Allegro CL version 10.1
Unrevised from 10.0 to 10.1.
10.0 version

Allegro CL Socket Library

This document contains the following sections:

1.0 Introduction and background
2.0 Support for Internet Protocol version 6 (IPv6)
3.0 IP addresses in Allegro CL
4.0 Characteristics
5.0 Stream Sockets
   5.1 Connections
   5.2 Host Naming
6.0 Variables
7.0 Functions
8.0 Errors
9.0 Sockets and SMP
10.0 Examples
11.0 Secure Socket Layer (SSL)
   11.1 The algorithm for loading the OpenSSL library
   11.2 SSL History
   11.3 Secure connections
   11.4 Client/Server
   11.5 Authentication
   11.6 Certificates
   11.7 CRL support
   11.8 TLS 1.3 support and ciphersuites
   11.9 The Allegro CL SSL API

1.0 Introduction and background

Sockets are a mechanism for interprocess communication designed at U.C. Berkeley for use in their version of Unix. Sockets have been added to many other versions of Unix and there is an implementation of sockets for Windows called Winsock. This document describes the Allegro interface to sockets. This interface works on Unix and on Windows.

Symbols naming objects in the socket utility are in the acl-socket package. It has the nickname socket.

The socket module is not included in all versions of Allegro CL. If it is present, it is (by default) included in a development image (one built with the include-devel-env argument to build-lisp-image specified true). To load the socket module if it is not present in an image, evaluate

(require :sock)

Note that runtime images cannot include the development environment (so include-devel-env must be specified nil when a runtime image is being built). If the socket module is needed, it must be loaded when the image is built. See runtime.htm, building-images.htm and delivery.htm for more information.

2.0 Support for Internet Protocol version 6 (IPv6)

Allegro CL supports Internet Protocol version 6 sockets (IPv6 sockets). As part of this support, several new functions have been added and several functions have been modified. The new functions are ipv6, get-ip-interfaces, ipaddrp, ipaddr-equalp, ipv6-address-p, and (added in a June, s006 update) dotted-address-p. The modified functions are dotted-to-ipaddr, dns-query, lookup-hostname, make-socket, and send-to. There is also a new variable *ipv6*.

The feature :ipv6 is added to the *features* list to indicate IPv6 support.

IPv6 support may be unusable on MacOS X 64-bit

Because of apparent bugs in Mac OS X 64-bit, certain IPv6 functionality may be restricted or unusable. In particular:

3.0 IP addresses in Allegro CL

Throughout the socket documentation, we make use of the term IP address. But what exactly is an IP address? Unless further clarified in the context in which it is used, an IP address is either an unsigned 32-bit integer or an ipv6 address structure.

The function socket:ipaddrp returns true when passed an IP adress. That function can be used to identify an object as an IP address. (Unsigned 32 bit integers obviously have other uses that representing IP addresses. The function simply determines whether the type and form of its argument is suitable as an IP address.)

4.0 Characteristics

There are three independent characteristics of sockets:


Valid values: :stream or :datagram.

A :stream socket offers a reliable, two-way, stream connection between sockets. Reliable means that what you send is received at the other end in the exact order you sent it. Stream means that the receiver reads a stream of bytes and sees no record boundaries. It uses the internet protocol TCP.

A :datagram socket offers unreliable, one-way, connectionless packet communication. Unreliable means that the packet may or may not be delivered. Packets may be delivered in an order other than in the order they were sent. Record boundaries are maintained: if the sender sends two ten byte packets and if the packets get through, the receiver will receive two ten byte packets rather than one twenty byte packet. For each packet you send you must give the destination address. It uses the internet protocol UDP.

address family

Valid values: :internet or :file.

In order to send to another socket the socket must have a name.

An :internet socket is named by a 32-bit host number and a 16-bit port number or an IPv6 address.

On Unix, port numbers less than 1024 can only be allocated by a process with the user id of root. A :file socket is named by a file on a local disk. This is called the Unix address family but we've chosen to call it the :file address family since it really isn't Unix specific. This address family can only permit processes on the same machine to communicate.

Note that the current version of the socket interface on Windows (Winsock, version 1.1), does not support the :file address family.


Valid values: :text or :binary, or, for :stream sockets only, :bivalent (see note below)

This isn't a property of the Unix socket implementation but is instead something we've added for the Common Lisp implementation since a Lisp stream is either binary (supports read-byte, etc.) or text (supports read-char, etc.).

Note on bivalent format:

Starting in release 5.0.1, the bivalent format is accepted for stream sockets. Bivalent means that the stream will accept text and binary stream functions. That is, you can write-byte or write-char, read-byte or read-char. A bivalent stream is useful in the http protocol (used between web browsers and web servers) since in that protocol the header data is sent in text format and the body can be in binary data (image files, for example).

Internally a bivalent socket stream is configured like a binary socket stream with 8 bit bytes. Character position is not maintained.

Bivalent socket streams have very efficient read-sequence and write-sequence implementations (as long as the sequence is either a vector of element-type character, (unsigned-byte 8) or (signed-byte 8)).

Bivalent socket streams also support the chunking protocol found in http/1.1. This protocol allows the sender to signal end of file without closing down the stream.

5.0 Stream Sockets

5.1 Connections

Stream sockets have a fourth characteristic called connect, with a value :active or :passive. In order to use stream sockets you have to set up a link between two of them. That link is called a connection. You set up a connection in this way:

  1. Machine A: create a passive socket at port port-b:
    (setq s-a (make-socket :connect :passive :local-port port-b))
  2. Machine B: create an active socket telling it to connect to Machine A, port port-b:
    (setq s-b (make-socket :remote-host "machine-a" 
                           :remote-port port-b))
  3. Machine A: wait for a connect request from anyone and when it occurs return a stream for I/O:
    (setq str-a (accept-connection s-a))
  4. When the accept-connection returns, machine A can use stream str-a to send messages to machine B and machine B can use stream s-b to send messages to machine A.

Note that steps 2 and 3 can occur in either order.

Note the asymmetry: a passive socket is not a Lisp stream (you can't do read and write to it). An active socket is a Lisp stream.

When accept-connection is called on a passive socket, it does not return until a connection is made to the passive socket. The value accept-connection returns is a stream.

As long as the passive socket is not closed, new connections can still be made to the port of that socket.

An active socket can be used for only one connection. Once that connection has been made, the socket should be closed and a new active socket created.

5.2 Host Naming

Host naming conventions: this package supports three conventions for naming a host:

hostname A string using the domain naming convention, e.g. "". The domain naming system is case-insensitive.
dotted A string which is the printed representation of the numeric address: e.g. "". We also support the non standard Berkeley extensions to this format for class A addresses: "23.3" (which is the same as "") and class B addresses "128.1.3" (which is the same as ""). IPv6 colon hex format, e.g., "fe80::209:5bff:fe8e:61c1", is also supported. See dotted-to-ipaddr.
ipaddr An unsigned 32-bit number, representing the IPv4 address in the native byte order for the host. Thus is 192*2^24 + 132*2^16 + 95*2^8 + 84 = 3229900628.
IPv6 An IPv6 address structure.

6.0 Variables

The variables defined by the interface are:


Please provide the value of this variable when asking for technical support with sockets as it tells us whether you have the latest version.


This variable controls whether the socket printing code converts the ip address of a socket into a hostname. This is usually what you want, however this can be a slow process (taking up to a minute to accomplish). The default value for this variable is t. See the full description for a discussion of the causes of the possible slowdown when the value is t.


Specifies the default value of the ipv6 keyword argument to lookup-hostname and make-socket.

7.0 Functions

The first table shows general functions defined by the interface and the second shows accessors.

Function Arguments Notes (follow function link for full description)
accept-connection (sock passive-socket) &key wait Generic function. Establishes a connection. If wait is nil and no connection is pending, returns nil and does nothing further. If wait is true (the default), waits until a connection is established. When a connection is established, returns the stream that communicates with the socket.
dotted-to-ipaddr dotted &key errorp Function. Converts a string like "" or similar format to an unsigned 32-bit IP address.

IPv6 "colon hex" address notation, including the %scopeid extension is also supported as is IPv4-mapped IPv6 address notation (::ffff:w.x.y.z).

dotted-address-p object Function. Returns true if its argument is a string in dotted IP address form.
get-ip-interfaces Function. Returns a list of conses of interface id's and names.
ipaddr-to-dotted ipaddr &key values Function. Convert a 32-bit unsigned IP address, ipaddr, to a string in dotted form. This function works on IPv6 address structures as well.
ipaddr-equalp add1 add2 &key compare-scope-id Function. Returns true if its two internet address arguments match.
ipaddr-to-hostname ipaddr Function. Returns, as a string, the hostname of the machine with address ipaddr. ipaddr should be a 32-bit IP address or an IPv6 address structure or IPv6 colon hex strings.
ipaddrp object Function. Returns true if its argument is an IP address.
ipv6 internet-socket Generic function. Returns true if its argument is an IPv6 socket.
ipv6-address-p object Function. Returns true if its argument is an IPv6 address structure.
lookup-hostname hostname Given a string naming a host, a 32-bit IP address, a string in dotted form, or a IPv6 address structure or IPv6 colon hex strings, return the 32-bit IP address for the host.
lookup-port portname protocol Function. Finds the port number using the symbolic name and the protocol.
make-socket &key type format address-family connect eol ipv6 scope-id &allow-other-keys Function. See the full description for details.
with-pending-connect &body body Macro. See the full description for details.
receive-from (sock datagram-socket) size &key buffer extract Generic function. This is used to read from a datagram socket.
send-to sock &key remote-host remote-port ipv6 scope-id Generic function with methods for internet-datagram-sockets and file-datagram-sockets
set-socket-options sock &key Generic function for modifying existing sockets.
shutdown sock &key direction Generic function that closes down the specified half of the bidirectional socket connection.
socket-control stream &key output-chunking output-chunking-eof input-chunking This function modifies the state of the socket stream, controlling input and output chunking.
socket-os-fd sock Generic function. Return the operating system file descriptor associated with this socket.

Socket Accessors

These functions retrieve slot values from socket instances. The values of these slots are set when the socket is created.

Function Arguments Notes (follow function link for full description)
remote-host socket Generic function. Returns an IP address.
local-host socket Generic function. Returns an IP address.
local-port socket

All are generic functions. All return the values of the particular attribute for socket.

Note: Both internet stream and internet datagram sockets use 16-bit port numbers.

Note that stream (tcp) port N is totally distinct from datagram (udp) port N.

remote-filename socket
local-filename socket
remote-port socket
socket-address-family socket
socket-connect socket
socket-format socket
socket-type socket
ipv6 internet-socket

8.0 Errors

When errors are raised by the socket interface, Lisp conditions are signaled. This section describes those conditions.

A condition is a CLOS class and thus fits into the hierarchy of CLOS classes. The condition socket-error is a subclass of the condition stream-error.

socket-error is the superclass for all socket related errors. See More on cl:stream-error in errors.htm.

socket-error denotes operating system detected socket errors. It has the following slots:

Name Reader function What
excl::identifier stream-error-identifier Symbol denoting this error (see table below)
excl::code stream-error-code Operating system dependent error code (if any)
excl::action stream-error-action String describing the operation in progress when the error occurred

Handling socket error is difficult because the error returned in exceptional situations can depend on the operating system and the address of the other side of the connection. For example, attempting to make a connection to a machine that is down may result in a "Connection Timed Out" or a "Host Unreachable" error, or maybe something else on certain systems.

The error codes assigned to socket errors vary from operating system to operating system. We translate a large set of the common error codes from a machine dependent number to a symbol which we call the identifier to make it easier for you to write portable code. Condition handling code should check the identifier field (using stream-error-identifier) If the identifier value is :unknown then this is not a common socket error and the operating system dependent code value of the condition must be used.

Possible identifier values and their meanings:

Identifier Meaning
:address-in-use Local socket address already in use
:address-not-available Local socket address not available
:network-down Network is down
:network-reset Network has been reset
:connection-aborted Connection aborted
:connection-reset Connection reset by peer
:no-buffer-space No buffer space
:shutdown Connection shut down
:connection-timed-out Connection timed out
:connection-refused Connection refused
:host-down Host is down
:host-unreachable Host is unreachable
:protocol-not-available Protocol not available
:unknown Unknown error

9.0 Sockets and SMP

An SMP (symmetric multiprocessing) Lisp runs one of more OS threads which can use more than one processor (see multiprocessing.htm). When a socket is opened in one SMP process, do not close it from another process.

If you have a process which is reading from a socket and that socket fails so the reading process hangs, you can wake the process up using a process interrupt. So assuming hung-process is the reading process and hung-socket is the socket, the following form executed in another process breaks the read (with an error) and closes the socket:

   (mp:process-interrupt hung-process 'close hung-socket)

You can also wrap the read in a (catch 'dead-partner ...) and the from another process do:

   (mp:process-run-function hung-process
                            #'(lambda ()
                                 (throw 'dead-partner t)))

10.0 Examples

Create an active stream socket connection to a socket that just prints characters to whomever connects to it. After connecting, read the first five characters and print them out.
USER(1): (let ((s (make-socket :remote-host "vapor" :remote-port "chargen")))
           (dotimes (i 5) (print (read-char s))) (close s))

Sending a message from frisky to vapor:

on vapor:

USER(1): (print (read (accept-connection
                       (make-socket :connect :passive :local-port 9933))))
.. this hangs ...

on frisky:

USER(1): (let ((s (make-socket :remote-host "vapor" :remote-port 9933)))
    (format s "Secret-message~%") (close s)) 

Then you see on vapor:


A flaw in this example is that on vapor we've left the socket and the stream open and we lost track of the objects to close them. So, while concise, this is not a good programming style.

Another problem with this example is that when we created the port on vapor we used a specific port number (9933). This means our program will fail if port 9933 is already in use. If possible, it is best to let the system choose a port number (this is done by not specifying a :local-port argument) and then using the local-port function to find out which port was chosen.

If we just want to send a simple message then datagrams might be more appropriate (although the program must guarantee that the message made it because datagram communication is unreliable).

on vapor:

user(2): (setq s (make-socket :type :datagram :local-port 9999))
#<text datagram socket waiting for connection at */9999 @ #x20664e82>

on frisky:

user(10): (setq x (make-socket :type :datagram))
#<text datagram socket waiting for connection at */45602 @ #x20717fb2>
user(11): (send-to x "foo-the-bar" 11 :remote-host "vapor" :remote-port 9999)

on vapor:

user(3): (receive-from s 100 :extract t)
11 ;; length of result
3229900653 ;; frisky's IP address
45602 ;; the port number chosen for the socket by frisky

11.0 Secure Socket Layer (SSL)

Allegro CL supports Secure Socket layers as described in this section. See also aserve/aserve.html, which describes Webserver support in Allegro CL. Using any https feature in aserve triggers loading of the :ssl module (i.e. evaluates (require :ssl)).

Allegro CL loads OpenSSL libraries dynamically when the :ssl module is loaded. Also loaded is the appropriate one of the Allegro CL-specific shared libraries aclssl, aclissl, aclssl11, and aclissl11. See *aclssl-name* for descriptions of these shared library files.

The system must be told either the version of SSL to be loaded (the system knows whether it uses 8-bit or 16-bit characters) or exactly which of the libraries should be loaded. How this information is determined is described in the next section. If the incorrect Allegro CL library is loaded (say aclissl rather than aclissl11 when SSL 1.1 is loaded) there will not be an immediate error but difficult to debug errors will likely occur later.

Newly installed versions of OpenSSL libraries will be used when installed on your computer without (usually) requiring an Allegro CL update. It is however the user's responsibility to ensure that OpenSSL libraries are available, properly updated, and findable.

To support OpenSSL 1.1, there is a new patch (released in August, 2019) that handles that version as well as continuing to handle SSL 1.0 (in both cases, all minor SSL releases -- the third number in release X.Y.Z -- are also handled.) This documentation has been updated to reflect behavior assuming the new patch has been installed. See sys:update-allegro for information on getting patches.

Users must themselves download and install OpenSSL libraries for their computers, and make sure these are updated when necessary. Further, users must ensure that Allegro CL knows where to find the OpenSSL libraries. On some platforms, there are standard locations where Allegro CL will look. On all platforms, environment variables or Lisp variables can be used to specify the library location.

Here is how to set things up for using OpenSSL:

In all cases, if (require :ssl) loads the :ssl module and the aclssl/aclissl library and loads the OpenSSL library without error, you are ready to use SSL.

The function openssl-version returns information about the version of the OpenSSL library which is loaded and the version used to build the aclssl/acl1ssl/aclssl11/aclissl11 library.

Most Allegro CL platforms support SSL. All that do have :ssl-support included on the *features* list.

The SSL functionality is in the ssl module. To ensure it is loaded, evaluate (require :ssl). Calling either of the two SSL functions, make-ssl-client-stream and make-ssl-server-stream, automatically loads that module.

If you are including the SSL facility in an application intended for delivery, be sure to include the :ssl module by adding the keyword :ssl to the list which is the value of the input-files argument to generate-application. If the copy-shared-libraries argument to that function is false, then the aclssl/aclissl shared library must be copied explicitly from the Allegro CL installation directory to the generated application directory. When the generated application is run, it must be possible to locate the the OpenSSL shared libraries in the same way that is described above.

11.1 The algorithm for loading the OpenSSL library

In this section, we give the exact steps on the various platforms for finding and loading the OpenSSL library. Usually user want the latest version available on their machines (so 1.1 if it is available, otherwise 1.0) but there may be cases where 1.0 is wanted even though 1.1 is also installed.

To repeat what we said in the previous section: try evaluating (require :ssl). If it works and openssl-version indicates the desired version of OpenSSL has been loaded, you are done.

If (require :ssl) signals an error or results in the wrong version of OpenSSL being loaded, then you must take further action.

Allegro CL must know the following things in order to load the :ssl module successfully:

With those four pieces of information, Allegro CL can load the :ssl module and associated library files. Standard defaults are used and usually if (require :ssl) succeeds, all the needed files are successfully found and loaded. If (require :ssl) fails or loads a version of OpenSSL that is not the desired version, then corrected information must be provided so the right files and libraries can be found. We describe how to do that in the remainder of this section.

As described, you can set these variables:

Finding OpenSSL library on Windows:

The procedure above is used. Note only that the value of *ssl-library-names*, if set, should be a list of two items, and the value of ACL_SSL_LIBRARY_NAMES, if set, should be a string concatenating two filenames (with extensions) separated by a single space.

Finding OpenSSL library on UNIX and the Mac:

The procedure above is used. Note only that the value of *ssl-library-names*, if set, should be a two-item list. The items can be a string naming a file (with extensions) or a list of strings naming files (with extensions). ACL_SSL_LIBRARY_NAMES, if set, a string concatenating two filenames (with extensions) separated by a single space.

11.2 SSL History

In 1994 Netscape Corporation designed the Secure Socket Layer (SSL) protocol to provide a means of safely and securely transporting private data (such as credit card numbers) between a Web Browser and a Web Server. Rather than tie SSL to the http protocol, Netscape wrote it as a protocol for making any TCP/IP connection secure.

At the end of 1994 version 2 of SSL was introduced and this was the first version shipped with a commercial web browser (Netscape Navigator (r)). In 1995 version 3 of SSL was introduced. At that point an international standards organization (IETF) took over work on SSL and introduced Transport Layer Security (or TLS) protocol (which is based on SSL but has a different handshake protocol). The IETF introduced TLS version 1.0 in 1999.

Allegro CL, starting in release 6.0, provides an interface that supports SSL version 2, SSL version 3 and TLS version 1 (subversions 1.2 and 1.3). When we use the name SSL, we mean SSL or TLS.

11.3 Secure connections

A secure TCP connection exists between two processes when both agree on the following:

These three items are determined via negotiation when the connection is made and the first data is to be sent.

11.4 Client/Server

In an SSL connection, one side is the client and the other side is the server. In the http environment, the web browser is the client and the web server is the server.

When a secure connection is started, the client starts the negotation by telling the server all the possible ways that it can communicate securely. The server then chooses one of the possible ways and informs the client.

Then the server sends its certificate and possibly other certificates if they are needed to prove that its certificate can be trusted. The important item in the certificate is the public key for the server. The client will use this public key to encrypt a random value which will be used by both the client and server to create the keys needed for the cipher chosen for data transmission.

In theory a certificate isn't necessary if both the client and server side support a key exchange algorithm that can generate a public key on the fly. The SSL libraries we use do not have this capability, thus you must always supply a server certificate.

Once both sides know the keys the other side will use to transmit, the secure data transmission can occur.

11.5 Authentication

The SSL protocol also permits each side of the connection to declare who they are. This is done by the exchange of certificates. The server must send a certificate describing itself to the client. The server can request that the client send a certificate to the server (although in the use of SSL on the web this is never done).

11.6 Certificates

A certificate is a digital document that stores information about an entity in such a way that it can be verified to be true. The primary use of certificates is to store the public key that can be used to send encrypted messages to the entity.

In the SSL protocol certificates have two uses:

  1. Encrpytion - by providing a public key they enable encrypted messages to be sent.
  2. Authentication - the certificate proves that the entity on the other end of the socket is who it claims to be.

Strictly speaking a certificate isn't required for SSL communication if both sides support a certain key exchange protocol. The OpenSSL libraries we use do not support this protocol thus whenever you create a server SSL stream you must supply a certificate (if you don't have your own we supply one in <Allegro directory>/examples/ssl/server.pem that you can use).

While certificates support authentication, the SSL protocol doesn't require that you take advantage of this facility.

A certificate contains the following:

  1. A Subject Identifier: a set of fields describing where the subject is geographically and its role within an organization.
  2. A Subject Public Key: the key that can be used to encrypt messages that only the Subject can decrypt since only the Subject has the associated private key.
  3. A Valid Time Interval: the interval of time during which this certificate is valid.
  4. An Issuer Identifier: just like the Subject Identifier but describing the entity that certifies that the Subject is who it says it is and that the public key is the correct one for the subject.
  5. An Issuer signature: a value which can be used by anyone to verify that the Issuer and only the Issuer signed this document testifying to it being correct.
  6. Various other fields: like serial numbers, version numbers, and other minor things.

A certificate is a combination of text and binary data and in order to make it easy to transport certificates they are usually encoded in a form called PEM which turns them into a sequence of printable characters.

When a web browser connects to a site via SSL (which is caused by the use of the 'https:' at the beginning of the url), it checks three things about the certificate:

  1. Does it know the Issuer and did the Issuer sign the certificate? A web browser knows about a set of Issuers (called Certificate Authorities) when it's installed on the machine (the Issuer certificates are part of the files that make up the web browser).
  2. Is the certificate valid right now or has it expired?
  3. Is the certificate for the machine we've contacted? If the url was then the certificate must be for The convention used is to store the name of the server machine in the CommonName slot of the Subject Identifier field of the certificate.

If all three tests pass then the web browser silently accepts the certificate and does a secure web page access. If any of the tests fail then the web browser notifies the user and waits for a response. Each browser displays the failure differently. For example, the Microsoft Internet Explorer (r) shows which of the three tests passed and which failed while the Netscape Navigator (r) just says that it received an invalid certificate. In both cases the person using the web browser is given the option of continuing with the web access. Transmission will still be secure if it is elected to continue. The only issue in doubt is the authenticity of the web server.

11.7 CRL support

The SSL implementation includes certificate revocation list (CRL) support. CRL checking is controlled by the crl-check and crl-file keyword arguments to make-ssl-client-stream and make-ssl-server-stream.

If you enable CRL checking, you must supply a proper PEM-encoded CRL, even if it contains zero revocations. If you do not supply a CRL, peer verification will never succeed.

11.8 TLS 1.3 support and ciphersuites

The TLS 1.3 protocols use ciphersuites. The currently defined (when this document was created) ciphersuite tokens are:

Any subset of these may be specified to the :cypersuites keyword argument to make-ssl-client-stream, make-ssl-server-stream, make-ssl-server-context, and make-ssl-client-context.

The argument may be unspecified or given the value nil, in which case the built-in OpenSSL defaults will be used. Otherwise the value should be a string containing the desired tokens delimited by colons, like "TLS_AES_128_GCM_SHA256:TLS_AES_128_CCM_SHA256:TLS_CHACHA20_POLY1305_SHA256".

The :ciphers keyword argument to those functions still exists and specifies the ciphers to be used with TLS1.2 and lower protocols.

For more information on TLS 1.3, see and

11.9 The Allegro CL SSL API

The following operators, variable and class comprise the SSL API. make-ssl-client-stream and make-ssl-server-stream create the streams that are used for communication. All symbols naming these objects are in the acl-socket (nicknamed socket) package.

The following function, variable, and classes (naming conditions) are related to loading and using OpenSSL libraries. The conditions are signaled when there are problems loading the OpenSSL library. All these symbols are in the excl package.

The file <Allegro directory>/examples/ssl/server.pem is a sample certificate and private key file. You can use this file when starting the server side of an SSL connection. The AllegroServe facility uses SSL. It is described in aserve/aserve.html.

Copyright (c) 1998-2022, Franz Inc. Lafayette, CA., USA. All rights reserved.
This page was not revised from the 10.0 page.
Created 2019.8.20.

Allegro CL version 10.1
Unrevised from 10.0 to 10.1.
10.0 version