Curl_easy struct is the one returned to the outside in the external API as an opaque CURL *. This pointer is usually known as an easy handle in API documentations and examples.connectdata struct. When a transfer is about to be made, libcurl will either create a new connection or re-use an existing one. The current connectdata that is used by this handle is pointed out by Curl_easy->conn.SingleRequest sub-struct.Curl_easy struct is added to a multi handle, as it must be in order to do any transfer, the ->multi member will point to the Curl_multi struct it belongs to. The ->prev and ->next members will then be used by the multi code to keep a linked list of Curl_easy structs that are added to that same multi handle. libcurl always uses multi so ->multi will point to a Curl_multi when a transfer is in progress.->mstate is the multi state of this particular Curl_easy. When multi_runsingle() is called, it will act on this handle according to which state it is in. The mstate is also what tells which sockets to return for a specific Curl_easy when [curl_multi_fdset()][12] is called etc.data everywhere for the local variable that points to the Curl_easy struct.Curl_easy is associated with an individual stream, sharing the same connectdata struct. Multiplexing makes it even more important to keep things associated with the right thing!connectdata struct identifies a single physical connection to a server. If the connection cannot be kept alive, the connection will be closed after use and then this struct can be removed from the cache and freed.Curl_easy can be used multiple times and each time select another connectdata struct to use for the connection. Keep this in mind, as it is then important to consider if options or choices are based on the connection or the Curl_easy.Curl_easy may no longer be around when the time comes to shut down a particular connection. For this purpose, libcurl holds a special dummy closure_handle Curl_easy in the Curl_multi struct to use when needed.conn for the local variable that points to the connectdata.Curl_multi is the multi handle struct exposed as the opaque CURLM * in external APIs.Curl_easy structs that have been added to this handle with [curl_multi_add_handle()][13]. The start of the list is ->easyp and ->num_easy is a counter of added Curl_easys.->msglist is a linked list of messages to send back when [curl_multi_info_read()][14] is called. Basically a node is added to that list when an individual Curl_easy's transfer has completed.->hostcache points to the name cache. It is a hash table for looking up name to IP. The nodes have a limited lifetime in there and this cache is meant to reduce the time for when the same name is wanted within a short period of time.->timetree points to a tree of Curl_easys, sorted by the remaining time until it should be checked - normally some sort of timeout. Each Curl_easy has one node in the tree.->sockhash is a hash table to allow fast lookups of socket descriptor for which Curl_easy uses that descriptor. This is necessary for the multi_socket API.->conn_cache points to the connection cache. It keeps track of all connections that are kept after use. The cache has a maximum size.->closure_handle is described in the connectdata section.multi for the variable that points to the Curl_multi struct.Curl_handler struct. It defines what the protocol is called and what functions the main code should call to deal with protocol specific issues. In general, there's a source file named [protocol].c in which there's a struct Curl_handler Curl_handler_[protocol] declared. In url.c there's then the main array with all individual Curl_handler structs pointed to from a single array which is scanned through when a URL is given to libcurl to work with.lib/urldata.h.->scheme is the URL scheme name, usually spelled out in uppercase. That is "HTTP" or "FTP" etc. SSL versions of the protocol need their own Curl_handler setup so HTTPS separate from HTTP.->setup_connection is called to allow the protocol code to allocate protocol specific data that then gets associated with that Curl_easy for the rest of this transfer. It gets freed again at the end of the transfer. It will be called before the connectdata for the transfer has been selected/created. Most protocols will allocate its private struct [PROTOCOL] here and assign Curl_easy->req.p.[protocol] to it.->connect_it allows a protocol to do some specific actions after the TCP connect is done, that can still be considered part of the connection phase. Some protocols will alter the connectdata->recv[] and connectdata->send[] function pointers in this function.->connecting is similarly a function that keeps getting called as long as the protocol considers itself still in the connecting phase.->do_it is the function called to issue the transfer request. What we call the DO action internally. If the DO is not enough and things need to be kept getting done for the entire DO sequence to complete, ->doing is then usually also provided. Each protocol that needs to do multiple commands or similar for do/doing needs to implement their own state machines (see SCP, SFTP, FTP). Some protocols (only FTP and only due to historical reasons) have a separate piece of the DO state called DO_MORE.->doing keeps getting called while issuing the transfer request command(s)->done gets called when the transfer is complete and DONE. That is after the main data has been transferred.->do_more gets called during the DO_MORE state. The FTP protocol uses this state when setting up the second connection.->proto_getsock, ->doing_getsock, ->domore_getsock, ->perform_getsock Functions that return socket information. Which socket(s) to wait for which I/O action(s) during the particular multi state.->disconnect is called immediately before the TCP connection is shutdown.->readwrite gets called during transfer to allow the protocol to do extra reads/writes->attach attaches a transfer to the connection.->defport is the default report TCP or UDP port this protocol uses->protocol is one or more bits in the CURLPROTO_* set. The SSL versions have their "base" protocol set and then the SSL variation. Like "HTTP|HTTPS".->flags is a bitmask with additional information about the protocol that will make it get treated differently by the generic engine:PROTOPT_SSL - will make it connect and negotiate SSLPROTOPT_DUAL - this protocol uses two connectionsPROTOPT_CLOSEACTION - this protocol has actions to do before closing the connection. This flag is no longer used by code, yet still set for a bunch of protocol handlers.PROTOPT_DIRLOCK - "direction lock". The SSH protocols set this bit to limit which "direction" of socket actions that the main engine will concern itself with.PROTOPT_NONETWORK - a protocol that does not use the network (read file:)PROTOPT_NEEDSPWD - this protocol needs a password and will use a default one unless one is providedPROTOPT_NOURLQUERY - this protocol cannot handle a query part on the URL (?foo=bar)Curl_easy has a pointer to its connection cache. Each multi handle sets up a connection cache that all added Curl_easys share by default.Curl_share struct, exposed to the external API as CURLSH *.CURLOPT_SHARE option, those specific Curl_easys will use the caches/pools that this share handle holds.Curl_easy structs can be made to share specific things that they otherwise would not, such as cookies.Curl_share struct can currently hold cookies, DNS cache and the SSL session cache.Curl_easy has its own private CookieInfo even when they are added to a multi handle. They can be made to share cookies by using the share API.