[swift-dev] [RFC] UnsafeBytePointer API for In-Memory Layout

Joe Groff jgroff at apple.com
Thu May 12 17:21:58 CDT 2016

> On May 12, 2016, at 11:21 AM, John McCall <rjmccall at apple.com> wrote:
>> On May 12, 2016, at 10:45 AM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
>>> On May 12, 2016, at 10:44, Joe Groff <jgroff at apple.com> wrote:
>>>> On May 12, 2016, at 9:27 AM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
>>>> - I’m uncomfortable with using the term “undefined behavior” as if it’s universally understood. Up until now we haven't formally had that notion in Swift, just “type safety” and “memory safety” and “invariant-preserving” and the like. Maybe we need it now, but I think it needs to be explicitly defined. (I’d actually talk to Dave about exactly what terms make the most sense for users.)
>>> We do have undefined behavior, and use that term in the standard library docs where appropriate:
>>> stdlib/public/core/Optional.swift-  /// `!` (forced unwrap) operator. However, in optimized builds (`-O`), no
>>> stdlib/public/core/Optional.swift-  /// check is performed to ensure that the current instance actually has a
>>> stdlib/public/core/Optional.swift-  /// value. Accessing this property in the case of a `nil` value is a serious
>>> stdlib/public/core/Optional.swift:  /// programming error and could lead to undefined behavior or a runtime
>>> stdlib/public/core/Optional.swift-  /// error.
>>> stdlib/public/core/Optional.swift-  ///
>>> stdlib/public/core/Optional.swift-  /// In debug builds (`-Onone`), the `unsafelyUnwrapped` property has the same
>>> --
>>> stdlib/public/core/StringBridge.swift-  /// The caller of this function guarantees that the closure 'body' does not
>>> stdlib/public/core/StringBridge.swift-  /// escape the object referenced by the opaque pointer passed to it or
>>> stdlib/public/core/StringBridge.swift-  /// anything transitively reachable form this object. Doing so
>>> stdlib/public/core/StringBridge.swift:  /// will result in undefined behavior.
>>> stdlib/public/core/StringBridge.swift-  @_semantics("self_no_escaping_closure")
>>> stdlib/public/core/StringBridge.swift-  func _unsafeWithNotEscapedSelfPointer<Result>(
>>> stdlib/public/core/StringBridge.swift-    _ body: @noescape (OpaquePointer) throws -> Result
>>> --
>>> stdlib/public/core/Unmanaged.swift-  /// reference's lifetime fixed for the duration of the
>>> stdlib/public/core/Unmanaged.swift-  /// '_withUnsafeGuaranteedRef' call.
>>> stdlib/public/core/Unmanaged.swift-  ///
>>> stdlib/public/core/Unmanaged.swift:  /// Violation of this will incur undefined behavior.
>>> stdlib/public/core/Unmanaged.swift-  ///
>>> stdlib/public/core/Unmanaged.swift-  /// A lifetime of a reference 'the instance' is fixed over a point in the
>>> stdlib/public/core/Unmanaged.swift-  /// programm if:
>> Those latter two are in stdlib-internal declarations. I think I have the same objection with using the term for 'unsafelyUnwrapped'.
> Well, we can say "A program has undefined behavior if it does X or Y", or we can say "A program which does X or Y lacks type safety".  In all cases we are referring to a concept defined elsewhere.  If we say "undefined behavior", we are using an easily-googled term whose popular discussions will quickly inform the reader of the consequences of the violation.  If we say "type safety", we are using a term with that's popularly used in very vague, hand-wavey ways and whose consequences aren't usually discussed outside of formal contexts.  If we say "memory safety", we're using a term that doesn't even have that precedent.  So we can use the latter two terms if we want, but that just means we need to have a standard place where we define them and describe the consequences of violating them, probably with at least a footnote saying "this is analogous to the undefined behavior rules of C and C++".

In other places where the standard library intentionally has undefined behavior, it looks like we use the term "serious programming error", for instance in the the doc comment for `assert`:

/// * In -Ounchecked builds, `condition` is not evaluated, but the
///   optimizer may assume that it *would* evaluate to `true`. Failure
///   to satisfy that assumption in -Ounchecked builds is a serious
///   programming error.

which feels a bit colloquial to me, and doesn't provide much insight into the full consequences of UB. I think we're better off using an established term.


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