S07. Receive Multipart Data as a Stream - Cpp Httplib

A naive upload handler puts the whole request into req.body, which blows up memory for large files. Use HandlerWithContentReader to receive the body chunk by chunk.

Basic usage

cpp

svr.Post("/upload",
  [](const httplib::Request &req, httplib::Response &res,
     const httplib::ContentReader &content_reader) {
    if (req.is_multipart_form_data()) {
      content_reader(
        // headers of each part
        [&](const httplib::FormData &file) {
          std::cout << "name: " << file.name
                    << ", filename: " << file.filename << std::endl;
          return true;
        },
        // body of each part (called multiple times)
        [&](const char *data, size_t len) {
          // write to disk here, for example
          return true;
        });
    } else {
      // plain request body
      content_reader([&](const char *data, size_t len) {
        return true;
      });
    }

    res.set_content("ok", "text/plain");
  });

The content_reader has two call shapes. For multipart data, pass two callbacks (one for headers, one for body). For plain bodies, pass just one.

Write directly to disk

Here's how to stream an uploaded file to disk.

cpp

svr.Post("/upload",
  [](const httplib::Request &req, httplib::Response &res,
     const httplib::ContentReader &content_reader) {
    std::ofstream ofs;

    content_reader(
      [&](const httplib::FormData &file) {
        if (!file.filename.empty()) {
          ofs.open("uploads/" + file.filename, std::ios::binary);
        }
        return static_cast<bool>(ofs);
      },
      [&](const char *data, size_t len) {
        ofs.write(data, len);
        return static_cast<bool>(ofs);
      });

    res.set_content("uploaded", "text/plain");
  });

Only a small chunk sits in memory at any moment, so gigabyte-scale files are no problem.

Warning: When you use HandlerWithContentReader, req.body stays empty. Handle the body yourself inside the callbacks.

For the client side of multipart uploads, see C07. Upload a file as multipart form data.