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;
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.