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
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
pipe for a pipe connection, or
real for a
process-status function returns
network connections. For a network server, the status is always
listen. Except for
stop, 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
:server keyword of
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-
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.
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 your own 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 like
80or an integer string like
The remaining arguments parameters are keyword/argument pairs that are mainly relevant to encrypted connections:
- If non-
nil, try to make an asynchronous connection.
- The type of connection. Options are:
- An ordinary, unencrypted connection.
- A TLS (Transport Layer Security) connection.
- 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.
- As for
nil, but if STARTTLS fails drop the connection.
- A shell connection.
- If non-
nil, always ask for the server's capabilities, even when doing a ‘plain’ connection.
- Command string to query the host capabilities.
- Regular expression matching the end of a command, or the end of the command capability-command. The latter defaults to the former.
- Function of one argument (the response to capability-command), which returns either
nil, or the command to activate STARTTLS if supported.
- Regular expression matching a successful STARTTLS negotiation.
- If non-
nil, do opportunistic STARTTLS upgrades even if Emacs doesn't have built-in TLS support.
- If non-
:return-valueis also non-
nil, Emacs will warn if the connection isn't encrypted. This is useful for protocols like IMAP and the like, where most users would expect the network traffic to be encrypted.
- Either a list of the form
), naming the certificate key file and certificate file itself, or
t, meaning to query
auth-sourcefor this information (see Overview). Only used for TLS or STARTTLS.
- The return value of this function. If omitted or
nil, return a process object. Otherwise, a cons of the form
), where plist has keywords:
- If non-
nil, the greeting string returned by the host.
- If non-
nil, the host's capability string.
- The connection type: ‘plain’ or ‘tls’.
- If the connection
shell, this parameter will be interpreted as a format-spec string that will be executed to make the connection. The specs available are ‘%s’ for the host name and ‘%p’ for the port number. For instance, if you want to first ssh to ‘gateway’ before making a plain connection, then this parameter could be something like ‘ssh gateway nc %s %p’.