[erlang-questions] Wrong large 64bit return-values from Erlang Linkedin Driver

Thijs Terlouw <>
Fri Jun 18 08:41:14 CEST 2010


Hi Mikael,

I have attached a self-contained test. It consists of a few erlang
files (bignum.erl (driver) , gen_linkedin_driver.erl (generic driver)
bignumtest.erl (test to use the driver) and reloader.erl (for easy
debugging)). It also contains a modified version of the C driver. On
my machine (Linux 2.6.16.21-0.8 i686) I just run "make". I have Erlang
R13B04: (erts-5.7.5 [source] [smp:4:4] [rq:4] [async-threads:30]
[hipe] [kernel-poll:true]). The file also contains a readme.txt how to
start the test. It uses eunit for the unit test. Which strangely
enough succeeds!

usage:
tar -xzvf bignumtest.tar.gz
cd bignumtest
make
./start.sh

1st test:
bignumtest:test_driver().
result:
"2) 322122547200 =:= 4294967296 => true"
expected:
"2) 4294967296 =:= 4294967296 => true"

Obviously something is very wrong here. It prints a value that is not
equal, but when the values are compared, they are equal?

2nd test (unit test)
bignumtest:test().
result:
The first run all unit tests pass. The second run, it will always
result in a core-dump.
expected:
no core dump

A colleague (echouzhou) found a bug inside the function
"erts_sint64_to_big" in "erts/emulator/beam/big.c". On lines 1518 and
1528 the *hpp pointer is not incremented correctly. It should be "*hpp
+= 3;" and "*hpp += 2;". After we changed this and recompiled Erlang,
the above tests all succeed without crashing and print the correct
values. I didn't look if the same bug is also in other parts of the
code.


Thijs




On Thu, Jun 17, 2010 at 8:40 PM, Mikael Pettersson <> wrote:
> Thijs Terlouw writes:
>  > Hello,
>  >
>  > I'm trying to return signed 64bit integers (on a 32bit system) from a
>  > custom Erlang linkedin driver. I try to use the ERL_DRV_INT64 type on
>  > the C-side. Because ERL_DRV_INT64 requires a pointer to ErlDrvSInt64,
>  > I first malloc a buffer, store it there, get a pointer to the result
>  > and then return that pointer. For small values this works great.
>  >
>  > Whenever I use values >134217727 (0x7FFFFFF) it will return wrong
>  > values.
>
> Can you please provide a self-contained test? I.e., an Erlang
> module that loads your driver, invokes it, and checks the result?
> And whatever build instructions are needed for your driver.
>
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