cpp-httplib v0.42.0
DRAFT

W04. Send and Receive Binary Frames

WebSocket has two frame types: text and binary. JSON and plain text go in text frames; images and raw protocol bytes go in binary. In cpp-httplib, send() picks the right type via overload.

How to pick a frame type

ws.send(std::string("Hello"));           // text
ws.send("Hello", 5);                      // binary
ws.send(binary_data, binary_data_size);   // binary

ws.send(std::string("Hello"));           // text
ws.send("Hello", 5);                      // binary
ws.send(binary_data, binary_data_size);   // binary

The std::string overload sends as text. The const char* + size overload sends as binary. A bit subtle, but once you know it, it sticks.

If you have a std::string and want to send it as binary, pass .data() and .size() explicitly.

std::string raw = build_binary_payload();
ws.send(raw.data(), raw.size()); // binary frame

std::string raw = build_binary_payload();
ws.send(raw.data(), raw.size()); // binary frame

Detect frame type on receive

The return value of ws.read() tells you whether the received frame was text or binary.

std::string msg;
auto result = ws.read(msg);

switch (result) {
  case httplib::ws::ReadResult::Text:
    std::cout << "text: " << msg << std::endl;
    break;
  case httplib::ws::ReadResult::Binary:
    std::cout << "binary: " << msg.size() << " bytes" << std::endl;
    handle_binary(msg.data(), msg.size());
    break;
  case httplib::ws::ReadResult::Fail:
    // error or closed
    break;
}

std::string msg;
auto result = ws.read(msg);

switch (result) {
  case httplib::ws::ReadResult::Text:
    std::cout << "text: " << msg << std::endl;
    break;
  case httplib::ws::ReadResult::Binary:
    std::cout << "binary: " << msg.size() << " bytes" << std::endl;
    handle_binary(msg.data(), msg.size());
    break;
  case httplib::ws::ReadResult::Fail:
    // error or closed
    break;
}

Binary frames still come back in a std::string, but treat its contents as raw bytes — use msg.data() and msg.size().

When binary is the right call

  • Images, video, audio: No Base64 overhead
  • Custom protocols: protobuf, MessagePack, or any structured binary format
  • Game networking: When latency matters
  • Sensor data streams: Push numeric arrays directly

Ping is binary-ish, but hidden

WebSocket Ping/Pong frames are close cousins of binary frames at the opcode level, but cpp-httplib handles them automatically — you don't touch them. See W02. Set a WebSocket heartbeat.

Example: send an image

// Server: push an image
svr.WebSocket("/image", [](const auto &req, auto &ws) {
  auto img = read_image_file("logo.png");
  ws.send(img.data(), img.size());
});

// Server: push an image
svr.WebSocket("/image", [](const auto &req, auto &ws) {
  auto img = read_image_file("logo.png");
  ws.send(img.data(), img.size());
});

// Client: receive and save
httplib::ws::WebSocketClient cli("ws://localhost:8080/image");
cli.connect();

std::string buf;
if (cli.read(buf) == httplib::ws::ReadResult::Binary) {
  std::ofstream ofs("received.png", std::ios::binary);
  ofs.write(buf.data(), buf.size());
}

// Client: receive and save
httplib::ws::WebSocketClient cli("ws://localhost:8080/image");
cli.connect();

std::string buf;
if (cli.read(buf) == httplib::ws::ReadResult::Binary) {
  std::ofstream ofs("received.png", std::ios::binary);
  ofs.write(buf.data(), buf.size());
}

You can mix text and binary in the same connection. A common pattern: JSON for control messages, binary for the actual data — you get efficient handling of metadata and payload both.

Note: WebSocket frames don't have an infinite size limit. For very large data, chunk it in your application code. cpp-httplib can handle a big frame in one shot, but it does load it all into memory at once.

ESC