"I guess from that intepretation,..." <- https://registry.khronos.org/vulkan/specs/1.3-extensions/html/vkspec.html#fundamentals-validusage Among other things, this notes "the behavior of the core layer to an application using the API incorrectly is undefined, and may include program termination." and "Valid usage conditions do not cover conditions where well-defined behavior (including returning an error code) exists." I guess the idea is the caller must satisfy "Valid usage" requirements or get undefined behavior. And a goal of Vulkan is to require limited error checking (with validation layers for use during development). So returning success for an invalid type of fd is completely consistent with what one expects from the spec.