Tuesday, March 31, 2009

RPC from Erlang Linked-in driver port

Developing SAX based XML parser (using libexpat) as a linked-in driver for Erlang, I came across the requirement to do the callback within linked-in driver. I have done it earlier in the c-node via ei_rpc C API. This require linked-in driver to run as a c-node. I was looking for a way to avoid it.

Then I came across this function driver_send_term. This can be used to send a term to any PID within the local VM. It is easy to send the term to the same process which did port_cmd. The PID for that process can be obtained using driver_caller C API.

My requirement is to send it to some other PID than the calling process. There is no standard API to do so. But going through the source code for erlang, I figured the way to convert erlang_pid sent by the caller to the one usable within driver_send_term call.

ErlDrvTermData pid = ((ErlDrvTermData) ( ((callbackPid.serial << 15 | callbackPid.num)) << 4 | (0x0 << 2 | 0x3)) );

This works with R12 and R13 version of Erlang. This is not guaranteed to work in the future releases. But till then I am going to use it.

I wonder why Erlang did not provide the standard interface to convert erlang_pid to ErlDrvTermData to be used within driver_send_term.

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