Requests are created in the usual way using a curl handle and added
to the scheduler with multi_add. This function returns immediately
and does not perform the request yet. The user needs to call multi_run
which performs all scheduled requests concurrently. It returns when all
requests have completed, or case of a timeout or SIGINT (e.g.
if the user presses ESC or CTRL+C in the console). In case of
the latter, simply call multi_run again to resume pending requests.
When the request succeeded, the done callback gets triggered with
the response data. The structure if this data is identical to curl_fetch_memory.
When the request fails, the fail callback is triggered with an error
message. Note that failure here means something went wrong in performing the
request such as a connection failure, it does not check the http status code.
Just like curl_fetch_memory, the user has to implement application logic.
Raising an error within a callback function stops execution of that function
but does not affect other requests.
A single handle cannot be used for multiple simultaneous requests. However
it is possible to add new requests to a pool while it is running, so you
can re-use a handle within the callback of a request from that same handle.
It is up to the user to make sure the same handle is not used in concurrent
requests.
The multi_cancel function can be used to cancel a pending request.
It has no effect if the request was already completed or canceled.
The multi_fdset function returns the file descriptors curl is
polling currently, and also a timeout parameter, the number of
milliseconds an application should wait (at most) before proceeding. It
is equivalent to the curl_multi_fdset and
curl_multi_timeout calls. It is handy for applications that is
expecting input (or writing output) through both curl, and other file
descriptors.