Emacs Lisp programs can open stream (TCP) and datagram (UDP) network
connections to other processes on the same machine or other machines.
A network connection is handled by Lisp much like a subprocess, and is
represented by a process object. However, the process you are
communicating with is not a child of the Emacs process, so it has no
process ID, and you can't kill it or send it signals. All you
can do is send and receive data.
delete-process closes the
connection, but does not kill the program at the other end; that
program must decide what to do about closure of the connection.
Lisp programs can listen for connections by creating network servers. A network server is also represented by a kind of process object, but unlike a network connection, the network server never transfers data itself. When it receives a connection request, it creates a new network connection to represent the connection just made. (The network connection inherits certain information, including the process plist, from the server.) The network server then goes back to listening for more connection requests.
Network connections and servers are created by calling
make-network-process with an argument list consisting of
keyword/argument pairs, for example
:server t to create a
server process, or
:type 'datagram to create a datagram
connection. See Low-Level Network, for details. You can also use
open-network-stream function described below.
To distinguish the different types of processes, the
process-type function returns the symbol
network for a
network connection or server,
serial for a serial port
real for a real subprocess.
process-status function returns
failed for network
connections. For a network server, the status is always
listen. None of those values is possible for a real
subprocess. See Process Information.
You can stop and resume operation of a network process by calling
continue-process. For a server
process, being stopped means not accepting new connections. (Up to 5
connection requests will be queued for when you resume the server; you
can increase this limit, unless it is imposed by the operating
system.) For a network stream connection, being stopped means not
processing input (any arriving input waits until you resume the
connection). For a datagram connection, some number of packets may be
queued but input may be lost. You can use the function
process-command to determine whether a network connection or
server is stopped; a non-
nil value means yes.
This function opens a TCP connection, and returns a process object that represents the connection.
The name argument specifies the name for the process object. It is modified as necessary to make it unique.
The buffer-or-name argument is the buffer to associate with the connection. Output from the connection is inserted in the buffer, unless you specify a filter function to handle the output. If buffer-or-name is
nil, it means that the connection is not associated with any buffer.
The arguments host and service specify where to connect to; host is the host name (a string), and service is the name of a defined network service (a string) or a port number (an integer).