Can I extend lisp with c++?

Question

Can I call a function from lisp from a library written in c or c++? How can I extend lisp? This is useful when you want to do some system calls or stuff like that.

Answer 1

It is unusual to call non-lisp code from lisp, and rarely necessary. CLX (the X11 client implementation for CL) doesn't link to the Xlib implementation but "speaks" X11 directly. On any system, your CL implementation is likely to already have excellent operating system hooks rendering this unnecessary.

That said, the answer depends on the lisp implementation:

In ECL you can actually host a CL environment under C and simply call cl_eval() with the code to execute. This can let you write your application (or application host) in C (or C++) and "call to" lisp code.

In CCL, there is a C-compatible calling interface which lets you do something like this:

(with-cstrs ((x "Hello World"))
  (#_puts x))

In most other CL implementations (like SBCL; and yes this works in ECL and CCL as well) you can use UFFI (or CFFI) which simply lets you call C functions, which is what other people are talking about. If this is all you want to do, then CFFI is a good, safe place to start.