PGM_IO_STATUS_TIMER_PENDING

Under ideal conditions there is a constant stream of data on the network and every call to pgm_recv returns data and there is no data loss or dropped packets.  Ideal conditions are rare though, we might see bursty data from senders, senders may close or crash, packets may be lost.
At the most basic level we need to be maintained that the senders exist, senders notify their presence by repeated broadcast of SPM packets.  Packet loss, closed or crashed applications would cause an absence of SPM broadcasts and this situation can be caught by a timer.  If no packets are seen within say 30 seconds we consider the sender to no longer to be operational.
The receive window extends beyond that to monitor every incoming packet, NAK elimination to prevent transmission of duplicate NAKs from the same or different receivers, and retransmission of NAKs for when the retransmit request itself was lost in the network.  Each state is driven by configurable timers or timeouts.

This means that as soon as a single packet from a sender is received the common return values expected are PGM_IO_STATUS_NORMAL for data and PGM_IO_STATUS_TIMER_PENDING for no-data or receive-state transitions.

PGM_IO_STATUS_WOULD_BLOCK

Assuming non-blocking sockets, the return value PGM_IO_STATUS_WOULD_BLOCK appears when the call to pgm_recv cannot immediately return due to no available contiguous data.

The important note about this return value is that it indicates that there are no known senders and the receive state engine is waiting for starting data packets to begin processing.

PGM_IO_STATUS_NORMAL

When calling pgm_recv the return value for indicating data was successfully read is PGM_IO_STATUS_NORMAL, this is obviously the ideal case and assuming a constant stream of data to read from the network.

OpenPGM follows a reactor model, calling pgm_recv will then read a packet from the underlying UDP or RAW socket.  If the packet is an original data packet, called ODATA, it will be inserted into the receive window and if-and-only-if the sequence is contiguous to the current lead the payload can be returned to the application.