Show how to call a function in a shared library (without dynamically linking to it at compile-time). In particular, show how to call the shared library function if the library is available, otherwise use an internal equivalent function.

This is a special case of calling a foreign language function where the focus is close to the ABI level and not at the normal API level.



Related task

 OpenGL -- OpenGL is usually maintained as a shared library.


CL-USER> (cffi:load-foreign-library "libX11.so")
#<CFFI::FOREIGN-LIBRARY {1004F4ECC1}>
CL-USER> (cffi:foreign-funcall "XOpenDisplay"
                               :string #+sbcl (sb-posix:getenv "DISPLAY")
                                       #-sbcl ":0.0"
                               :pointer)
#.(SB-SYS:INT-SAP #X00650FD0)