The t_httpc_req_open_cb definition specifies the format of the callback function that is called before a PUT or POST request is sent.
The user implementation (based on the request method) can decide whether a resource needs to be allocated (for example, a file in the PUT or POST cases). If the resource it tries to allocate is not currently available (for example, because there is no free file handle), it can signal that a later retry is required.
|http_req_hdl||The handle of the current request.||uint16_t|
|req_clen||The length of the data to be sent.||uint32_t *|
|p_hdl||Where to write the internal handle. This handle is used by the req_read() and req_close() calls.||t_httpc_user_hdl *|
|HTTPC_RETRY||The client must retry later.|