[erlang-questions] "erl_drv_output_term" vs "driver_output_binary" ?

[Frank Muller]

One more question Raimo: how about the opposite scenario: Erlang sending
big binary to the driver?

1. Is the binary simply reference-counted?
2. What if the Erlang side change the binary after
sending it? Should I worry about that?

/Frank


Thanks Raimo. I'll stick with "driver_output_binary" for now.
>
> /Frank
>
> Le ven. 21 juil. 2017 à 11:44, Raimo Niskanen <
> raimo+erlang-questions@REDACTED> a écrit :
>
>> On Fri, Jul 21, 2017 at 09:11:24AM +0000, Frank Muller wrote:
>> > Hi there,
>> >
>> > I would like to know the difference between the two calls.
>> >
>> > My linked-in driver has to send big (~4MB in average) binaries to the
>> > Erlang owning process.
>> >
>> > 1. Which call is better suited from that ?
>>
>> driver_output_binary(port, NULL, 0, bin, offset, len) will be received by
>> the owning process as {Port,{data,Bin}}.  A containing integer list can be
>> added using hbuf != NULL.
>>
>> erl_drv_output_term(port, term, n) will be received by the owning process
>> simply as Term.  This involves parsing the term in ErlDrvTermData[] format
>> to be able to produce any term.
>>
>> > 2. Does every binary gets copied or just reference counted?
>>
>> Binaries from a driver gets reference counted with either of these APIs.
>>
>> > 3. Any better alternative to use?
>>
>> I think these are fairly equivalent except that one produces a term on a
>> predefined format.
>>
>> > 4. Anything else I should be aware of?
>>
>> If your driver needs the binary after sending it to Erlang it will have to
>> increment the reference count itself, and free it later after decrementing
>> the reference count.
>>
>> The driver can not change the data in the binary after sending it to
>> Erlang.  That has very undefined consequences.
>>
>> >
>> > Many thanks...
>> > /Frank
>>
>>
>> --
>>
>> / Raimo Niskanen, Erlang/OTP, Ericsson AB
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>>
>
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