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Samples that did not compile or run as shown: - res.user_data.get<T>() inside a generic lambda needs the `template` keyword; use explicit parameter types (tour 09, cookbook s15). - listen() on a Unix domain socket fails with port 0 (tour 09, s22). - "*.dev.local" is not a NO_PROXY pattern (c16). - ssl_backend_error() holds a verify result, not an ERR_get_error() value, after a verification failure; decode each with the matching OpenSSL function (c18). - The content provider's `length` is everything that remains, so the sample read the whole file in one call (s05). Statements corrected: - Client keep-alive is off by default; c14 is rewritten around set_keep_alive(true). - Mounted files are looked up before GET handlers (tour 04, s04). - Params keep insertion order, and to_string(Error::Connection) reads "Could not establish connection" (tour 02). - A chunked provider ends with sink.done(), and post_routing_handler runs before the response is sent (tour 09). - Timeouts surface as Error::Read; Error::Timeout comes from the stream API (c17). The max timeout cuts off the wait for the response only (c13). The progress callback needs Content-Length (c11). - Encoding selection follows q-values, then Brotli, gzip, Zstd (s08), and the client compresses with the first of those it was built with (c15). - stop() cuts a provider-driven response short (s19); a rejected content_reader already gets 400 or 413 (s07); user_data values must be copyable (s12); Client accepts a client certificate too (t04); on_message() is the fallback for every unhandled event and 204/403/404 end reconnection (e04); the pong timeout takes two to three intervals and ends a waiting read() (w02). In the LLM app tutorial, an uncaught exception does not crash the server, so say what it does instead. Drop the server and client timeout settings whose stated purpose, covering inference and download time, they do not serve: those timeouts bound a single socket wait. Update the llama.cpp server layout in chapter 7.
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title, order, status
| title | order | status |
|---|---|---|
| C11. Use the Progress Callback | 11 | draft |
To display download or upload progress, pass a DownloadProgress or UploadProgress callback. Both take two arguments: (current, total).
Download progress
httplib::Client cli("http://localhost:8080");
auto res = cli.Get("/large-file",
[](size_t current, size_t total) {
auto percent = (total > 0) ? (current * 100 / total) : 0;
std::cout << "\rDownloading: " << percent << "% ("
<< current << "/" << total << ")" << std::flush;
return true; // return false to abort
});
std::cout << std::endl;
The callback fires each time data arrives. total comes from the Content-Length header. For a response without one (chunked transfer, for example), the progress callback is not called at all.
Upload progress
Uploads work the same way. Pass an UploadProgress as the last argument to Post() or Put().
httplib::Client cli("http://localhost:8080");
std::string body = load_large_body();
auto res = cli.Post("/upload", body, "application/octet-stream",
[](size_t current, size_t total) {
auto percent = current * 100 / total;
std::cout << "\rUploading: " << percent << "%" << std::flush;
return true;
});
std::cout << std::endl;
Cancel mid-transfer
Return false from the callback to abort the transfer. This is how you wire up a "Cancel" button in a UI — flip a flag, and the next progress tick stops the transfer.
std::atomic<bool> cancelled{false};
auto res = cli.Get("/large-file",
[&](size_t current, size_t total) {
return !cancelled.load();
});
Note: A response without Content-Length never calls the progress callback, so it cannot be cancelled this way. Return
falsefrom aContentReceiverinstead.
Note:
ContentReceiverand the progress callback can be used together. When you want to stream to a file and show progress at the same time, pass both.
For a concrete example of saving to a file, see C01. Get the response body / save to a file.