$ZKEY contains a list of sockets in the current SOCKET device which are ready for use. Its contents include both non selected but ready sockets from the prior WRITE /WAITs and any sockets with unread data in their GT.M buffer. $ZKEY can be used any time a SOCKET device is current. Once an incoming socket (that is, "LISTENING") has been accepted either by being selected by WRITE /WAIT or by USE socdev:socket="listeningsocket", it is removed from $ZKEY.

$ZKEY contains any one of the following values:

"LISTENING|<listening_socket_handle>|{<portnumber>|</path/to/LOCAL_socket>}"

"READ|<socket_handle>|<address>"

If $ZKEY contains one or more "READ|<socket_handle>|<address>" entries, it means there are ready to READ sockets that were selected by WRITE /WAIT or were partially read and there is data left in their buffer. Each entry is delimited by a ";".

$ZKEY is empty if no sockets have data in the buffer and there are no unaccepted incoming sockets from previous WRITE /WAITs.