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yhirose
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A failed request could reach the client's error logger twice: once from the step that failed, without a request when that was the TLS setup or the proxy CONNECT, and again from send_() with the request. A content receiver or progress callback that canceled was reported twice too, a CONNECT answered with 407 only without a request, and a stream that failed to open not at all unless TLS failed. Report a failure in one place instead. The outermost send() reports the request's error once, with the request the caller made, also when the failure happened on a redirect (a redirect client no longer gets the error logger) and when an answer carries an error, such as a proxy's 407. open_stream() reports a stream that fails to open the same way. Only an upload body compressed before send() still reports where it fails.
58 lines
2.2 KiB
Markdown
58 lines
2.2 KiB
Markdown
---
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title: "C19. Set a Logger on the Client"
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order: 19
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status: "draft"
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---
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To log requests sent and responses received by the client, use `set_logger()`. If you only care about errors, there's a separate `set_error_logger()`.
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## Log requests and responses
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```cpp
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httplib::Client cli("https://api.example.com");
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cli.set_logger([](const httplib::Request &req, const httplib::Response &res) {
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std::cout << req.method << " " << req.path
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<< " -> " << res.status << std::endl;
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});
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auto res = cli.Get("/users");
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```
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The callback you pass to `set_logger()` fires once for each completed request. You get both the request and the response as arguments — so you can log the method, path, status, headers, body, or whatever else you need.
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## Catch errors only
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When a network-layer error happens (like `Error::Connection`), `set_logger()` is **not** called — there's no response to log. For those cases, use `set_error_logger()`.
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```cpp
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cli.set_error_logger([](const httplib::Error &err, const httplib::Request *req) {
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std::cerr << "error: " << httplib::to_string(err);
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if (req) {
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std::cerr << " (" << req->method << " " << req->path << ")";
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}
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std::cerr << std::endl;
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});
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```
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Each failed request is reported once, with the request you made, also when it failed on a redirect. A stream that fails to open with `open_stream()` or `stream::Get()` is reported the same way; errors while reading its body come back through `read_error()` instead. `req` is null only in a server's error logger, for errors that belong to no request.
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## Use both together
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A nice pattern is to log successes through one, failures through the other.
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```cpp
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cli.set_logger([](const auto &req, const auto &res) {
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std::cout << "[ok] " << req.method << " " << req.path
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<< " " << res.status << std::endl;
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});
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cli.set_error_logger([](const auto &err, const auto *req) {
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std::cerr << "[ng] " << httplib::to_string(err);
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if (req) std::cerr << " " << req->method << " " << req->path;
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std::cerr << std::endl;
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});
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```
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> **Note:** The log callbacks run synchronously on the same thread as the request. Heavy work inside them slows the request down — push it to a background queue if you need to do anything expensive.
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