$K[EY] contains the string that terminated the most recent READ command from the current device (including any introducing and terminating characters). If no READ command was issued to the current device or if no terminator is used, the value of $KEY is an empty string. However, when input is terminated by typing a function key, the value of $KEY is equal to the string of characters that is transmitted by that function key.

The effect of a READ *glvn on $KEY is unspecified.

For terminals, $KEY and $ZB both have the terminator.

For SOCKET:

$KEY contains the socket handle and the state information of the current SOCKET device after certain I/O commands.

After a successful OPEN or USE with the LISTEN deviceparameter, $KEY contains for TCP sockets:

"LISTENING|<socket_handle>|<portnumber>"

and for LOCAL sockets:

"LISTENING|<socket_handle>|<address>"

After a successful OPEN or USE with the CONNECT device parameter or when GT.M was started with a socket as the $PRINCIPAL device, $KEY contains:

"ESTABLISHED|<socket handle>|<address>"

When WRITE /WAIT selects an incoming connection, $KEY contains:

"CONNECT|<socket_handle>|<address>"

When WRITE /WAIT selects a socket with data available for reading, $KEY contains:

"READ|<socket_handle>|<address>"

For TCP sockets, <address> is the numeric IP address for the remote end of the connection. For LOCAL sockets it is the path to the socket.

For TCP LISTENING sockets, <portnumber> is the local port on which socket_handle is listening for incoming connections. For LOCAL LISTENING sockets, it is the path of the socket.

If the WRITE /WAIT was timed, $KEY returns an empty value if the wait timed out or there was no established connection. $KEY only has the selected handle, if any, immediately after a WRITE /WAIT. $KEY is also used by other socket I/O commands such as READ which sets it to the delimiter or malformed UTF-8 character, if any, which terminated the read.

When a socket has enabled non-blocking WRITEs, additional values will show if it is possible to write at least one byte to the socket. If the second argument of WRITE /WAIT is omitted or specifies both "READ" and "WRITE" and the socket selected by WRITE /WAIT is ready for both READ and WRITE, $KEY contains:

READWRITE|<socket handle>|<address>

If the socket selected by a WRITE /WAIT which implicitly or explicitly requests the write state would block on a READ but not block on WRITE, $KEY contains:

WRITE|<socket handle>|<address>

If the socket selected by WRITE /WAIT which implicitly or explicitly requests the read state would not block on a READ but would block on a WRITE, $KEY contains:

READ|<socket handle>|<address>