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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 |
|---|---|---|
| C17. Handle Error Codes | 17 | draft |
cli.Get(), cli.Post(), and friends return a Result. When the request fails — can't reach the server, times out, etc. — the result is "falsy". To get the specific reason, use Result::error().
Basic check
httplib::Client cli("http://localhost:8080");
auto res = cli.Get("/api/data");
if (res) {
// the request was sent and a response came back
std::cout << "status: " << res->status << std::endl;
} else {
// the network layer failed
std::cerr << "error: " << httplib::to_string(res.error()) << std::endl;
}
Use if (res) to check success. On failure, res.error() returns a httplib::Error enum value. Pass it to to_string() to get a human-readable description.
Common errors
| Value | Meaning |
|---|---|
Error::Connection |
Couldn't connect to the server |
Error::ConnectionTimeout |
Connection timeout (set_connection_timeout) |
Error::Read / Error::Write |
Error during send or receive. A timeout from set_read_timeout or set_max_timeout is also reported as Error::Read |
Error::Timeout |
A body read timed out in stream::Get() or SSEClient |
Error::ExceedRedirectCount |
Too many redirects |
Error::SSLConnection |
TLS handshake failed |
Error::SSLServerVerification |
Server certificate verification failed |
Error::Canceled |
A progress callback returned false |
Network errors vs. HTTP errors
Even when res is truthy, the HTTP status code can still be 4xx or 5xx. These are two different things.
auto res = cli.Get("/api/data");
if (!res) {
// network error (no response received at all)
std::cerr << "network error: " << httplib::to_string(res.error()) << std::endl;
return 1;
}
if (res->status >= 400) {
// HTTP error (response received, but the status is bad)
std::cerr << "http error: " << res->status << std::endl;
return 1;
}
// success
std::cout << res->body << std::endl;
Keep them separated in your head: network-layer errors go through res.error(), HTTP-level errors through res->status.
To dig deeper into SSL-related errors, see C18. Handle SSL errors.