This is the usocket Common Lisp sockets library: a library to bring sockets access to the broadest of common lisp implementations as possible.

The library currently supports:

  • SBCL
  • ArmedBear Common Lisp
  • Allegro Common Lisp
  • LispWorks
  • Clozure CL
  • ECL
  • Scieneer Common Lisp
  • Macintosh Common Lisp
  • MOCL

If your favorite common lisp misses in the list above, please contact and submit a request. Please include references to available sockets functions in your lisp implementation.

The library has been ASDF ( enabled, meaning that you can tar up a checkout and use that to ASDF-INSTALL:INSTALL the package in your system package site. (Or use your usual ASDF tricks to use the checkout directly.)

Remarks on licensing

Even though the source code has an MIT style license attached to it, when compiling this code with some of the supported lisp implementations you may not end up with an MIT style binary version due to the licensing of the implementations themselves. ECL is such an example and - when it will become supported - GCL is like that too.

Non-support of :external-format

Because of its definition in the hyperspec, there's no common external-format between lisp implementations: every vendor has chosen a different way to solve the problem of newline translation or character set recoding.

Because there's no way to avoid platform specific code in the application when using external-format, the purpose of a portability layer gets defeated. So, for now, usocket doesn't support external-format.

The workaround to get reasonably portable external-format support is to layer a flexi-stream (from flexi-streams) on top of a usocket stream.

API definition

  • usocket (class)
  • stream-usocket (class; usocket derivative)
  • stream-server-usocket (class; usocket derivative)
  • socket-connect (function) [ to create an active/connected socket ] socket-connect host port &key element-type where `host' is a vectorized ip or a string representation of a dotted ip address or a hostname for lookup in the DNS system
  • socket-listen (function) [ to create a passive/listening socket ] socket-listen host port &key reuseaddress backlog element-type where `host' has the same definition as above
  • socket-accept (method) [ to create an active/connected socket ] socket-accept socket &key element-type returns (server side) a connected socket derived from a listening/passive socket.
  • socket-close (method) socket-close socket where socket a previously returned socket
  • socket (usocket slot accessor), the internal/implementation defined socket representation
  • socket-stream (usocket slot accessor), socket-stream socket the return value of which satisfies the normal stream interface
  • socket-shutdown


  • address-in-use-error
  • address-not-available-error
  • bad-file-descriptor-error
  • connection-refused-error
  • connection-aborted-error
  • connection-reset-error
  • invalid-argument-error
  • no-buffers-error
  • operation-not-supported-error
  • operation-not-permitted-error
  • protocol-not-supported-error
  • socket-type-not-supported-error
  • network-unreachable-error
  • network-down-error
  • network-reset-error
  • host-down-error
  • host-unreachable-error
  • shutdown-error
  • timeout-error
  • unkown-error

Non-fatal conditions:

  • interrupted-condition
  • unkown-condition

(for a description of the API methods and functions see

Test suite

The test suite unfortunately isn't mature enough yet to run without some manual configuration. Several elements are required which are hard to programatically detect. Please adjust the test file before running the tests, for these variables:

  • +non-existing-host+: The stringified IP address of a host on the same subnet. No physical host may be present.
  • +unused-local-port+: A port number of a port not in use on the machine the tests run on.
  • +common-lisp-net+: A vector with 4 integer elements which make up an IP address. This must be the IP "" resolves to.

Known problems

  • CMUCL error reporting wrt sockets raises only simple-errors meaning there's no way to tell different error conditions apart. All errors are mapped to unknown-error on CMUCL.

  • The ArmedBear backend doesn't do any error mapping (yet). Java defines exceptions at the wrong level (IMO), since the exception reported bares a relation to the function failing, not the actual error that occurred: for example 'Address already in use' (when creating a passive socket) is reported as a BindException with an error text of 'Address already in use'. There's no way to sanely map 'BindException' to a meaningfull error in usocket. [This does not mean the backend should not at least map to 'unknown-error'!]

  • When using the library with ECL, you need the C compiler installed to be able to compile and load the Foreign Function Interface. Not all ECL targets support DFFI yet, so on some targets this would be the case anyway. By depending on this technique, usocket can reuse the FFI code on all platforms (including Windows). This benefit currently outweighs the additional requirement. (hey, it's Embeddable Common Lisp, so, you probably wanted to embed it all along, right?)

Erik Enge & Erik Huelsmann, Chun Tian (binghe), Erik Enge
Chun Tian (binghe) & Hans Huebner, Chun Tian (binghe)