[erlang-questions] Port Driver, outputv, binaries.

Tony Rogvall <>
Thu Dec 10 14:30:12 CET 2009


On 10 dec 2009, at 12.20, Matthew Sackman wrote:

> Hi Tony,
> Thank you very much for your email - that's very helpful.
> I have one further question:
> If I construct and send a binary with port_command, can I be guaranteed
> that it'll not be split across two or more different binvs?

My guess is that you could probably assume that a single binary
will not be split up. But how knows ;-) 

> Eg
> a) port_command(Port, [<<1,2,3>>]), or
> b) port_command(Port, [<<"foo">>]), or
> c) port_command(Port, [<<"foo",0>>]), or
> c) is particularly important to me because I've noticed that if you send
> a string down, it doesn't get null terminated, so sticking the extra 0
> on the end is very important. It would be somewhat horrendous if that 0
> could end up in a different binv entry from the "foo".

The part with adding cstring zero termination sounds a bit error prone and dangerous.
I would suggest a length byte (bytes) and checking and use an API where you pass length instead.
(strncpy/memcpy instead of strcpy etc)

> How about:
> d) D = <<1>>, port_command(Port, [<<D/binary, 0, <<255>>/binary, <<"wibble">>/binary>>])
> Is it safe to rely a binary being an atomic unit?

If you mean that the binary parts are not split? then yes.
But as you know the binaries may be ProcBin or HeapBin (you do not really know)


More information about the erlang-questions mailing list