the
response at once. And this code can work. The only problem is sometimes
SystemStackError was raised.

Can you please use sample code to show me where is the problem?

The problem is in waitall. The sample code otherwise works ok. Try
this

trap “CLD” do
puts “#{Process.wait} exited”

alternative

p Process.wait2
end

5.times do |i|
pid = fork do
puts “proc #{i}”
end and puts “#{pid} created”
end

puts “all children created”

sleep 10

Here’s another approach

require ‘pp’

5.times do |i|
pid = fork do
puts “proc #{i}”
end and puts “#{pid} created”
end

puts “all children created”

pp Process.waitall

Kind regards

robert

Ok. I see. You mean that waitall shouldn’t be used in trap ‘CLD’ because
it would wait until all process exit.

But I use wait at first and get the SystemStackError exception. Then I
try to use waitall later. So I still don’t know why the SystemStackError
exception is raised.

I also try to write test code and please have a look:

Server code:
require ‘socket’

trap(‘CLD’) do
pid = Process.wait
puts “pid #{pid} exited”
end

class TestServer

def initialize
@tcpServer = TCPServer.new(‘0.0.0.0’, 7777)
end

def run
while tcp = @tcpServer.accept
fork do
begin
while str = tcp.gets()
tcp.puts(“hello”)
end
ensure
tcp.close_read
tcp.close_write
end
end
tcp.close
end
end
end

server = TestServer.new

server.run

Client code:

require ‘socket’

threads = []
100.times do
threads << Thread.new() do
while true
socket = TCPSocket.new(‘localhost’, 7777)
socket.puts(“hello”)
sleep 1
socket.gets
socket.close
sleep 10
end
end
end

threads.each {|thr| thr.join}

From the server output, I can see that CLD signal has been traped and
server puts the process exit. But from top I can see that zombie process
are getting more and more. Is there some problem in the code logic?

Thanks.

6

On 03.03.2008 21:54, Oliver P. wrote:

5.times do |i|

pp Process.waitall
try to use waitall later. So I still don’t know why the SystemStackError
puts “pid #{pid} exited”
fork do
end
require ‘socket’
sleep 10
end
end
end

threads.each {|thr| thr.join}

From the server output, I can see that CLD signal has been traped and
server puts the process exit. But from top I can see that zombie process
are getting more and more. Is there some problem in the code logic?

I can’t really see one. Can you post the exception with the stack
trace?

Kind regards

robert