UDP socket at webassembly

Question

I'm trying to port my desktop app written in C and C++ to webassembly platform and am investigating if it is possible at all. One of important things the app does is communicate by sending and receiving UDP messages. I have implemented minimal UDP client which just creates UDP socket and sends packets to server (which is build natively and is running as separate executable at the same machine). socket, bind and sendto APIs return no error and everything looks working but no messages are receiving on server side and wireshark shows no activity on that port.

Is UDP socket just stubbed at webassembly libc port, or it is implemented on top of some web standard connection (e.g. WebRTC)?

The client code is below. I checked that native build is working properly.

#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>

#define BUFLEN 512
#define NPACK 100
#define PORT 9930

void diep(char *s)
{
  perror(s);
  exit(1);
}

#define SRV_IP "127.0.0.1"


int main(void)
{
  struct sockaddr_in si_other;
  int s, i, slen=sizeof(si_other);
  char buf[BUFLEN];

  if ((s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
    diep("socket");

  memset((char *) &si_other, 0, sizeof(si_other));
  si_other.sin_family = AF_INET;
  si_other.sin_port = htons(PORT);
  if (inet_aton(SRV_IP, &si_other.sin_addr)==0) {
    fprintf(stderr, "inet_aton() failed\n");
    exit(1);
  }

  for (i=0; i<NPACK; i++) {
    printf("Sending packet %d\n", i);
    sprintf(buf, "This is packet %d\n", i);
    if (sendto(s, buf, BUFLEN, 0, (struct sockaddr*)&si_other, slen)==-1)
      diep("sendto()");
  }

  close(s);
  return 0;
}

I followed instructions from http://webassembly.org/getting-started/developers-guide/ to build and run it.

Thanks in advance for any help or clues!

Answer 1

I found how UDP sockets are implemented at webassembly. Actually, they are emulated by websockets. It probably would work if both client and server were webassemblies, but my server is built natively. As wasm doesn't support dynamic linking, all the code (including libc implementation) is bundled to one JS file, were we can find UDP sendto implementation:

// if we're emulating a connection-less dgram socket and don't have
      // a cached connection, queue the buffer to send upon connect and
      // lie, saying the data was sent now.
      if (sock.type === 2) {
        if (!dest || dest.socket.readyState !== dest.socket.OPEN) {
          // if we're not connected, open a new connection
          if (!dest || dest.socket.readyState === dest.socket.CLOSING || dest.socket.readyState === dest.socket.CLOSED) {
            dest = SOCKFS.websocket_sock_ops.createPeer(sock, addr, port);
          }
          dest.dgram_send_queue.push(data);
          return length;
        }
      }