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86 lines
2.8 KiB
Markdown
86 lines
2.8 KiB
Markdown
---
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title: "W04. Send and Receive Binary Frames"
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order: 54
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status: "draft"
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---
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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.
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## How to pick a frame type
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```cpp
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ws.send(std::string("Hello")); // text
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ws.send("Hello", 5); // binary
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ws.send(binary_data, binary_data_size); // binary
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```
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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.
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If you have a `std::string` and want to send it as binary, pass `.data()` and `.size()` explicitly.
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```cpp
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std::string raw = build_binary_payload();
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ws.send(raw.data(), raw.size()); // binary frame
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```
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## Detect frame type on receive
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The return value of `ws.read()` tells you whether the received frame was text or binary.
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```cpp
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std::string msg;
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auto result = ws.read(msg);
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switch (result) {
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case httplib::ws::ReadResult::Text:
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std::cout << "text: " << msg << std::endl;
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break;
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case httplib::ws::ReadResult::Binary:
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std::cout << "binary: " << msg.size() << " bytes" << std::endl;
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handle_binary(msg.data(), msg.size());
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break;
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case httplib::ws::ReadResult::Fail:
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// error or closed
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break;
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}
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```
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Binary frames still come back in a `std::string`, but treat its contents as raw bytes — use `msg.data()` and `msg.size()`.
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## When binary is the right call
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- **Images, video, audio**: No Base64 overhead
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- **Custom protocols**: protobuf, MessagePack, or any structured binary format
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- **Game networking**: When latency matters
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- **Sensor data streams**: Push numeric arrays directly
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## Ping is binary-ish, but hidden
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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](w02-websocket-ping).
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## Example: send an image
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```cpp
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// Server: push an image
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svr.WebSocket("/image", [](const auto &req, auto &ws) {
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auto img = read_image_file("logo.png");
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ws.send(img.data(), img.size());
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});
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```
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```cpp
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// Client: receive and save
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httplib::ws::WebSocketClient cli("ws://localhost:8080/image");
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cli.connect();
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std::string buf;
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if (cli.read(buf) == httplib::ws::ReadResult::Binary) {
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std::ofstream ofs("received.png", std::ios::binary);
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ofs.write(buf.data(), buf.size());
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}
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```
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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.
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> **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.
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