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CPPHTTPLIB_MULTIPART_FORM_DATA_FILE_MAX_COUNT is enforced only in Server::read_content(), where parts are accumulated into req.form. The streaming ContentReader path keeps nothing and was never in scope, but this was undocumented (GHSA-923p-8q8g-xcqj). Note the split in the README and show how to bound the part count from inside a ContentReader handler.
104 lines
3.3 KiB
Markdown
104 lines
3.3 KiB
Markdown
---
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title: "S07. Receive Multipart Data as a Stream"
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order: 26
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status: "draft"
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---
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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.
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## Basic usage
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```cpp
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svr.Post("/upload",
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[](const httplib::Request &req, httplib::Response &res,
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const httplib::ContentReader &content_reader) {
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if (req.is_multipart_form_data()) {
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content_reader(
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// headers of each part
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[&](const httplib::FormData &file) {
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std::cout << "name: " << file.name
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<< ", filename: " << file.filename << std::endl;
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return true;
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},
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// body of each part (called multiple times)
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[&](const char *data, size_t len) {
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// write to disk here, for example
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return true;
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});
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} else {
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// plain request body
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content_reader([&](const char *data, size_t len) {
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return true;
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});
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}
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res.set_content("ok", "text/plain");
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});
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```
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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.
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## Write directly to disk
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Here's how to stream an uploaded file to disk.
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```cpp
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svr.Post("/upload",
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[](const httplib::Request &req, httplib::Response &res,
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const httplib::ContentReader &content_reader) {
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std::ofstream ofs;
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content_reader(
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[&](const httplib::FormData &file) {
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if (!file.filename.empty()) {
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ofs.open("uploads/" + file.filename, std::ios::binary);
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}
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return static_cast<bool>(ofs);
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},
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[&](const char *data, size_t len) {
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ofs.write(data, len);
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return static_cast<bool>(ofs);
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});
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res.set_content("uploaded", "text/plain");
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});
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```
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Only a small chunk sits in memory at any moment, so gigabyte-scale files are no problem.
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## Count the parts yourself
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There is a cap on the number of parts, `CPPHTTPLIB_MULTIPART_FORM_DATA_FILE_MAX_COUNT` (1024 by default), but it only applies to the buffered path, where every part is accumulated into `req.form`. The `ContentReader` keeps nothing on the library side, so the cap does not apply here.
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If you want an upper bound, count the parts yourself and return `false` from the header callback. The parser stops right there.
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```cpp
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svr.Post("/upload",
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[](const httplib::Request &req, httplib::Response &res,
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const httplib::ContentReader &content_reader) {
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size_t count = 0;
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auto ok = content_reader(
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[&](const httplib::FormData &file) {
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if (++count > 100) { return false; } // stop here
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return true;
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},
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[&](const char *data, size_t len) {
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return true;
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});
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if (!ok) {
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res.status = httplib::StatusCode::BadRequest_400;
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return;
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}
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res.set_content("ok", "text/plain");
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});
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```
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When `content_reader` returns `false`, set the response status yourself. The rest of the body is left unread and the connection is closed, so a client that is still sending sees the connection drop.
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> **Warning:** When you use `HandlerWithContentReader`, `req.body` stays **empty**. Handle the body yourself inside the callbacks.
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> For the client side of multipart uploads, see [C07. Upload a file as multipart form data](../c07-multipart-upload).
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