Port driver communication witout copy of binaries

Romain Lenglet <>
Wed Mar 29 05:29:40 CEST 2006


Raimo Niskanen wrote:
> What you really want to do can not be done (as far as I know)
> but you might get it done with some tricks...
>
> To avoid copying your driver must implement the
> ->outputv() entry point and you must send it I/O lists
> being lists of binaries (might even be an improper list,
> that is a binary in the tail). You will have to map
> your tuples into that.
>
> If you send [1,<<2,3,4>>,5,6|<<7,8>>] to the driver,
> void (*outputv)(ErlDrvData drv_data, ErlIOVec *ev) will get:
>
> ev->iov[0].iov_len = 1;
> ev->iov[0].iov_base -> {1};
> ev->binv[0] = NULL;
> ev->iov[1].iov_len = 3;
> ev->iov[1].iov_base -> ev->binv[1]->orig_bytes;
> ev->binv[1]->orig_size = 3;
> ev->binv[1]->orig_bytes = {2,3,4};
> ev->iov[2].iov_len = 2;
> ev->iov[2].iov_base -> {5,6};
> ev->binv[2] = NULL;
> ev->iov[3].iov_len = 2;
> ev->iov[3].iov_base -> ev->binv[3]->orig_bytes;
> ev->binv[3]->orig_size = 2;
> ev->binv[3]->orig_bytes = {7,8};
>
> approximately, excuse my syntax :-)
>
> Binaries will be binaries and intermediate bytes
> will be loose vectors. If your driver wants to
> hang on to the data, it will have to use the
> reference count in the binary to avoid premature freeing.

Very nice! Using lists is OK for me.
I don't have to stick to tuples, I used tuples only to explain my 
problem.


> To send data back without copying your driver will
> have to use driver_outputv() and it arrives to erlang as
> a header list of integers followed by a list of
> binaries. Conversion to tuple format will have to
> be done in erlang.

Yes, I wanted to use such functions, but I was not sure if 
binaries were copied.


> Keep on dreaming...
>
> Have a look at efile_drv.c in the sources...

Thanks a lot, I have a little hope now to do things cleanly. :)

-- 
Romain LENGLET



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