E116: An extern function can't have a body

extern fn doesn’t define a function, it announces one that already exists somewhere else (usually in C). The code is not in your file, so there is nothing to write in { ... }:

extern fn malloc(size: int): ptr {       // no body allowed    return null;}

Declare it with a semicolon instead:

extern fn malloc(size: int): ptr;

If you wanted to write the function yourself in Turf, drop the extern:

fn my_alloc(size: int): ptr { ... }

Run turf --explain E116 in a terminal to read this again.