cpp-httplib/docs-src/pages/en/cookbook/s05-stream-response.md

title, order, status

title	order	status
S05. Stream a Large File in the Response	24	draft

When the response is a huge file or data generated on the fly, loading the whole thing into memory isn't realistic. Use Response::set_content_provider() to produce data in chunks as you send it.

When the size is known

svr.Get("/download", [](const httplib::Request &req, httplib::Response &res) {
  size_t total_size = get_file_size("large.bin");

  res.set_content_provider(
    total_size, "application/octet-stream",
    [](size_t offset, size_t length, httplib::DataSink &sink) {
      auto data = read_range_from_file("large.bin", offset, length);
      sink.write(data.data(), data.size());
      return true;
    });
});

The lambda is called repeatedly with offset and length. Read just that range and write it to sink. Only a small chunk sits in memory at any given time.

Just send a file

If you only want to serve a file, set_file_content() is far simpler.

svr.Get("/download", [](const httplib::Request &req, httplib::Response &res) {
  res.set_file_content("large.bin", "application/octet-stream");
});

It streams internally, so even huge files are safe. Omit the Content-Type and it's guessed from the extension.

When the size is unknown — chunked transfer

For data generated on the fly, where you don't know the total size up front, use set_chunked_content_provider(). It's sent with HTTP chunked transfer encoding.

svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
  res.set_chunked_content_provider(
    "text/plain",
    [](size_t offset, httplib::DataSink &sink) {
      auto chunk = produce_next_chunk();
      if (chunk.empty()) {
        sink.done(); // done sending
        return true;
      }
      sink.write(chunk.data(), chunk.size());
      return true;
    });
});

Call sink.done() to signal the end.

Note: The provider lambda is called multiple times. Watch out for the lifetime of captured variables — wrap them in a std::shared_ptr if needed.

To serve the file as a download, see S06. Return a File Download Response.