[erlang-questions] NIFs and hooking file descriptors to Erlang-manged poll
Thu Jan 29 20:32:06 CET 2015
I understand your reasoning, though NIF (being the "most recent" successor
of C API) is a much cleaner API then the marshaling part of drivers.
Having written a fair number of ports/drivers/NIFs I should say that from
coding perspective, IMHO, NIFs are the easiest/cleanest to use when it
comes to bringing existing C libraries with Erlang. Consequently, I would
think that adding FD event awareness to that API (possibly prior to having
full-fledged native processes) would only make it more convenient to use as
it would cover more possible use cases.
On Thu, Jan 29, 2015 at 12:05 PM, Björn-Egil Dahlberg <egil@REDACTED>
> On 2015-01-29 15:20, Serge Aleynikov wrote:
> Is there a way to hook a file descriptor from within a NIF function to
> the select/epoll/poll loop managed by the emulator with a callback invoked
> in the "NIF-land" on activity detected on the file descriptor?
> I am looking for similar functionality available to NIF functions that
> is available when writing drivers using driver_select() call (*). More
> specifically, I have an eventfd file descriptor that I'd like to be
> notified about without allocating a separate OS thread to poll on that FD
> and return results by sending a message to a given Erlang Pid.
> I think you have discovered why we have drivers in Erlang. This is exactly
> the use case for drivers. Normally you don't do much in a driver. You react
> to an event and transfer the binary to erlang space and to what should be
> done in Erlang. Don't try to program Erlang in C (like erl_interface), do
> it in Erlang.
> And no, we don't have native processes (in which case drivers would be
> obsolete). And no, it's not an oversight in the NIF API, not really. You
> could see native processes as an extension to NIFs but it is something
> completely different.
> In some specific cases you could transfer a fd (socket) to Erlangs I/O
> system, i.e. select/poll. See gen_tcp:connect/3,4. I know of projects that
> open fd:s in NIFs which then get transferred to gen_tcp in this manner.
> // Björn-Egil
> (*) http://www.erlang.org/doc/man/erl_driver.html#driver_select
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