Extern

Extern declarations use external linkage: symbols resolve at link time by the system linker. Missing symbols fail the link step.

Functions

extern fn without a body declares an imported symbol. Calls use the C calling convention:

extern fn malloc(usize) *void;
extern fn free(*void);

fn main() {
  let ptr: *i32 = malloc(4);
  *ptr = 123;
  free(ptr);
}

extern fn with a body exports the symbol for native callers:

extern fn hello() {
  @println("hi from Zeen");
}

Varargs with ...:

extern fn printf([*]const char, ...);

C ABI

Struct arguments and returns follow the C ABI of the target:

• struct arguments pass by the C ABI rules (registers or memory),
• small struct returns are lowered per the C ABI,
• extern call sites carry the byval attribute,
• struct layout follows the target, not the host.

struct Vec2 {
  x: f32,
  y: f32,
}

extern fn vec_len(v: Vec2) f32;

fn main() {
  @println("{}", vec_len(Vec2 { .x = 3.0, .y = 4.0 }));
}

Variables

extern let declares an imported global. Storage lives in native code, no value allowed:

extern let errno: i32;

Link

extern link appends native files after the compiled object in the link step:

extern link "helpers.c";

Object-only toolchains skip these sources with a warning.