[erlang-questions] NIFs and hooking file descriptors to Erlang-manged poll

Björn-Egil Dahlberg egil@REDACTED
Thu Jan 29 18:05:43 CET 2015

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

> Thanks,
> Serge
> (*) http://www.erlang.org/doc/man/erl_driver.html#driver_select
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