Send message to GenServer from C

Question

How do I send a message to a remote Elixir GenServer and then receive the results of the call using the C Erlang Interface?

I want to run something in C analogous to

{result1, result2} = GenServer.call(MyModule.NodeName, {:dothing, "blah"})

Here's what I have so far. It compiles and connects to the remote server and runs ei_reg_send without causing an error, but the remote server doesn't receive a response. (I have a logger turned on so I know when the call comes through.)

#include <erl_interface.h>
#include <ei.h>

#define COOKIE "cookieval"
#define HOST "name@host"

int main() {
  ei_init();
  ei_cnode ec;
  int n = 0;
  int sockfd;
  int self;
  if((self = ei_connect_init(&ec, "nss", COOKIE, n++)) < 0) {
    return 1;
  }
  if((sockfd = ei_connect(&ec, HOST)) < 0) {
    return 2;
  }

  ei_x_buff request;
  ei_x_new(&request);
  ei_x_format(&request, "{dothing, ~a}", "blah");
  if(ei_reg_send(&ec, sockfd, "MyModule.NodeName", request.buff, request.index) < 0) {
    return 3;
  }
  ei_x_free(&request);
  // code makes it to here, but there's no indication that the genserver was called
  return 0;
}

Answer 1

You cannot do GenServer.call/2 in C Node. At least not directly, due to fact that this is not public API how the messages in that case look like (not that it changes a lot, but you cannot rely on that). Instead what you can do is that you send regular message and handle that within handle_info.

Answer 2

Calling a GenServer with a predetermined name directly from C was, as pointed out, more difficult than expected. What I ended up doing was creating a module function in Elixir that called the GenServer itself and returned the results to the caller. Then I interfaced with this in C using RPC functions.

  ei_x_buff request;                                                               
  ei_x_new(&request);                                                              
  ei_x_format_wo_ver(&request, "[~s]", "My Argument");                                      
  ei_x_buff response;                                                              
  ei_x_new(&response);                                                             
  ei_rpc(&ec, sockfd, "Elixir.MyModule", "dothing", request.buff, request.index, &response);

See http://erlang.org/doc/man/ei_connect.html#ei_rpc

Note that ei_rpc doesn't return a "version magic number" in its response buffer but ei_rpc_from does.