36.1. Adding a built-in function

36.1. Adding a built-in function
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In the rare cases when you really need to modify CLISP internals and add a truly built-in function, you should read the CLISP sources for inspiration and enlightenment, choose a file where your brand-new built-in function should go to, and then ...

add the LISPFUN form and the implementation there
add the LISPFUN header to file subr.d
declare the function name in file constsym.d in the appropriate package (probably “EXT”, if there is no specific package)
if your function accepts keyword arguments, then an appropriate pair of forms must be added to subrkw.d and you must make sure that the keyword symbols are declared in constsym.d;
see the example in Section 32.2.7.1, “Modprep” for argument passing and returning values
export your function name from the appropriate package in file init.lisp
when you are done, you should run make check-sources in your build directory: this will check that the definitions (source files) and the declarations (subr.d, subrkw.d and fsubr.d) are in sync.

Warning

Be very careful with the GC-unsafe functions! Always remember about GC-safety!

These instructions are intentionally terse - you are encouraged to use modules and/or “FFI” instead of adding built-ins directly.