Why isPFN_vkVoidFunction in a Option?

[Local-Trash]

Note: This is my first post in a discussion. Sorry if I mess up or anything. Please tell if I did something wrong.

I have been using the vulkanalia-sys crate and I have also tried out the ash crate. Through this time, I have notice that some of the function pointers are wrapped in a Option and that they are not pointers themselves(e.i. type x = *mut y). I was wondering why this is in the sys crate. Thank you for your time.

[KyleMayes]

The Rust type:

vulkanalia/vulkanalia-sys/src/functions.rs

Line 87 in 384a8d3

pub type PFN_vkVoidFunction = Option<unsafe extern "system" fn()>;

The C type (according to the Vulkan specification):

typedef void (VKAPI_PTR *PFN_vkVoidFunction)(void);

The reason the Rust type is a Option<unsafe extern "system" fn()> instead of just unsafe extern "system" fn() is to allow it to represent null function pointers. C function pointer types implicitly allow null values, but Rust function pointer types do not.

The Rust types for actual Vulkan commands in vulkanalia do not include Option, e.g.:

vulkanalia/vulkanalia-sys/src/commands.rs

Lines 3041 to 3042 in 384a8d3

	pub type PFN_vkGetDeviceProcAddr =
	unsafe extern "system" fn(_device: Device, _name: *const c_char) -> PFN_vkVoidFunction;