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Extension libraries

Extension libraries (eggs) are extensions to the core functionality provided by the basic CHICKEN system, to be built and installed separately. The mechanism for loading compiled extensions is based on dynamically loadable code and as such is only available on systems on which loading compiled code at runtime is supported. Currently these are most UNIX-compatible platforms that provide the libdl functionality like Linux, Solaris, BSD, Mac OS X and Windows using Cygwin.

Note: Extension may also be normal applications or shell scripts, but are usually libraries.

Extensions are technically nothing but dynamically loadable compiled files with added meta-data that describes dependencies to other extensions, version information and things like the author/maintainer of the extension. Three tools provide an easy to use interface for installing extensions, removing them and querying the current status of installed extensions.

Installing extensions

To install an extension library, run the chicken-install program with the extension name as argument. The extension archive is downloaded, its contents extracted and the contained setup script is executed. This setup script is a normal Scheme source file, which will be interpreted by chicken-install. The complete language supported by csi is available, and the library units srfi-1 regex utils posix tcp are loaded. Additional libraries can be loaded at run-time.

The setup script should perform all necessary steps to build the new library (or application). After a successful build, the extension can be installed by invoking one of the procedures install-extension, install-program or install-script. These procedures will copy a number of given files into the local extension repository or in the path where the CHICKEN executables are located (in the case of executable programs or scripts). Additionally the list of installed files, and user-defined metadata is stored in the repository.

If no extension name is given on the command-line, then all .setup scripts in the current directory are processed, in the order given on the command line.

Installing extensions that use libraries

Sometimes an extension requires a C library to compile. Compilation can fail when your system has this library in a nonstandard location. Normally the C compiler searches in the default locations /usr and /usr/local, and in the prefix where Chicken itself was installed. Sometimes this is not enough, so you'll need to supply chicken-install with some extra hints to the C compiler/linker. Here's an example:

 CSC_OPTIONS='-I/usr/pkg/include/mysql -L/usr/pkg/lib/mysql -L -R/usr/pkg/lib/mysql' chicken-install mysql

This installs the mysql egg with the extra compiler options -I and -L to set the include path and the library search path. The second -L switch passes the -R option directly to the linker, which causes the library path to get hardcoded into the resulting extension file (for systems that do not use

The environment variables CHICKEN_C_INCLUDE_PATH and CHICKEN_C_LIBRARY_PATH can also be used to override include- and linker-paths. Each of these variables may contain one or more directory names, separated by : or ; and will be passed using -I and -L to the C compiler.

Creating extensions

An extension can be created by placing its code and some special files in a directory named after it. For example, if your extension is called foo, create a directory called foo and put the extension code in it.

Extensions need two special files: an <extension name>.setup file and an <extension name>.meta file (where <extension name> is the name of your extension). The former indicates how the egg is to be copiled and the latter provides some information about the extension (author, license, dependencies etc). See the eggs tutorial for further information about how to create extensions.

Procedures and macros available in setup scripts


[procedure] (install-extension ID FILELIST [INFOLIST])

Installs the extension library with the name ID. All files given in the list of strings FILELIST will be copied to the extension repository. It should be noted here that the extension id has to be identical to the name of the file implementing the extension. The extension may load or include other files, or may load other extensions at runtime specified by the require-at-runtime property.

FILELIST may be a filename, a list of filenames, or a list of pairs of the form (SOURCE DEST) (if you want to copy into a particular sub-directory - the destination directory will be created as needed). If DEST is a relative pathname, it will be copied into the extension repository.

The optional argument INFOLIST should be an association list that maps symbols to values, this list will be stored as ID.setup-info at the same location as the extension code. Currently the following properties are used:

[extension property] (syntax)

Marks the extension as syntax-only. No code is compiled, the extension is intended as a file containing macros to be loaded at compile/macro-expansion time.

[extension property] (require-at-runtime ID ...)

Specifies extensions that should be loaded (via require) at runtime. This is mostly useful for syntax extensions that need additional support code at runtime.

 [extension property] (import-only)

Specifies that this extension only provides a expansion-time code in an import library and does not require code to be loaded at runtime.

[extension property] (version STRING)

Specifies version string.


[procedure] (install-program ID FILELIST [INFOLIST])

Similar to install-extension, but installs an executable program in the executable path (usually /usr/local/bin).


[procedure] (install-script ID FILELIST [INFOLIST])

Similar to install-program, but additionally changes the file permissions of all files in FILELIST to executable (for installing shell-scripts).


[procedure] (standard-extension ID [VERSION] #!key info)

A convenience procedure that combines the compilation and installation of a simple single-file extension. This is roughly equivalent to:

 (compile -s -O2 -d1 ID.scm -j ID)
 (compile -s -O2 -d0 ID.import.scm)
  '( "" "")
  '((version 1.0)
    ... `INFO' ...

VERSION may be #f or can be omitted, in that case the version obtained from where the extension has been retrieved wil be taken. If installed directly from a local directory, the version will default to "unknown".


[syntax] (run FORM ...)

Runs the shell command FORM, which is wrapped in an implicit quasiquote. (run (csc ...)) is treated specially and passes -v (if -verbose has been given to chicken-install) and -feature compiling-extension options to the compiler.


[syntax] (compile FORM ...)

Equivalent to (run (csc FORM ...)).


[syntax] (make ((TARGET (DEPENDENT ...) COMMAND ...) ...) ARGUMENTS)

A make macro that executes the expressions COMMAND ..., when any of the dependents DEPENDENT ... have changed, to build TARGET. This is the same as the make extension, which is available separately. For more information, see make.


[procedure] (patch WHICH REGEX SUBST)

Replaces all occurrences of the regular expression REGEX with the string SUBST, in the file given in WHICH. If WHICH is a string, the file will be patched and overwritten. If WHICH is a list of the form OLD NEW, then a different file named NEW will be generated.


[procedure] (copy-file FROM TO)

Copies the file or directory (recursively) given in the string FROM to the destination file or directory TO.


[procedure] (move-file FROM TO)

Moves the file or directory (recursively) given in the string FROM to the destination file or directory TO.


[procedure] (remove-file* PATH)

Removes the file or directory given in the string PATH, if it exists.


[procedure] (find-library NAME PROC)

Returns #t if the library named libNAME.[a|so] (unix) or NAME.lib (windows) could be found by compiling and linking a test program. PROC should be the name of a C function that must be provided by the library. If no such library was found or the function could not be resolved, #f is returned.


[procedure] (find-header NAME)

Returns #t if a C include-file with the given name is available, or #f otherwise.


[procedure] (try-compile CODE #!key cc cflags ldflags compile-only c++)

Returns #t if the C code in CODE compiles and links successfully, or #f otherwise. The keyword parameters cc (compiler name, defaults to the C compiler used to build this system), cflags and ldflags accept additional compilation and linking options. If compile-only is true, then no linking step takes place. If the keyword argument c++ is given and true, then the code will be compiled in C++ mode.


[procedure] (create-directory/parents PATH)

Creates the directory given in the string PATH, with all parent directories as needed.


[parameter] extension-name-and-version

Returns a list containing the name and version of the currently installed extension as strings. If the setup script is not invoked via chicken-install, then name and version will be empty.


[procedure] (version>=? V1 V2)

Compares the version numbers V1 and V2 and returns #t if V1 is "less" than V2 or #f otherwise. A version number can be an integer, a floating-point number or a string. version>=? handles dot-separated version-indicators of the form "X.Y. ..".

If one version number is the prefix of the other, then the shorter version is considered "less" than the longer.


[procedure] (installation-prefix)

An alternative installation prefix that will be prepended to extension installation paths if specified. It is set by the -prefix option or environment variable CHICKEN_INSTALL_PREFIX.


[parameter] (program-path [PATH])

Holds the path where executables are installed and defaults to either $CHICKEN_PREFIX/bin, if the environment variable CHICKEN_PREFIX is set or the path where the CHICKEN binaries (chicken, csi, etc.) are installed.


[parameter] (setup-root-directory [PATH])

Contains the path of the directory where chicken-install was invoked.


[parameter] (setup-install-mode [BOOL])

Reflects the setting of the -no-install option, i.e. is #f, if -no-install was given to chicken-install.


[parameter] host-extension

For a cross-compiling CHICKEN, when compiling an extension, then it should be built for the host environment (as opposed to the target environment). This parameter is controlled by the -host command-line option. A setup script should perform the proper steps of compiling any code by passing -host when invoking csc or using the compile macro.

Examples for extensions

A simple library

The simplest case is a single file that does not export any syntax. For example

;;;; hello.scm

(define (hello name)
  (print "Hello, " name " !") )

We need a .setup script to build and install our nifty extension:

;;;; hello.setup

;; compile the code into a dynamically loadable shared object
;; (will generate
(compile -s hello.scm)

;; Install as extension library
(install-extension 'hello "")

Lastly, we need a file hello.meta defining a minimal set of properties:

;;;; hello.meta

((author "Me")
 (synopsis "A cool hello-world library")
 (license "GPLv3")
 (files "hello.scm" "hello.setup"))

(for more information about available properties, see the metafile reference)

After entering

$ chicken-install

at the shell prompt (and in the same directory where the two files exist), the file hello.scm will be compiled into a dynamically loadable library. If the compilation succeeds, will be stored in the repository, together with a file named hello.setup-info containing an a-list with metadata (what you stored above in hello.meta). If no extension name is given to chicken-install, it will simply execute the any files with the .setup extension it can find.

Use it like any other CHICKEN extension:

$ csi -q
#;1> (require-library hello)
; loading /usr/local/lib/chicken/4/ ...
#;2> (hello "me")
Hello, me!

An application

Here we create a simple application:

;;;; hello2.scm

(print "Hello, ")
(for-each (lambda (x) (printf "~A " x)) (command-line-arguments))
(print "!")

We also need a setup script:

;;;; hello2.setup

(compile hello2.scm)  ; compile `hello2'
(install-program 'hello2 "hello2") ; name of the extension and files to be installed
;;;; hello2.meta

((author "Me")
 (synopsis "A cool hello-world application")
 (license "proprietary")
 (files "hello.scm" "hello.setup"))

To use it, just run chicken-install in the same directory:

$ chicken-install

(Here we omit the extension name)

Now the program hello2 will be installed in the same location as the other CHICKEN tools (like chicken, csi, etc.), which will normally be /usr/local/bin. Note that you need write-permissions for those locations and may have to run chicken-install with administrative rights or use the -sudo option.

The extension can be used from the command line:

$ hello2 one two three
one two three !

De-installation is just as easy - use the chicken-uninstall program to remove one or more extensions from the local repository:

$ chicken-uninstall hello2

A module exporting syntax

The hello module was just a shared library, and not a module.

To create an extension that exports syntax see the chapter on Modules and macros. We will show a simple example here: a module my-lib that exports one macro (prog1) and one procedure (my-sum):

;;; my-lib.scm

(module my-lib
  (import scheme chicken)

(define-syntax prog1
  (syntax-rules ()
    ((_ e1 e2 ...)
     (let ((result e1))
       (begin e2 ...)

(define my-sum
  (lambda (numbers)
      (apply + numbers)
      (display "my-sum used one more time!")


The prog1 macro is similar to Common Lisp's prog1: it evaluates a list of forms, but returns the value of the first form.

The meta file:

;;; my-lib.meta

((files "my-lib.setup"
 (licence "BSD")
 (author "Me again")
 (synopsis "My own cool libraries"))

The setup file is:

;;; my-lib.setup

(compile -s -O3 -d1 "my-lib.scm" -j my-lib)
(compile -s -O3 -d0 "my-lib.import.scm")

 '("" "")
 '((version 1.0)))

The first line tells the compiler to create a shared (-s) library and to create an import file (my-lib.import.scm, because of the -j flag). The second line compiles the import file created by the first one.

IMPORTANT: the module name exported by my-lib.scm must be the same module name passed to the compiler using the -j option, otherwise the imports file will not be generated!

Running chicken-install on the same directory will install the extension.

Next, it should be possible to load the library:

$ csi -q
#;1> (use my-lib)
; loading /usr/local/lib/chicken/6/ ...
; loading /usr/local/lib/chicken/6/ ...
; loading /usr/local/lib/chicken/6/ ...
; loading /usr/local/lib/chicken/6/ ...
#;2> (my-sum '(10 20 30))
my-sum used one more time!
#;3> (my-sum '(-1 1 0))
my-sum used one more time!
#;4> (prog1 (+ 2 2) (print "---"))

Notes on chicken-install

When running chicken-install with an argument NAME, for which no associated .setup file exists, then it will try to download the extension via HTTP from the CHICKEN code repository at Extensions that are required to compile and/or use the requested extension are downloaded and installed automatically.

To query the list of currently installed extensions, use chicken-status. It can list what extensions are installed and what files belong to a particular installed extension.

chicken-install reference

Available options:

-h -help
show this message and exit
-v -version
show version and exit
don't ask, install even if versions don't match
-k -keep
keep temporary files
-l -location LOCATION
install from given location instead of default
-t -transport TRANSPORT
use given transport instead of default
-proxy HOST[:PORT]
connect via HTTP proxy
-s -sudo
use sudo(1) for installing or removing files
-r -retrieve
only retrieve egg into current directory, don't install
-n -no-install
do not install, just build (implies -keep)
-p -prefix PREFIX
change installation prefix to PREFIX
when cross-compiling, compile extension for host only
when cross-compiling, compile extension for target only
run included test-cases, if available
-username USER
set username for transports that require this
-password PASS
set password for transports that require this
-i -init DIRECTORY
initialize empty alternative repository
-u -update-db
update export database
print path to extension repository
install extension in the application directory for a deployed application (see Deployment for more information)
build trunk instead of tagged version (only local)
-D -feature FEATURE
pass this on to subinvocations of csi and csc (when done via compile or (run (csc ...)))
print full call-trace when encountering errors in the setup script
continue installation, even if a dependency fails
display a list of foreign dependencies for the given egg(s)

chicken-install recognizes the http_proxy environment variable, if set.

chicken-uninstall reference

-h -help
show usage information and exit
-v -version
show version and exit
don't ask, delete whatever matches
-s -sudo
use sudo(1) for deleting files
when cross-compiling, remove extensions for host system only
when cross-compiling, remove extensions for target system only
match extension-name exactly (do not match as pattern)

chicken-status reference

-h -help
show usage information and exit
-v -version
show version and exit
-f -files
list installed files
when cross-compiling, show extensions for host system only
when cross-compiling, show extensions for target system only
match extension-name exactly (do not match as pattern)


When extensions are downloaded and installed one is executing code from potentially compromised systems. This applies also when chicken-install executes system tests for required extensions. As the code has been retrieved over the network effectively untrusted code is going to be evaluated. When chicken-install is run as root the whole system is at the mercy of the build instructions (note that this is also the case every time you install software via sudo make install, so this is not specific to the CHICKEN extension mechanism).

Security-conscious users should never run chicken-install as root. A simple remedy is to keep the repository inside a user's home directory (see the section "Changing repository location" below). Alternatively obtain write/execute access to the default location of the repository (usually /usr/local/lib/chicken) to avoid running as root. chicken-install also provides a -sudo option to perform the last installation steps as root user, but do building and other .setup script processing as normal. A third solution is to override VARDIR when building the system (for example by passing "VARDIR=/foo/bar" on the make command line, or by modifying config.make. Eggs will then be installed in $(VARDIR)/chicken/5.

Changing repository location

When Chicken is installed a repository for eggs is created and initialized in a default location (usually something like /usr/local/lib/chicken/6/). It is possible to keep an eggs repository in another location. This can be configured at build-time by passing VARDIR=<directory> to make(3) or by modifying the config.make configuration file. If you want to override this location after chicken is installed, you can create an initial repository directory with some default extensions and set the CHICKEN_REPOSITORY environment variable:

Note that your binary version can differ from the examples here, if your chicken version is older or newer than the one used in these examples. Check your default location for the correct binary-version number.

First, initialize the new repository with

 mkdir -p ~/myeggs/lib/chicken/6               # to create directory structure
 chicken-install -init ~/myeggs/lib/chicken/6

Then set this environment variable:

 export CHICKEN_REPOSITORY=~/myeggs/lib/chicken/6

CHICKEN_REPOSITORY is the place where extensions are to be loaded from for all chicken-based programs (which includes all the tools).

If you want to install eggs somewhere other than the default or your environment variable, you can use

 chicken-install -p ~/myeggs <package>

See that the argument to chicken-install is just ~/myeggs, while everywhere else it's ~/myeggs/lib/chicken/5.

When you load eggs from the interpreter, you will see messages showing where libraries are being loaded from:

#;1> (use numbers)
; loading /home/jdoe/myeggs/lib/chicken/6/ ...
; loading /home/jdoe/myeggs/lib/chicken/6/ ...
; loading /home/jdoe/myeggs/lib/chicken/6/ ...
; loading /home/jdoe/myeggs/lib/chicken/6/ ...
; loading /home/jdoe/myeggs/lib/chicken/6/ ...
; loading /home/jdoe/myeggs/lib/chicken/6/ ...


After a main upgrade, you need to reinstall all the eggs. First, reset the repository environment variable:

 export CHICKEN_REPOSITORY=/usr/lib/chicken/6

Then, make a new directory that reflects the systematic one:

 mkdir ~/myeggs/lib/chicken/6

After that, do the same steps above for the new directory (initialize, set repository environment variable) and reinstall all the eggs.

Other modes of installation

It is possible to install extensions directly from a Subversion repository or from a local checkout of the repository tree by using the -transport and -location options when invoking chicken-install. Three possible transport mechanisms are currently supported:

download extension sources via HTTP from a web-server (this is the default)
perform an svn export from the central extension repository; this will require a svn(1) client to be installed on the machine
use sources from the local filesystem and build directly in the source directory

The -location option specifies where to look for the source repository and names a web URL, a subversion repository URL or a filesystem path, respectively. A list of locations to try when retrieving extensions is stored in the file setup.defaults (usually installed in /usr/local/share/chicken). For http transports, chicken-install will detect networking timeouts and try alternative locations, as listed in the file.

Dependency information, which is necessary to ensure required extensions are also installed, is processed automatically.

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