-@page
-@node Modules
-@chapter Modules
-@cindex modules
-
-When programs become large, naming conflicts can occur when a function
-or global variable defined in one file has the same name as a function
-or global variable in another file. Even just a @emph{similarity}
-between function names can cause hard-to-find bugs, since a programmer
-might type the wrong function name.
-
-The approach used to tackle this problem is called @emph{information
-encapsulation}, which consists of packaging functional units into a
-given name space that is clearly separated from other name spaces.
-@cindex encapsulation
-@cindex information encapsulation
-@cindex name space
-
-The language features that allow this are usually called @emph{the
-module system} because programs are broken up into modules that are
-compiled separately (or loaded separately in an interpreter).
-
-Older languages, like C, have limited support for name space
-manipulation and protection. In C a variable or function is public by
-default, and can be made local to a module with the @code{static}
-keyword. But you cannot reference public variables and functions from
-another module with different names.
-
-More advanced module systems have become a common feature in recently
-designed languages: ML, Python, Perl, and Modula 3 all allow the
-@emph{renaming} of objects from a foreign module, so they will not
-clutter the global name space.
-@cindex name space - private
-
-@menu
-* Scheme and modules:: How modules are handled in standard Scheme.
-* The Guile module system:: How Guile does it.
-* Dynamic Libraries:: Loading libraries of compiled code at run time.
-@end menu
-
-
-@node Scheme and modules
-@section Scheme and modules
-
-Scheme, as defined in R5RS, does @emph{not} have a module system at all.
-
-Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
-implemented a provide/require mechanism for many Scheme implementations.
-Library files in SLIB @emph{provide} a feature, and when user programs
-@emph{require} that feature, the library file is loaded in.
-
-For example, the file @file{random.scm} in the SLIB package contains the
-line
-
-@smalllisp
-(provide 'random)
-@end smalllisp
-
-so to use its procedures, a user would type
-
-@smalllisp
-(require 'random)
-@end smalllisp
-
-and they would magically become available, @emph{but still have the same
-names!} So this method is nice, but not as good as a full-featured
-module system.
-
-
-@node The Guile module system
-@section The Guile module system
-
-In 1996 Tom Lord implemented a full-featured module system for Guile which
-allows loading Scheme source files into a private name space. This system has
-been in available since Guile version 1.4.
-@c fixme: Actually, was it available before? 1.4 seems a bit late...
-
-For Guile version 1.5.0 and later, the system has been improved to have better
-integration from C code, more fine-grained user control over interfaces, and
-documentation.
-
-Although it is anticipated that the module system implementation will
-change in the future, the Scheme programming interface described in this
-manual should be considered stable. The C programming interface is
-considered relatively stable, although at the time of this writing,
-there is still some flux.
-@c fixme: Review: Need better C code interface commentary.
-
-@menu
-* General Information about Modules:: Guile module basics.
-* Using Guile Modules:: How to use existing modules.
-* Creating Guile Modules:: How to package your code into modules.
-* More Module Procedures:: Low-level module code.
-* Module System Quirks:: Strange things to be aware of.
-* Included Guile Modules:: Which modules come with Guile?
-@end menu
-
-@node General Information about Modules
-@subsection General Information about Modules
-
-A Guile module is a collection of named procedures, variables and
-macros, altogether called the @dfn{bindings}, since they bind, or
-associate, a symbol (the name) to a Scheme object (procedure, variable,
-or macro). Within a module, all bindings are visible. Certain bindings
-can be declared @dfn{public}, in which case they are added to the
-module's so-called @dfn{export list}; this set of public bindings is
-called the module's @dfn{public interface} (@pxref{Creating Guile
-Modules}).
-
-A client module @dfn{uses} a providing module's bindings by either
-accessing the providing module's public interface, or by building a
-custom interface (and then accessing that). In a custom interface, the
-client module can @dfn{select} which bindings to access and can also
-algorithmically @dfn{rename} bindings. In contrast, when using the
-providing module's public interface, the entire export list is available
-without renaming (@pxref{Using Guile Modules}).
-
-To use a module, it must be found and loaded. All Guile modules have a
-unique @dfn{module name}, which is a list of one or more symbols.
-Examples are @code{(ice-9 popen)} or @code{(srfi srfi-11)}. When Guile
-searches for the code of a module, it constructs the name of the file to
-load by concatenating the name elements with slashes between the
-elements and appending a number of file name extensions from the list
-@code{%load-extensions} (REFFIXME). The resulting file name is then
-searched in all directories in the variable @code{%load-path}. For
-example, the @code{(ice-9 popen)} module would result in the filename
-@code{ice-9/popen.scm} and searched in the installation directory of
-Guile and in all other directories in the load path.
-
-@c FIXME::martin: Not sure about this, maybe someone knows better?
-Every module has a so-called syntax transformer associated with it.
-This is a procedure which performs all syntax transformation for the
-time the module is read in and evaluated. When working with modules,
-you can manipulate the current syntax transformer using the
-@code{use-syntax} syntactic form or the @code{#:use-syntax} module
-definition option (@pxref{Creating Guile Modules}).
-
-Please note that there are some problems with the current module system
-you should keep in mind (@pxref{Module System Quirks}). We hope to
-address these eventually.
-
-
-@node Using Guile Modules
-@subsection Using Guile Modules
-
-To use a Guile module is to access either its public interface or a
-custom interface (@pxref{General Information about Modules}). Both
-types of access are handled by the syntactic form @code{use-modules},
-which accepts one or more interface specifications and, upon evaluation,
-arranges for those interfaces to be available to the current module.
-This process may include locating and loading code for a given module if
-that code has not yet been loaded (REFFIXME %load-path).
-
-An @dfn{interface specification} has one of two forms. The first
-variation is simply to name the module, in which case its public
-interface is the one accessed. For example:
-
-@smalllisp
-(use-modules (ice-9 popen))
-@end smalllisp
-
-Here, the interface specification is @code{(ice-9 popen)}, and the
-result is that the current module now has access to @code{open-pipe},
-@code{close-pipe}, @code{open-input-pipe}, and so on (@pxref{Included
-Guile Modules}).
-
-Note in the previous example that if the current module had already
-defined @code{open-pipe}, that definition would be overwritten by the
-definition in @code{(ice-9 popen)}. For this reason (and others), there
-is a second variation of interface specification that not only names a
-module to be accessed, but also selects bindings from it and renames
-them to suit the current module's needs. For example:
-
-@smalllisp
-(use-modules ((ice-9 popen)
- :select ((open-pipe . pipe-open) close-pipe)
- :rename (symbol-prefix-proc 'unixy:)))
-@end smalllisp
-
-Here, the interface specification is more complex than before, and the
-result is that a custom interface with only two bindings is created and
-subsequently accessed by the current module. The mapping of old to new
-names is as follows:
-
-@c Use `smallexample' since `table' is ugly. --ttn
-@smallexample
-(ice-9 popen) sees: current module sees:
-open-pipe unixy:pipe-open
-close-pipe unixy:close-pipe
-@end smallexample
-
-This example also shows how to use the convenience procedure
-@code{symbol-prefix-proc}.
-
-@c begin (scm-doc-string "boot-9.scm" "symbol-prefix-proc")
-@deffn procedure symbol-prefix-proc prefix-sym
-Return a procedure that prefixes its arg (a symbol) with
-@var{prefix-sym}.
-@c Insert gratuitous C++ slam here. --ttn
-@end deffn
-
-@c begin (scm-doc-string "boot-9.scm" "use-modules")
-@deffn syntax use-modules spec @dots{}
-Resolve each interface specification @var{spec} into an interface and
-arrange for these to be accessible by the current module. The return
-value is unspecified.
-
-@var{spec} can be a list of symbols, in which case it names a module
-whose public interface is found and used.
-
-@var{spec} can also be of the form:
-
-@smalllisp
- (MODULE-NAME [:select SELECTION] [:rename RENAMER])
-@end smalllisp
-
-in which case a custom interface is newly created and used.
-@var{module-name} is a list of symbols, as above; @var{selection} is a
-list of selection-specs; and @var{renamer} is a procedure that takes a
-symbol and returns its new name. A selection-spec is either a symbol or
-a pair of symbols @code{(ORIG . SEEN)}, where @var{orig} is the name in
-the used module and @var{seen} is the name in the using module. Note
-that @var{seen} is also passed through @var{renamer}.
-
-The @code{:select} and @code{:rename} clauses are optional. If both are
-omitted, the returned interface has no bindings. If the @code{:select}
-clause is omitted, @var{renamer} operates on the used module's public
-interface.
-
-Signal error if module name is not resolvable.
-@end deffn
-
-
-@c FIXME::martin: Is this correct, and is there more to say?
-@c FIXME::martin: Define term and concept `system transformer' somewhere.
-
-@deffn syntax use-syntax module-name
-Load the module @code{module-name} and use its system
-transformer as the system transformer for the currently defined module,
-as well as installing it as the current system transformer.
-@end deffn
-
-
-@node Creating Guile Modules
-@subsection Creating Guile Modules
-
-When you want to create your own modules, you have to take the following
-steps:
-
-@itemize @bullet
-@item
-Create a Scheme source file and add all variables and procedures you wish
-to export, or which are required by the exported procedures.
-
-@item
-Add a @code{define-module} form at the beginning.
-
-@item
-Export all bindings which should be in the public interface, either
-by using @code{define-public} or @code{export} (both documented below).
-@end itemize
-
-@c begin (scm-doc-string "boot-9.scm" "define-module")
-@deffn syntax define-module module-name [options @dots{}]
-@var{module-name} is of the form @code{(hierarchy file)}. One
-example of this is
-
-@smalllisp
-(define-module (ice-9 popen))
-@end smalllisp
-
-@code{define-module} makes this module available to Guile programs under
-the given @var{module-name}.
-
-The @var{options} are keyword/value pairs which specify more about the
-defined module. The recognized options and their meaning is shown in
-the following table.
-
-@c fixme: Should we use "#:" or ":"?
-
-@table @code
-@item #:use-module @var{interface-specification}
-Equivalent to a @code{(use-modules @var{interface-specification})}
-(@pxref{Using Guile Modules}).
-
-@item #:use-syntax @var{module}
-Use @var{module} when loading the currently defined module, and install
-it as the syntax transformer.
-
-@item #:autoload @var{module} @var{symbol}
-Load @var{module} whenever @var{symbol} is accessed.
-
-@item #:export @var{list}
-Export all identifiers in @var{list}, which must be a list of symbols.
-This is equivalent to @code{(export @var{list})} in the module body.
-
-@item #:no-backtrace
-Tell Guile not to record information for procedure backtraces when
-executing the procedures in this module.
-
-@item #:pure
-Create a @dfn{pure} module, that is a module which does not contain any
-of the standard procedure bindings except for the syntax forms. This is
-useful if you want to create @dfn{safe} modules, that is modules which
-do not know anything about dangerous procedures.
-@end table
-
-@end deffn
-@c end
-
-@deffn syntax export variable @dots{}
-Add all @var{variable}s (which must be symbols) to the list of exported
-bindings of the current module.
-@end deffn
-
-@c begin (scm-doc-string "boot-9.scm" "define-public")
-@deffn syntax define-public @dots{}
-Equivalent to @code{(begin (define foo ...) (export foo))}.
-@end deffn
-@c end
-
-
-@node More Module Procedures
-@subsection More Module Procedures
-
-@c FIXME::martin: Review me!
-
-@c FIXME::martin: Should this procedure be documented and supported
-@c at all?
-
-The procedures in this section are useful if you want to dig into the
-innards of Guile's module system. If you don't know precisely what you
-do, you should probably avoid using any of them.
-
-@deffn primitive standard-eval-closure module
-Return an eval closure for the module @var{module}.
-@end deffn
-
-
-@node Module System Quirks
-@subsection Module System Quirks
-
-Although the programming interfaces are relatively stable, the Guile
-module system itself is still evolving. Here are some situations where
-usage surpasses design.
-
-@itemize @bullet
-
-@item
-When using a module which exports a macro definition, the other module
-must export all bindings the macro expansion uses, too, because the
-expanded code would otherwise not be able to see these definitions and
-issue a ``variable unbound'' error, or worse, would use another binding
-which might be present in the scope of the expansion.
-
-@item
-From C, you need to construct a @code{module-export!} call using
-@code{gh_eval_str}. This is cumbersome.
-
-@item
-When two or more used modules export bindings with the same names, the
-last accessed module wins, and the exported binding of that last module
-will silently be used. This might lead to hard-to-find errors because
-wrong procedures or variables are used. To avoid this kind of
-@dfn{name-clash} situation, use a custom interface specification
-(@pxref{Using Guile Modules}). (We include this entry for the possible
-benefit of users of Guile versions previous to 1.5.0, when custom
-interfaces were added to the module system.)
-
-@item
-[Add other quirks here.]
-
-@end itemize
-
-
-@node Included Guile Modules
-@subsection Included Guile Modules
-
-@c FIXME::martin: Review me!
-
-Some modules are included in the Guile distribution; here are references
-to the entries in this manual which describe them in more detail:
-
-@table @strong
-@item boot-9
-boot-9 is Guile's initialization module, and it is always loaded when
-Guile starts up.
-
-@item (ice-9 debug)
-Mikael Djurfeldt's source-level debugging support for Guile
-(@pxref{Debugger User Interface}).
-
-@item (ice-9 threads)
-Guile's support for multi threaded execution (@pxref{Scheduling}).
-
-@item (ice-9 rdelim)
-Line- and character-delimited input (@pxref{Line/Delimited}).
-
-@item (ice-9 documentation)
-Online documentation (REFFIXME).
-
-@item (srfi srfi-1)
-A library providing a lot of useful list and pair processing
-procedures (@pxref{SRFI-1}).
-
-@item (srfi srfi-2)
-Support for @code{and-let*} (@pxref{SRFI-2}).
-
-@item (srfi srfi-6)
-Support for some additional string port procedures (@pxref{SRFI-6}).
-
-@item (srfi srfi-8)
-Multiple-value handling with @code{receive} (@pxref{SRFI-8}).
-
-@item (srfi srfi-9)
-Record definition with @code{define-record-type} (@pxref{SRFI-9}).
-
-@item (srfi srfi-10)
-Read hash extension @code{#,()} (@pxref{SRFI-10}).
-
-@item (srfi srfi-11)
-Multiple-value handling with @code{let-values} and @code{let-values*}
-(@pxref{SRFI-11}).
-
-@item (srfi srfi-13)
-String library (@pxref{SRFI-13}).
-
-@item (srfi srfi-14)
-Character-set library (@pxref{SRFI-14}).
-
-@item (srfi srfi-17)
-Getter-with-setter support (@pxref{SRFI-17}).
-
-@item (ice-9 slib)
-This module contains hooks for using Aubrey Jaffer's portable Scheme
-library SLIB from Guile (@pxref{SLIB}).
-
-@c FIXME::martin: This module is not in the distribution. Remove it
-@c from here?
-@item (ice-9 jacal)
-This module contains hooks for using Aubrey Jaffer's symbolic math
-packge Jacal from Guile (@pxref{JACAL}).
-@end table
-
-
-@node Dynamic Libraries
-@section Dynamic Libraries
-
-Most modern Unices have something called @dfn{shared libraries}. This
-ordinarily means that they have the capability to share the executable
-image of a library between several running programs to save memory and
-disk space. But generally, shared libraries give a lot of additional
-flexibility compared to the traditional static libraries. In fact,
-calling them `dynamic' libraries is as correct as calling them `shared'.
-
-Shared libraries really give you a lot of flexibility in addition to the
-memory and disk space savings. When you link a program against a shared
-library, that library is not closely incorporated into the final
-executable. Instead, the executable of your program only contains
-enough information to find the needed shared libraries when the program
-is actually run. Only then, when the program is starting, is the final
-step of the linking process performed. This means that you need not
-recompile all programs when you install a new, only slightly modified
-version of a shared library. The programs will pick up the changes
-automatically the next time they are run.
-
-Now, when all the necessary machinery is there to perform part of the
-linking at run-time, why not take the next step and allow the programmer
-to explicitly take advantage of it from within his program? Of course,
-many operating systems that support shared libraries do just that, and
-chances are that Guile will allow you to access this feature from within
-your Scheme programs. As you might have guessed already, this feature
-is called @dfn{dynamic linking}@footnote{Some people also refer to the
-final linking stage at program startup as `dynamic linking', so if you
-want to make yourself perfectly clear, it is probably best to use the
-more technical term @dfn{dlopening}, as suggested by Gordon Matzigkeit
-in his libtool documentation.}
-
-As with many aspects of Guile, there is a low-level way to access the
-dynamic linking apparatus, and a more high-level interface that
-integrates dynamically linked libraries into the module system.
-
-@menu
-* Low level dynamic linking::
-* Compiled Code Modules::
-* Dynamic Linking and Compiled Code Modules::
-@end menu
-
-@node Low level dynamic linking
-@subsection Low level dynamic linking
-
-When using the low level procedures to do your dynamic linking, you have
-complete control over which library is loaded when and what get's done
-with it.
-
-@deffn primitive dynamic-link library
-Find the shared library denoted by @var{library} (a string) and link it
-into the running Guile application. When everything works out, return a
-Scheme object suitable for representing the linked object file.
-Otherwise an error is thrown. How object files are searched is system
-dependent.
-
-Normally, @var{library} is just the name of some shared library file
-that will be searched for in the places where shared libraries usually
-reside, such as in @file{/usr/lib} and @file{/usr/local/lib}.
-@end deffn
-
-@deffn primitive dynamic-object? val
-Determine whether @var{val} represents a dynamically linked object file.
-@end deffn
-
-@deffn primitive dynamic-unlink dynobj
-Unlink the indicated object file from the application. The argument
-@var{dynobj} should be one of the values returned by
-@code{dynamic-link}. When @code{dynamic-unlink} has been called on
-@var{dynobj}, it is no longer usable as an argument to the functions
-below and you will get type mismatch errors when you try to.
-@end deffn
-
-@deffn primitive dynamic-func function dynobj
-Search the C function indicated by @var{function} (a string or symbol)
-in @var{dynobj} and return some Scheme object that can later be used
-with @code{dynamic-call} to actually call this function. Right now,
-these Scheme objects are formed by casting the address of the function
-to @code{long} and converting this number to its Scheme representation.
-
-Regardless whether your C compiler prepends an underscore @samp{_} to
-the global names in a program, you should @strong{not} include this
-underscore in @var{function}. Guile knows whether the underscore is
-needed or not and will add it when necessary.
-@end deffn
-
-@deffn primitive dynamic-call function dynobj
-Call the C function indicated by @var{function} and @var{dynobj}. The
-function is passed no arguments and its return value is ignored. When
-@var{function} is something returned by @code{dynamic-func}, call that
-function and ignore @var{dynobj}. When @var{function} is a string (or
-symbol, etc.), look it up in @var{dynobj}; this is equivalent to
-
-@smallexample
-(dynamic-call (dynamic-func @var{function} @var{dynobj} #f))
-@end smallexample
-
-Interrupts are deferred while the C function is executing (with
-@code{SCM_DEFER_INTS}/@code{SCM_ALLOW_INTS}).
-@end deffn
-
-@deffn primitive dynamic-args-call function dynobj args
-Call the C function indicated by @var{function} and @var{dynobj}, just
-like @code{dynamic-call}, but pass it some arguments and return its
-return value. The C function is expected to take two arguments and
-return an @code{int}, just like @code{main}:
-
-@smallexample
-int c_func (int argc, char **argv);
-@end smallexample
-
-The parameter @var{args} must be a list of strings and is converted into
-an array of @code{char *}. The array is passed in @var{argv} and its
-size in @var{argc}. The return value is converted to a Scheme number
-and returned from the call to @code{dynamic-args-call}.
-@end deffn
-
-When dynamic linking is disabled or not supported on your system,
-the above functions throw errors, but they are still available.
-
-Here is a small example that works on GNU/Linux:
-
-@smallexample
-(define libc-obj (dynamic-link "libc.so"))
-libc-obj
-@result{} #<dynamic-object "libc.so">
-(dynamic-args-call 'rand libc-obj '())
-@result{} 269167349
-(dynamic-unlink libc-obj)
-libc-obj
-@result{} #<dynamic-object "libc.so" (unlinked)>
-@end smallexample
-
-As you can see, after calling @code{dynamic-unlink} on a dynamically
-linked library, it is marked as @samp{(unlinked)} and you are no longer
-able to use it with @code{dynamic-call}, etc. Whether the library is
-really removed from you program is system-dependent and will generally
-not happen when some other parts of your program still use it. In the
-example above, @code{libc} is almost certainly not removed from your
-program because it is badly needed by almost everything.
-
-The functions to call a function from a dynamically linked library,
-@code{dynamic-call} and @code{dynamic-args-call}, are not very powerful.
-They are mostly intended to be used for calling specially written
-initialization functions that will then add new primitives to Guile.
-For example, we do not expect that you will dynamically link
-@file{libX11} with @code{dynamic-link} and then construct a beautiful
-graphical user interface just by using @code{dynamic-call} and
-@code{dynamic-args-call}. Instead, the usual way would be to write a
-special Guile<->X11 glue library that has intimate knowledge about both
-Guile and X11 and does whatever is necessary to make them inter-operate
-smoothly. This glue library could then be dynamically linked into a
-vanilla Guile interpreter and activated by calling its initialization
-function. That function would add all the new types and primitives to
-the Guile interpreter that it has to offer.
-
-From this setup the next logical step is to integrate these glue
-libraries into the module system of Guile so that you can load new
-primitives into a running system just as you can load new Scheme code.
-
-There is, however, another possibility to get a more thorough access to
-the functions contained in a dynamically linked library. Anthony Green
-has written @file{libffi}, a library that implements a @dfn{foreign
-function interface} for a number of different platforms. With it, you
-can extend the Spartan functionality of @code{dynamic-call} and
-@code{dynamic-args-call} considerably. There is glue code available in
-the Guile contrib archive to make @file{libffi} accessible from Guile.
-
-@node Compiled Code Modules
-@subsection Putting Compiled Code into Modules
-
-The new primitives that you add to Guile with @code{gh_new_procedure} or
-with any of the other mechanisms are normally placed into the same
-module as all the other builtin procedures (like @code{display}).
-However, it is also possible to put new primitives into their own
-module.
-
-The mechanism for doing so is not very well thought out and is likely to
-change when the module system of Guile itself is revised, but it is
-simple and useful enough to document it as it stands.
-
-What @code{gh_new_procedure} and the functions used by the snarfer
-really do is to add the new primitives to whatever module is the
-@emph{current module} when they are called. This is analogous to the
-way Scheme code is put into modules: the @code{define-module} expression
-at the top of a Scheme source file creates a new module and makes it the
-current module while the rest of the file is evaluated. The
-@code{define} expressions in that file then add their new definitions to
-this current module.
-
-Therefore, all we need to do is to make sure that the right module is
-current when calling @code{gh_new_procedure} for our new primitives.
-Unfortunately, there is not yet an easy way to access the module system
-from C, so we are better off with a more indirect approach. Instead of
-adding our primitives at initialization time we merely register with
-Guile that we are ready to provide the contents of a certain module,
-should it ever be needed.
-
-@deftypefun void scm_register_module_xxx (char *@var{name}, void (*@var{initfunc})(void))
-Register with Guile that @var{initfunc} will provide the contents of the
-module @var{name}.
-
-The function @var{initfunc} should perform the usual initialization
-actions for your new primitives, like calling @code{gh_new_procedure} or
-including the file produced by the snarfer. When @var{initfunc} is
-called, the current module is a newly created module with a name as
-indicated by @var{name}. Each definition that is added to it will be
-automatically exported.
-
-The string @var{name} indicates the hierachical name of the new module.
-It should consist of the individual components of the module name
-separated by single spaces. That is, the Scheme module name @code{(foo
-bar)}, which is a list, should be written as @code{"foo bar"} for the
-@var{name} parameter.
-
-You can call @code{scm_register_module_xxx} at any time, even before
-Guile has been initialized. This might be useful when you want to put
-the call to it in some initialization code that is magically called
-before main, like constructors for global C++ objects.
-
-An example for @code{scm_register_module_xxx} appears in the next section.
-@end deftypefun
-
-Now, instead of calling the initialization function at program startup,
-you should simply call @code{scm_register_module_xxx} and pass it the
-initialization function. When the named module is later requested by
-Scheme code with @code{use-modules} for example, Guile will notice that
-it knows how to create this module and will call the initialization
-function at the right time in the right context.
-
-@node Dynamic Linking and Compiled Code Modules
-@subsection Dynamic Linking and Compiled Code Modules
-
-The most interesting application of dynamically linked libraries is
-probably to use them for providing @emph{compiled code modules} to
-Scheme programs. As much fun as programming in Scheme is, every now and
-then comes the need to write some low-level C stuff to make Scheme even
-more fun.
-
-Not only can you put these new primitives into their own module (see the
-previous section), you can even put them into a shared library that is
-only then linked to your running Guile image when it is actually
-needed.
-
-An example will hopefully make everything clear. Suppose we want to
-make the Bessel functions of the C library available to Scheme in the
-module @samp{(math bessel)}. First we need to write the appropriate
-glue code to convert the arguments and return values of the functions
-from Scheme to C and back. Additionally, we need a function that will
-add them to the set of Guile primitives. Because this is just an
-example, we will only implement this for the @code{j0} function, tho.
-
-@smallexample
-#include <math.h>
-#include <guile/gh.h>
-
-SCM
-j0_wrapper (SCM x)
-@{
- return gh_double2scm (j0 (gh_scm2double (x)));
-@}
-
-void
-init_math_bessel ()
-@{
- gh_new_procedure1_0 ("j0", j0_wrapper);
-@}
-@end smallexample
-
-We can already try to bring this into action by manually calling the low
-level functions for performing dynamic linking. The C source file needs
-to be compiled into a shared library. Here is how to do it on
-GNU/Linux, please refer to the @code{libtool} documentation for how to
-create dynamically linkable libraries portably.
-
-@smallexample
-gcc -shared -o libbessel.so -fPIC bessel.c
-@end smallexample
-
-Now fire up Guile:
-
-@smalllisp
-(define bessel-lib (dynamic-link "./libbessel.so"))
-(dynamic-call "init_math_bessel" bessel-lib)
-(j0 2)
-@result{} 0.223890779141236
-@end smalllisp
-
-The filename @file{./libbessel.so} should be pointing to the shared
-library produced with the @code{gcc} command above, of course. The
-second line of the Guile interaction will call the
-@code{init_math_bessel} function which in turn will register the C
-function @code{j0_wrapper} with the Guile interpreter under the name
-@code{j0}. This function becomes immediately available and we can call
-it from Scheme.
-
-Fun, isn't it? But we are only half way there. This is what
-@code{apropos} has to say about @code{j0}:
-
-@smallexample
-(apropos 'j0)
-@print{} the-root-module: j0 #<primitive-procedure j0>
-@end smallexample
-
-As you can see, @code{j0} is contained in the root module, where all
-the other Guile primitives like @code{display}, etc live. In general,
-a primitive is put into whatever module is the @dfn{current module} at
-the time @code{gh_new_procedure} is called. To put @code{j0} into its
-own module named @samp{(math bessel)}, we need to make a call to
-@code{scm_register_module_xxx}. Additionally, to have Guile perform
-the dynamic linking automatically, we need to put @file{libbessel.so}
-into a place where Guile can find it. The call to
-@code{scm_register_module_xxx} should be contained in a specially
-named @dfn{module init function}. Guile knows about this special name
-and will call that function automatically after having linked in the
-shared library. For our example, we add the following code to
-@file{bessel.c}:
-
-@smallexample
-void scm_init_math_bessel_module ()
-@{
- scm_register_module_xxx ("math bessel", init_math_bessel);
-@}
-@end smallexample
-
-The general pattern for the name of a module init function is:
-@samp{scm_init_}, followed by the name of the module where the
-individual hierarchical components are concatenated with underscores,
-followed by @samp{_module}. It should call
-@code{scm_register_module_xxx} with the correct module name and the
-appropriate initialization function. When that initialization function
-will be called, a newly created module with the right name will be the
-@emph{current module} so that all definitions that the initialization
-functions makes will end up in the correct module.
-
-After @file{libbessel.so} has been rebuild, we need to place the shared
-library into the right place. When Guile tries to autoload the
-@samp{(math bessel)} module, it looks not only for a file called
-@file{math/bessel.scm} in its @code{%load-path}, but also for
-@file{math/libbessel.so}. So all we need to do is to create a directory
-called @file{math} somewhere in Guile's @code{%load-path} and place
-@file{libbessel.so} there. Normally, the current directory @file{.} is
-in the @code{%load-path}, so we just use that for this example.
-
-@smallexample
-% mkdir maths
-% cd maths
-% ln -s ../libbessel.so .
-% cd ..
-% guile
-guile> (use-modules (math bessel))
-guile> (j0 2)
-0.223890779141236
-guile> (apropos 'j0)
-@print{} bessel: j0 #<primitive-procedure j0>
-@end smallexample
-
-That's it!
-
-Note that we used a symlink to make @file{libbessel.so} appear in the
-right spot. This is probably not a bad idea in general. The
-directories that the @file{%load-path} normally contains are supposed to
-contain only architecture independent files. They are not really the
-right place for a shared library. You might want to install the
-libraries somewhere below @samp{exec_prefix} and then symlink to them
-from the architecture independent directory. This will at least work on
-heterogenous systems where the architecture dependent stuff resides in
-the same place on all machines (which seems like a good idea to me
-anyway).
-
-
-@c Local Variables:
-@c TeX-master: "guile.texi"
-@c End:
HCoop / bpt / guile.git / commitdiff