Can read() be non-blocking?

[Thomas Christensen]

This issue has been raised a couple of times I am sure. But I have yet
to find a satisfying answer.

I am reading from a subprocess and this subprocess sometimes hang, in
which case a call to read() call will block indefinite, keeping me from
killing it.

The folloing sample code illustrates the problem:

proc = subprocess.Pope n(['/usr/bin/foo', '/path/to/some/file'],
stdout=subproce ss.PIPE)
output = StringIO.String IO()
while True:
r = select.select([proc.stdout.fil eno()], [], [], 5)[0]
if r:
# NOTE: This will block since it reads until EOF
data = proc.stdout.rea d()
if not data:
break # EOF from process has been reached
else:
output.write(da ta)
else:
os.kill(proc.pi d, signal.SIGKILL)
proc.wait()

<Process the output...>

As the NOTE: comment above suggests the call to read() will block here.

I see two solutions:

1. Read one byte at a time, meaning call read(1).
2. Read non-blocking.

I think reading one byte at a time is a waste of CPU, but I cannot find
a way to read non-blocking.

Is there a way to read non-blocking? Or maybe event a better way in
generel to handle this situation?

Thanks

Thomas

[Lawrence D'Oliveiro]

Re: Can read() be non-blocking?

In message <mailman.3600.1 226012406.3487. python-list@python.org >, Thomas
Christensen wrote:

r = select.select([proc.stdout.fil eno()], [], [], 5)[0]
if r:
# NOTE: This will block since it reads until EOF
data = proc.stdout.rea d()

No, it will read what data is available.

[Lawrence D'Oliveiro]

Re: Can read() be non-blocking?

In message <gf07sh$inr$3@l ust.ihug.co.nz> , Lawrence D'Oliveiro wrote:

In message <mailman.3600.1 226012406.3487. python-list@python.org >, Thomas
Christensen wrote:
>

> r = select.select([proc.stdout.fil eno()], [], [], 5)[0]
> if r:
> # NOTE: This will block since it reads until EOF
> data = proc.stdout.rea d()

>
No, it will read what data is available.

Sorry, maybe not. But you can set O_NOBLOCK on the fd.

[=?GB2312?B?0rvK18qr?=]

Re: Can read() be non-blocking?

On Nov 7, 6:54 am, Thomas Christensen <thom...@thomas christensen.org >
wrote:

This issue has been raised a couple of times I am sure.  But I have yet
to find a satisfying answer.
>
I am reading from a subprocess and this subprocess sometimes hang, in
which case a call to read() call will block indefinite, keeping me from
killing it.
>
The folloing sample code illustrates the problem:
>
  proc = subprocess.Pope n(['/usr/bin/foo', '/path/to/some/file'],
                          stdout=subproce ss.PIPE)
  output = StringIO.String IO()
  while True:
      r = select.select([proc.stdout.fil eno()], [], [], 5)[0]
      if r:
          # NOTE: This will block since it reads until EOF
          data = proc.stdout.rea d()
          if not data:
              break  # EOF from process has been reached
          else:
              output.write(da ta)
      else:
          os.kill(proc.pi d, signal.SIGKILL)
  proc.wait()
>
  <Process the output...>
>
As the NOTE: comment above suggests the call to read() will block here.
>
I see two solutions:
>
1. Read one byte at a time, meaning call read(1).
2. Read non-blocking.
>
I think reading one byte at a time is a waste of CPU, but I cannot find
a way to read non-blocking.
>
Is there a way to read non-blocking?  Or maybe event a better way in
generel to handle this situation?
>
Thanks
>
                Thomas

As far as I know, you can use '''fctnl''' to make a file handle non-
blocking.

But :

1. I don't know if it works on standard io
2. If it works in python

[Karthik Gurusamy]

Re: Can read() be non-blocking?

On Nov 6, 2:54 pm, Thomas Christensen <thom...@thomas christensen.org >
wrote:

This issue has been raised a couple of times I am sure.  But I have yet
to find a satisfying answer.
>
I am reading from a subprocess and this subprocess sometimes hang, in
which case a call to read() call will block indefinite, keeping me from
killing it.
>
The folloing sample code illustrates the problem:
>
  proc = subprocess.Pope n(['/usr/bin/foo', '/path/to/some/file'],
                          stdout=subproce ss.PIPE)
  output = StringIO.String IO()
  while True:
      r = select.select([proc.stdout.fil eno()], [], [], 5)[0]
      if r:
          # NOTE: This will block since it reads until EOF
          data = proc.stdout.rea d()
          if not data:
              break  # EOF from process has been reached
          else:
              output.write(da ta)
      else:
          os.kill(proc.pi d, signal.SIGKILL)
  proc.wait()
>
  <Process the output...>
>
As the NOTE: comment above suggests the call to read() will block here.
>
I see two solutions:
>
1. Read one byte at a time, meaning call read(1).
2. Read non-blocking.
>
I think reading one byte at a time is a waste of CPU, but I cannot find
a way to read non-blocking.
>
Is there a way to read non-blocking?  Or maybe event a better way in
generel to handle this situation?

From what I understand, you want a way to abort waiting on a blocking
read if the process is hung.
There are some challenges about how you decide if the process is hung
or just busy doing work without generating output for a while (or may
be the system is busy and the process didn't get enough CPU due to
other CPU hungry processes).
Assuming you have a valid way to figure this out, one option is to
have a timeout on the read.
If the timeout exceeds, you abort the read call. No, the read doesn't
provide a timeout, you can build one using alarm.

def alarm_handler(* args):
""" This signal stuff may not work in non unix env """
raise Exception("time out")

signal.signal(s ignal.SIGALRM, alarm_handler)

try:
signal.alarm(ti meout) # say timeout=60 for a max wait of 1
minute
data = proc.stdout.rea d()
except Exception, e:
if not str(e) == 'timeout': # something else went wrong ..
raise
# got the timeout exception from alarm .. proc is hung; kill it

Karthik

>
Thanks
>
                Thomas

[saju.pillai@gmail.com]

Re: Can read() be non-blocking?

On Nov 7, 9:09 am, Lawrence D'Oliveiro <l...@geek-
central.gen.new _zealandwrote:

In message <gf07sh$in...@l ust.ihug.co.nz> , Lawrence D'Oliveiro wrote:
>

In message <mailman.3600.1 226012406.3487. python-l...@python.org >, Thomas
Christensen wrote:

>

      r = select.select([proc.stdout.fil eno()], [], [], 5)[0]
      if r:
          # NOTE: This will block since it reads until EOF
          data = proc.stdout.rea d()

>

No, it will read what data is available.

>
Sorry, maybe not. But you can set O_NOBLOCK on the fd.

Set O_NONBLOCK on proc.fileno() and try using os.read() on that
descriptor.

-srp