[erlang-questions] os:cmd hang on OTP 18

Dániel Szoboszlay <>
Thu Jul 27 09:21:20 CEST 2017


Thanks Dmytro, this really helped a lot!

I think the commit you pointed to is not directly related: it only changes
Erlang code, and if the behaviour depends on whether you are using a
release/debug build, the root cause is most probably somewhere in the C
code of erts.

But the commit message talks about the emulator no longer using vfork, and
it was a good clue: disabling vfork on 18 prevents the problem. So this one
will finish:

ERL_NO_VFORK=true erl +A0 +S 1:1 -noinput -noshell -eval 'os:cmd("tar -C
/tmp/ -xf /tmp/tartest --use-compress-program=lbzip2"), init:stop().'

Thanks again for the clue, I will look into the difference between using
fork/vfork in OTP 18!

Daniel

On Thu, 27 Jul 2017 at 02:09 Dmytro Lytovchenko <
> wrote:

> I could observe the behaviour only in R18, but not in R19 and not in R20
> I also could not reproduce it in debug flavour of R18 emulator, but it
> reproduces reliably in release SMP variant.
>
> The changes to os.erl between 18.3.4.5 and 19.0 include removal of os:cmd
> server which might somehow be related (commit
> *200247f972b012ced0c4b2c6611f091af66ebedd*). This commit *possibly* fixes
> the behavior — in R19 (build 19.0 by Kerl) the behaviour does not happen.
>
> 2017-07-26 21:47 GMT+02:00 Dániel Szoboszlay <>:
>
>> Honestly, I didn't try with other command variations. There are many
>> commands that do not hang when run from os:cmd, regardless of the OTP
>> version. But this particular command does hang with one OTP version, and
>> not with the other OTP version. So the difference is in OTP, and I want to
>> find out what has changed.
>>
>> Daniel
>>
>> On Wed, 26 Jul 2017 at 21:34 Dmytro Lytovchenko <
>> > wrote:
>>
>>> Is it something lbzip2 related?
>>> Did you try normal single-thread bzip2? (-j flag or --bzip2)
>>> What is you use gzip? (-z or --gzip)
>>>
>>> 2017-07-26 21:27 GMT+02:00 Dániel Szoboszlay <>:
>>>
>>>> Hi,
>>>>
>>>> I've encountered a strange problem with os:cmd when running tar and
>>>> lbzip2. Steps to reproduce:
>>>>
>>>> # create some lbzip2 compressed data
>>>>
>>>> dd if=/dev/urandom of=/tmp/testfile count=10
>>>> tar -cf - -C /tmp testfile | lbzip2 -6 -n 4 | dd of=/tmp/tartest status=none
>>>>
>>>>
>>>> # try to extract the archive from Erlang with os:cmd
>>>>
>>>> erl -noinput -eval 'os:cmd("tar -C /tmp/ -xf /tmp/tartest --use-compress-program=lbzip2"), init:stop().'
>>>>
>>>>
>>>> This worked fine with OTP 17.5.6.7, but with OTP 18.3.4.5 the command
>>>> hangs: lbzip2 just sits in a rt_sigsuspend syscall waiting for a USR2, PIPE
>>>> or XFSZ signal. And its parent, the tar process waits in a wait4 syscall
>>>> for lbzip2 to terminate.
>>>>
>>>> I don't have at the moment any newer OTP version installed, I'm not
>>>> sure how OTP 19 or 20 would behave.
>>>>
>>>> I tried to strace the processes, but there's too much noise, I couldn't
>>>> yet figure out anything interesting there.
>>>>
>>>> I also tried to diff OTP 17 & 18, but os:cmd/1 and friends didn't
>>>> change. I'm not sure about the port code, but at least the release notes
>>>> didn't mention anything major. Or did I miss something? Does anyone have an
>>>> idea what may have changed between these OTP versions?
>>>>
>>>> Thanks,
>>>> Daniel
>>>>
>>>> _______________________________________________
>>>> erlang-questions mailing list
>>>> 
>>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>>
>>>>
>>>
>
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