ext_sysmidi: Sending and receiving MIDI messages in C

I am writing an external where it is important for me to be able to send and receive midi messages from the C code. It looks as if the "ext_sysmidi.h" public header provides the necessary functionality, but there are no comments or documentation that explain the behavior of the functions. I presume that standard objects such as midiinfo and midiin depend on this interface, but perhaps it was considered unlikely that third party externals would need to. I was successfully able to get the names of input and output ports using this API, but I would appreciate it if someone could provide me with information about how to receive input (either by polling or callback registration) and send messages. Thanks in advance.