SSE using cpp-httplib

client.cc

#include <httplib.h>
#include <cstdio>
#include <chrono>

using namespace httplib;

bool reconnect = true;
unsigned long retry = 3000;
//warning: events do not necessarily correspond with chunks
//a chunk may contain more than one event
bool chunk_callback(const char *data, uint64_t len){
  //warning: this callback runs async
  std::string msg(data, len);
  if(msg.rfind("retry: ", 8) == 0){
    retry = std::stoul(msg.substr(7));
    printf("detected timeout: %lu\n", retry);
  };
  printf("new chonk: %s\n", msg.c_str());
  return true;
});

int main(){
  // http://samshull.com/server-sent/events.php
  Client cli("samshull.com");
  cli.follow_location(true);
  const httplib::Headers headers = {
    { "Accept", "text/event-stream" },
    { "Connection", "Keep-Alive"}
  };
  while(reconnect){
    auto res = cli.Get("/server-sent/events.php", headers, chunk_callback);
    if(res){
      printf("HTTP %i\n", res->status);
      //check that response code is 2xx
      reconnect = (res->status / 100) == 2;
    }
    else{
      printf("SSE stream ended. Will retry in %f seconds\n", retry/1000.0);
      std::this_thread::sleep_for(std::chrono::milliseconds(retry));
      printf("Retrying\n");
    }
  }
  printf("closed connection\n");
  return 0;
}

server.cc

#include <chrono>
#include <cstdio>
#include <httplib.h>

using namespace httplib;

uint64_t event_id;
void create_event(uint64_t offset, DataSink sink, Done done){
  std::string eventdata;
  std::this_thread::sleep_for(std::chrono::seconds(3));
  if(event_id == 0)
    //tell client to retry in 15s if disconnected
    eventdata = "retry: 15000\n\n";
  else
  {
    eventdata += "id: ";
    eventdata += std::to_string(event_id);
    eventdata += "\ndata: This is an event message\n\n";
  }
  event_id++;
  sink(eventdata.c_str(), eventdata.length());
}

int main(void) {
  Server svr;
  svr.Get("/eventsrc",[](const Request &req, Response &res){
    res.set_chunked_content_provider(create_event);
    res.set_header("Content-Type", "text/event-stream");
  });
  
  svr.listen("localhost", 8080);

  return 0;
}

Unfortunately your program can not compile:
error: ‘struct httplib::Response’ has no member named ‘content_producer’
res.content_producer = create_event;
I have the newest httplib.h!
Gerald

@kreativnix sorry! yhirose updated httplib.h upstream and changed the content streaming interface on the server side. I have edited this gist to reflect his changes.

Hi PixlRainbow, I thank you very much for the quick reaction. I did not believe that in advance. Now I can continue to test.
My goal is to find a relative simple possibility to program in a RasPi 3B+ a program including webserver functionality with SSE, because I have measurment data to display. Maybe this httplib will help!
Many thanks and a nice weekend
Gerald