PicoLisp FFI with Zig

You can get it on GitHub.

This repo provides a simple example of how to use PicoLisp with Zig using PicoLisp’s FFI (native) functionality.

This is similar to picolisp-rust except written for Zig.

Requirements

• Zig v0.6+
• PicoLisp 64-bit v17.12+ or pil21

Getting started

Once you’ve setup PicoLisp and Zig, simply type make to build and test the shared library.

Output

Before I explain what’s going on, here’s what the output should look like:

Received struct: pil.PilStruct{ .byte1 = 32, .byte2 = 33, .character1 = 67, .character2 = 68, .int = -1, .long = 1, .string = u8@4847464544434241, .array = { 1, 2, 3, 4, 5, 6, 7, 8 } }
Result code: 0
Extracted struct: 
(de Extracted (42 43)
   ("A" "B")
   65535
   9223372036854775807
   "pilzig"
   (80 105 99 111 76 105 115 112) )

Explain

The code can be found in extract.l and pil.zig. The Zig code is designed as a shared library and can be called by PicoLisp’s (native) function to pass data to/from between both languages.

PicoLisp code explanation

First, the code allocates 32 bytes of memory, which will be used to store data in a struct.

It then creates a struct named P with the following specification:

• 2 arbitrary bytes
• 2-bytes containing valid UTF-8 characters
• 1x 32-bit (4 bytes) signed integer
• 1x 64-bit (8 bytes) signed long
• 1x 8-byte null-terminated string
• 1x 8-byte arbitrary bytes array

Then the following native call is made and its result is stored in the Res variable:

(native "./libpil.so" "extract" 'I P)

This calls the extract function from the Zig library, with the P struct as its only parameter. It expects a 32-bit signed integer I as the return value (it will be either 0 or -1).

Next, the code will extract the P structure using the specification described above:

(struct P '((B . 2) (C . 2) I N S (B . 8)))

Finally, the code will free the previously allocated memory and print the result of the P structure.

Some tests run at the end to ensure the data received from Zig is what we expected.

Note

• The values sent to the Zig library will be printed by Zig as Received struct:.
• The values received from the Zig library will be printed by PicoLisp as Extracted struct:.

Zig code explanation

The Zig code defines the struct for the received data; it is named PilStruct and contains the exact same specification as the P struct in the PicoLisp code explanation.

The extract() function creates a new struct in the variable newstruct which contains some new values, different from what was received by PicoLisp.

Luckily, the Zig pointers cannot be null so there’s no need to check for that.

The code then dereferences the pointer and prints what it received (the entire struct) from PicoLisp as Received struct: (mentioned earlier).

Finally, it writes the newstruct struct to the pointer and returns 0. PicoLisp can then read the return code and the new struct data.

Thoughts

There isn’t much to this code, but I thought it would be fun to create a working FFI library that’s not written in C or Rust and which works perfectly with PicoLisp.