Foreign function interface - learning guile 3.0 ffi

test6

seems like guile 3.0.9 documentation needs a pull request

Good Code - section 5.3.1 A Sample Guile Extension

Bad Code - section 6.19.2 Foreign Extensions

001 - math bessel

to be able to call bessel j0 procedure from guile scheme

bit of a walk around park of foreign function interfacing to get this all to work

a ) need to generate the shared library libguile-bessel.so b ) write a guile module exports j0 c ) start guile , load bessel.scm d ) (use-modules (math bessel)) e ) finally call j0

(j0 3) => -0.2600519549019335

for what its worth , i have no idea what the bessel function does.

[primitive procedures] (https://www.gnu.org/software/guile/manual/html_node/Primitive-Procedures.html)

bessel.c calls this on init_math_bessel c code scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);

scm_c_define_gsubr can be used to register c code to be used from scheme.

BUT only if the procedure j0_wrapper takes scheme values SCM(s) and returns scheme values SCM(s) .

This means anything call from scheme needs to be wrapped up in a slice of C code to do these conversions.

At heart of foreign function interface is understanding how to convert c code to scheme compatible code and visa versa.

002 - simple boolean procedure

Boolean datatype

scm_from_double c code -> scheme scm_to_double scheme -> c code

in c booleans are just small ints , common 0 means false. Anything else means true.

Integers , Char , Big Nums

In general conversion from c to scheme / scheme to c basic types is simply a matter of choosing given routine from libguile

scm_from_ c code -> scheme scm_to_ scheme -> c code

Foreign function interface really concerned with c structures , c unions and c pointers .

definitely a big step forward , if we can get a handle on understanding this topic.

009 - foreign c image structure

c image type with width , height and pointer to byte array

challenge is to get guile scheme to somehow load the shared library and use the library to do something.

load_extension loads foreign library and then looks for a thunk procedure no args to run first

missing was call to init_image_type

without this foreign type being declared , guile scheme simply segfaults without explanation.

010 - a simple pair structure of two integers

struct pair {
 int a;
 int b;
 }