For Socket devices:

$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>"

"WRITE|<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 ";".

If $ZKEY contains one or more "LISTENING|<listening_socket_handle>|{<portnumber|/path/to/LOCAL_socket>}" entries, it means that there are pending connections and a USE s:socket=listening_socket_handle will accept a pending connection and remove the LISTENING|<listening_socket_handle> entry from $ZKEY.

If $ZKEY contains one or more "WRITE|<socket_handle>|<address>" entries, it means that the prior WRITE /WAIT considered the non-blocking sockets writeable. This is likely to be the case most of the time. If $ZKEY contains one WRITE and one READ entry, it means that the non-blocking socket is both readable and writeable.

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

For Sequential File Device:

$ZKEY contains the current position in the file based on the last READ. This is in bytes for STREAM and VARIABLE formats, and in a record,byte pair for FIXED format. For FIXED format, SEEKs and normal READs always produce a zero byte position; a non-zero byte position in $ZKEY for FIXED format operation indicates a partially read record, caused by a READ # or READ *. In FIXED mode, the information returned for $ZKEY is a function of record size, and, if a USE command changes record size by specifying the WIDTH deviceparameter while the file is open, $ZKEY offsets change accordingly; if record size changes, previously saved values of $ZKEY are likely inappropriate for use with SEEK.