Statically compile that callback too

build.rs

@@ -24,6 +24,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         .define("VB_SIGNED_PROPAGATE", None)
         .define("VB_DIV_GENERIC", None)
         .define("VB_DIRECT_EXECUTE", "on_execute")
+        .define("VB_DIRECT_FRAME", "on_frame")
         .define("VB_DIRECT_READ", "on_read")
         .define("VB_DIRECT_WRITE", "on_write")
         .file(Path::new("shrooms-vb-core/core/vb.c"))

src/emulator/shrooms_vb_core.rs

@@ -170,6 +170,7 @@ extern "C" {
     fn vb_write(sim: *mut VB, address: u32, _type: VBDataType, value: i32) -> i32;
 }
 
+#[no_mangle]
 extern "C" fn on_frame(sim: *mut VB) -> c_int {
     // SAFETY: the *mut VB owns its userdata.
     // There is no way for the userdata to be null or otherwise invalid.