FFI with dependencies in Racket?

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I developed a very simple FFI for the Wiring PI library, to be able to control a Raspberry Pi using Racket.

This worked well, but only covers the base functionality. I wanted to extend this to also use the lcd.h from the libwiringPiDev.so, so that I could create an FFI to control an LCD.

Normally, when you compile a C file that uses the LCD, you tag both the standard library and the development library when compiling:

-lwiringPi -lwiringPiDev

In my LCD FFI I did the following:

(define-ffi-definer define-lcd
  (ffi-lib "usr/lib/libwiringPiDev.so"))

However, when I try to require that racket file, I run into:

; ffi-lib: couldn't open "/usr/lib/libwiringPiDev.so"
;   (/usr/lib/libwiringPiDev.so: undefined symbol: digitalRead) [,bt for
;   context]

The digitalRead function exists in the base library but not in the dev library...how do I do a "FFI with dependencies" so that Racket can properly manage both libraries to be able to find the necessary symbols?

wiringpi raspberry-pi racket ffi
1 Answers

Try loading libwiringPi.so first in global mode, then load libwiringPiDev.so:

(define-ffi-definer define-lcd
  (ffi-lib "/usr/lib/libwiringPi.so" #:global? #t))
(define-ffi-definer define-lcd-dev
  (ffi-lib "/usr/lib/libwiringPiDev.so"))

The #:global? #t argument corresponds to setting the RTLD_GLOBAL flag in the call to dlopen, and the man page for dlopen (on my Linux system) says

RTLD_GLOBAL
The symbols defined by this shared object will be made available for symbol resolution of subsequently loaded shared objects.

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