Attribute Macro interoptopus_proc::ffi_service_method [−][src]
#[ffi_service_method]
Expand description
Inside a #[ffi_service]
block, provide special directives to functions.
This is an optional attribute that can be applied to some methods.
By default service methods
must return a Result<(), Error>
return type that will be mapped to an FFIError
and
transparently checked in languages supporting the pattern.
However, sometimes you might want to return an actual value. Using this attribute and specifying
on_panic
behavior you can opt out of error mapping and instead return values as-is.
See the service module for an introduction into services.
Parameters
The following attributes can be provided:
Parameter | Explanation |
---|---|
on_panic | Determines what will happen on a panic (ffi_error , return_default , undefined_behavior ) and, as a side effect, also determine how return values will be handled. See below. |
Wrapping Behavior
Details what on_panic
means:
Mode | Explanation |
---|---|
ffi_error | Method must return Result<(), Error> and maps that to an FFIError . Default behavior. |
return_default | Method can return any T: Default . If panic occurred T::default() will be returned, see below. |
undefined_behavior | Method can return any T . If panic occurs undefined behavior happens. Slightly faster (nanoseconds) and mostly an escape hatch when running into lifetime issues in autogenerated code, e.g., when returning an AsciiPointer from a service. In the long term our proc macro code gen should be fixed to handle this situation. |
Panic Behavior
⚠️ Generated methods add panic guards when used with ffi_error
and return_default
. However, since return_default
methods
have no other way to signal errors they will return Default::default()
instead if a panic
is encountered. If you compiled Interoptopus with the log
feature a message will be emitted
in that case.
Safety
⚠️ You must ensure that methods marked with on_panic = "undefined_behavior"
will never panic. Failure to do so will lead to
undefined behavior.
Example
use interoptopus::{ffi_type, ffi_service, ffi_service_ctor, ffi_service_method};
#[ffi_type(opaque)]
pub struct SimpleService { }
#[ffi_service(error = "MyFFIError", prefix = "simple_service_")]
impl SimpleService {
#[ffi_service_ctor]
pub fn new_with(some_value: u32) -> Result<Self, Error> {
Ok(Self { })
}
#[ffi_service_method(on_panic = "return_default")]
#[allow(unconditional_panic)]
pub fn oops(&self, x: u32) -> u32 {
let array = [0, 1, 2];
// This will panic. The method will return 0 instead.
array[5]
}
#[ffi_service_method(on_panic = "undefined_behavior")]
pub fn return_value(&self) -> u32 {
// If this method ever panicked the entire application calling
// you (not just your library) will crash or worse.
123
}
}