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37.14 Network Connections

Emacs Lisp programs can open stream (TCP) and datagram (UDP) network connections (see Datagrams) 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, 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 the 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 connection, or real for a real subprocess.

The process-status function returns open, closed, connect, or 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 stop-process and 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—see the :server keyword of make-network-process, Network Processes.) 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.

Emacs can create encrypted network connections, using either built-in or external support. The built-in support uses the GnuTLS (“Transport Layer Security”) library; see the GnuTLS project page. If your Emacs was compiled with GnuTLS support, the function gnutls-available-p is defined and returns non-nil. For more details, see Overview. The external support uses the starttls.el library, which requires a helper utility such as gnutls-cli to be installed on the system. The open-network-stream function can transparently handle the details of creating encrypted connections for you, using whatever support is available.

— Function: open-network-stream name buffer host service &rest parameters

This function opens a TCP connection, with optional encryption, 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 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 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).

The remaining arguments parameters are keyword/argument pairs that are mainly relevant to encrypted connections:

:nowait boolean
If non-nil, try to make an asynchronous connection.
:type type
The type of connection. Options are:
plain
An ordinary, unencrypted connection.
tls
ssl
A TLS (“Transport Layer Security”) connection.
nil
network
Start with a plain connection, and if parameters ‘:success’ and ‘:capability-command’ are supplied, try to upgrade to an encrypted connection via STARTTLS. If that fails, retain the unencrypted connection.
starttls
As for nil, but if STARTTLS fails drop the connection.
shell
A shell connection.

:always-query-capabilities boolean
If non-nil, always ask for the server's capabilities, even when doing a ‘plain’ connection.
:capability-command capability-command
Command string to query the host capabilities.
:end-of-command regexp
:end-of-capability regexp
Regular expression matching the end of a command, or the end of the command capability-command. The latter defaults to the former.
:starttls-function function
Function of one argument (the response to capability-command), which returns either nil, or the command to activate STARTTLS if supported.
:success regexp
Regular expression matching a successful STARTTLS negotiation.
:use-starttls-if-possible boolean
If non-nil, do opportunistic STARTTLS upgrades even if Emacs doesn't have built-in TLS support.
:client-certificate list-or-t
Either a list of the form (key-file cert-file), naming the certificate key file and certificate file itself, or t, meaning to query auth-source for this information (see Overview). Only used for TLS or STARTTLS.
:return-list cons-or-nil
The return value of this function. If omitted or nil, return a process object. Otherwise, a cons of the form (process-object . plist), where plist has keywords:
:greeting string-or-nil
If non-nil, the greeting string returned by the host.
:capabilities string-or-nil
If non-nil, the host's capability string.
:type symbol
The connection type: ‘plain’ or ‘tls’.