connection_test.c

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

int main()
{
  int sockfd, connfd;
  struct sockaddr_in servaddr, cli;
  char buf[1460];

  sockfd = socket(AF_INET, SOCK_STREAM, 0);
  if (sockfd == -1) {
    printf("socket creation failed...\n");
    return 1;
  }

  printf("Socket successfully created..\n");
  memset(&servaddr, 0, sizeof(servaddr));

  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
  servaddr.sin_port = htons(4321);

  // connect the client socket to server socket
  if (connect(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) != 0) {
    printf("connection with the server failed...\n");
    return 1;
  }

  printf("connected to the server..\n");

  read(sockfd, buf, sizeof(buf));
  printf("From Server : %s", buf);
  close(sockfd);
  return 0;
}

Dockerfile

FROM debian:stretch

RUN apt-get update && apt-get install -y git python libxml2

RUN git clone https://github.com/emscripten-core/emsdk.git && \
    cd emsdk && \
    ./emsdk install latest && \
    ./emsdk activate latest

ENV PATH="/emsdk:/emsdk/node/12.9.1_64bit/bin:/emsdk/upstream/emscripten:${PATH}"

COPY files/src /src

WORKDIR src
RUN emcc connection_test.c -o connection_test.html

CMD emrun --no_browser --port 8080 .

Localhost socket request

GET / HTTP/1.1
Host: 127.0.0.1:4321
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:72.0) Gecko/20100101 Firefox/72.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Sec-WebSocket-Version: 13
Origin: http://10.6.0.159:8080
Sec-WebSocket-Protocol: binary
Sec-WebSocket-Extensions: permessage-deflate
Sec-WebSocket-Key: eQ1PRC4mT6pLSizRjTKlmA==
DNT: 1
Connection: keep-alive, Upgrade
Pragma: no-cache
Cache-Control: no-cache
Upgrade: websocket