Home/ Questions/Q 3672602
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-19T02:37:46+00:00

I am developing a custom LISP interpreter. It won’t support defining functions like in

  • 0

I am developing a custom LISP interpreter. It won’t support defining functions like in LISP, instead all functions are mapped to C functions. When it sees an expression like,

(substr 'input '1 '1)

it knows to call internal substr function and return the result.

Now I am planning to implement a message function which supports basic formatting and writes the output to stdout. Something like,

(message "Hello, %s" name)

%s will be replaced with value in variable name.

Current plan is to directly pass the format and arguments to functions like printf. In that way, I can support all formats that printf supports. But problem comes with variable number of arguments. One way to do will be something like,

if(argcount == 1)
   /* call printf with one arg */
else if(argcount == 2)
   /* call printf with two arg */
....

This works, but I am wondering is there a better way to achieve this?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    I doubt there is a way to do this. The reason is that the number of parameters to your lisp function is only known at runtime, but the number of arguments to a C function must be known at compile time.

    This includes va_lists unless you want to hack at them in some kind of platform specific way.

    The best you can really do is write a function in C which is capable of looping through the arguments one at a time and doing something with them. The only way I can see around this is to not only store a function pointer for each of your internal functions, but to also store a “calling convention” which will give information about whether it takes parameters in the ordinary way or whether it finishes with the equivalent of a va_list.

    Functions like printf would have a wrapper, printf_wrapper, say, and you’d store a function pointer to the wrapper. This wrapper would accept the format string as an ordinary parameter, followed by a list or array of other parameters (roughly analogous to a va_list).

    You might indicate that printf_wrapper finishes with a parameter that expects a list by specifying the calling conventions for the printf_wrapper function as “va_list_type”, meaning that it takes the usual fixed parameters, and that all remaining parameters must be bundled up and supplied to it as a list.

    Of course writing a printf_wrapper function which can split up and parse a format string into multiple format strings is a bit of work. Here’s an example of where I did precisely this so that I could add my own custom format specifiers:

    https://github.com/wbhart/bsdnt/blob/v0.26/helper.c

    • 0