Errors

Every fallible call returns an lwhttp_error_t, and on_complete receives one as the final result. lwhttp_error_string() turns any of them into a short message.

Result	Typical cause
LWHTTP_OK	Success. A finished request may still carry any HTTP status, including 404 or 500.
LWHTTP_ERR_ARG	NULL arguments, a bad method, target or header, or a user Content-Length / Transfer-Encoding.
LWHTTP_ERR_MEM	The lwIP-CE heap could not satisfy an allocation. See memory.
LWHTTP_ERR_STATE	A request was started while the client was still busy.
LWHTTP_ERR_URL	The URL has no scheme or host, a bad port, or spaces and control characters.
LWHTTP_ERR_DNS	The host name could not be resolved.
LWHTTP_ERR_CONNECT	The TCP connection could not be established, or timed out while connecting.
LWHTTP_ERR_TLS	The TLS handshake or certificate check failed. See HTTPS.
LWHTTP_ERR_SEND	lwIP-CE rejected a write on an open connection.
LWHTTP_ERR_CLOSED	The connection closed, reset or timed out before the response was complete.
LWHTTP_ERR_PROTOCOL	The server sent a malformed response: bad status line, bad header, inconsistent lengths, broken chunking.
LWHTTP_ERR_VERSION	The server answered with a major HTTP version other than 1.
LWHTTP_ERR_UNSUPPORTED	A scheme other than http/https, or a compressed or otherwise encoded response body.
LWHTTP_ERR_TOO_LARGE	A length or chunk size too big to represent, or a URL too long for its buffer.
LWHTTP_ERR_HEADERS_TOO_LARGE	The response head or a single header line does not fit in the header store.
LWHTTP_ERR_REDIRECT	A Location header that cannot be resolved into a URL.
LWHTTP_ERR_ABORTED	A callback returned false, the request was cancelled, or a body reader ended early.

Synchronous and asynchronous errors

Errors found before a request goes on the wire are returned by lwhttp_client_request() and never reach on_complete. Everything after that is reported once, through on_complete, and is also available afterwards from lwhttp_client_result().

lwhttp_error_t e = lwhttp_client_request(client, &request, &handlers, &state);
if (e) show_error(lwhttp_error_string(e));