(guile.info.gz) Low level dynamic linking
(guile.info.gz) Dynamic Libraries
31.4.1 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 gets done
-- Scheme Procedure: dynamic-link library
-- C Function: scm_dynamic_link (library)
Find the shared library denoted by 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, 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 `/usr/lib' and `/usr/local/lib'.
-- Scheme Procedure: dynamic-object? obj
-- C Function: scm_dynamic_object_p (obj)
Return `#t' if OBJ is a dynamic library handle, or `#f' otherwise.
-- Scheme Procedure: dynamic-unlink dobj
-- C Function: scm_dynamic_unlink (dobj)
Unlink the indicated object file from the application. The
argument DOBJ must have been obtained by a call to `dynamic-link'.
After `dynamic-unlink' has been called on DOBJ, its content is no
-- Scheme Procedure: dynamic-func name dobj
-- C Function: scm_dynamic_func (name, dobj)
Search the dynamic object DOBJ for the C function indicated by the
string NAME and return some Scheme handle that can later be used
with `dynamic-call' to actually call the function.
Regardless whether your C compiler prepends an underscore `_' to
the global names in a program, you should *not* include this
underscore in FUNCTION. Guile knows whether the underscore is
needed or not and will add it when necessary.
-- Scheme Procedure: dynamic-call func dobj
-- C Function: scm_dynamic_call (func, dobj)
Call the C function indicated by FUNC and DOBJ. The function is
passed no arguments and its return value is ignored. When
FUNCTION is something returned by `dynamic-func', call that
function and ignore DOBJ. When FUNC is a string , look it up in
DYNOBJ; this is equivalent to
(dynamic-call (dynamic-func FUNC DOBJ) #f)
Interrupts are deferred while the C function is executing (with
-- Scheme Procedure: dynamic-args-call func dobj args
-- C Function: scm_dynamic_args_call (func, dobj, args)
Call the C function indicated by FUNC and DOBJ, just like
`dynamic-call', but pass it some arguments and return its return
value. The C function is expected to take two arguments and
return an `int', just like `main':
int c_func (int argc, char **argv);
The parameter ARGS must be a list of strings and is converted into
an array of `char *'. The array is passed in ARGV and its size in
ARGC. The return value is converted to a Scheme number and
returned from the call to `dynamic-args-call'.
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:
(define libc-obj (dynamic-link "libc.so"))
=> #<dynamic-object "libc.so">
(dynamic-args-call 'rand libc-obj '())
=> #<dynamic-object "libc.so" (unlinked)>
As you can see, after calling `dynamic-unlink' on a dynamically
linked library, it is marked as `(unlinked)' and you are no longer able
to use it with `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, `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,
`dynamic-call' and `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 `libX11'
with `dynamic-link' and then construct a beautiful graphical user
interface just by using `dynamic-call' and `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 `libffi', a library that implements a "foreign
function interface" for a number of different platforms. With it, you
can extend the Spartan functionality of `dynamic-call' and
`dynamic-args-call' considerably. There is glue code available in the
Guile contrib archive to make `libffi' accessible from Guile.
(guile.info.gz) Dynamic Libraries
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