File Descriptor types as defined in net.h.
NET_UNKNOWN_FD (0) - The file descriptor type is unknown.
NET_LOCAL_FILE_FD (1) - The file descriptor is a local file descriptor.
NET_SOCKET_FD (2) - The file descriptor is a socket.
NET_EVERYTIME_TYPE (3) - special case
NET_ProcessNet() is the workhorse of netlib. It is repeatedly called to process urls (usually it is called from a front end loop; an onIdle loop for example). When NET_ProcessNet() is called, a file descriptor that is in the ready state is passed in as an argument. Of course a file descriptor ready for reading is meaningless unless you know where to put the data, so, NET_ProcessNet() looks up the active entry whose file descriptor is equal to the ready_fd passed in (eg. the url that is ready for ready for processing), and calls it's protocol implementation's process member function. This puts the responsiblity of reading and writing, from and to the file descriptor, on each protocol specific implementation.
Confused? Try this. A call to NET_GetURL() is made to retrieve a url. An entry is created for that url and put in netlib's active entry list. The caller then calls NET_ProcessNet() when a socket (file descriptor) is ready to receive or send data. NET_ProcessNet() figures out which active entry in the list is the owner of the ready socket. It then tells the protocol implementation of that entry, to process (read/write) the socket. The process member function reads/writes data, pumping the data it receives into a stream, or sending the data it wants to send down the socket.
Remember that NET_ProcessNet()'s only responsibility is to match ready sockets (file descriptors) with active entries (urls that need loading), it's up to each entry's protocol implementation to communicate commands between the client and the server and to send and receive the data.
Analogy: NET_ProcessNet() is like the lever of a water pump. The hinge (protocol implementation's process routine) is actually bearing the load (handling the hard stuff). You need the lever to get water, but, you also need the hinge.
Judson Valeski, 1998