[swift-evolution] [RFC] UnsafeBytePointer API for In-Memory Layout
Andrew Trick
atrick at apple.com
Thu May 19 02:08:54 CDT 2016
Hello Swift evolution,
I'm sending this proposal out again for another round of RFC. The first round did not get much specific feedback, and nothing has fundamentally changed. In this updated version I beefed up the explanation a bit and clarified the language.
-Andy
> On May 9, 2016, at 11:14 AM, Andrew Trick via swift-evolution <swift-evolution at swift.org> wrote:
>
> Hello Swift evolution,
>
> I sent this to swift-dev last week. Sorry to post on two lists!
>
> Swift does a great job of protecting against undefined behavior--as long as you avoid "unsafe" APIs, that is. However, unsafe APIs are important for giving developers control over implementation details and performance. Naturally, the contract between unsafe APIs and the optimizer is crucial. When a developer uses an unsafe API, the rules governing safe, well-defined behavior must be clear. On the opposite end, the optimizer must know which assumptions it can make based on those rules. Simply saying that anything goes because "unsafe" is in the name is not helpful to this effort.
>
> For a long time, I've wanted these rules nailed down. We have more users taking advantage of advanced features, and more optimizations that take advantage of assumptions guided by the type system. This seems like a particularly good time to resolve UnsafePointer semantics, considering the type system and UnsafePointer work that's been going on recently. Strict aliasing is something I would like addressed. If we do nothing here, then we will end up by default inheriting C/C++ semantics, as with any language that relies on a C/C++ backend. In other words, developers will be forced to write code with technically undefined behavior and rely on the compiler to be smart enough to recognize and recover from common patterns. Or we can take advantage of this opportunity and instead adopt a sound memory model with respect to aliasing.
>
> This proposal is only an RFC at this point. I'm sending it out now to allow for plenty of time for discussion (or advance warning). Keep in mind that it could change considerably before it goes up for review.
>
> -Andy
>
UnsafeBytePointer API for In-Memory Layout
Proposal: SE-NNNN <https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md>
Author(s): Andrew Trick <https://github.com/atrick>
Status: Awaiting review <https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#rationale>
Review manager: TBD
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#introduction>Introduction
UnsafePointer and UnsafeMutable refer to a typed region of memory, and the compiler must be able to assume that UnsafePointer element (Pointee) type is consistent with other access to the same memory. See proposed Type Safe Memory Access documentation <https://github.com/atrick/swift/blob/type-safe-mem-docs/docs/TypeSafeMemory.rst>. Consequently, conversion between UnsafePointer element types exposes an easy way to abuse the type system.
In the following example, takesUIntPtr accesses a memory location as a UInt, which is incompatible with the declared type of the pointer passed to takesIntPtr, yet the statement that performs the pointer conversion provides no indication that type punning may be taking place:
func takesUIntPtr(_ p: UnsafeMutablePointer<UInt>) -> UInt {
return p[0]
}
func takesIntPtr(q: UnsafeMutablePointer<Int>) -> UInt {
return takesUIntPtr(UnsafeMutablePointer(q))
}
If this pointer conversion was accidental, then it is likely a serious bug. Type punning via UnsafePointer is semantically undefined behavior and de facto undefined behavior given the optimizer's long-time treatment of UnsafePointer.
If the user's intention is to perform type punning, then UnsafePointer is the wrong API. Swift does not currently provide an API that permits safe, legal, type punning.
Swift-evolution thread: [RFC] UnsafeBytePointer API for In-Memory Layout <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160509/thread.html#16909>
In this document, all mentions of UnsafePointer also apply to UnsafeMutablePointer.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#motivation>Motivation
Type punning is not a normal use case for UnsafePointer and will likely lead to undefined behavior. To avoid accidental type punning, we should prohibit inferred conversion between UnsafePointer<T> and UnsafePointer<U> unless the target of the conversion is an untyped or nondereferenceable pointer (currently represented as UnsafePointer<Void>). An "inferred conversion" is one in which a generic type is initialized via type inferrence without the need to spell the destination type:
struct S {
let ptr : UnsafePointer<T>
}
let p = UnsafePointer<U>(...)
S(ptr: UnsafePointer(p))
To support this change we should introduce a new pointer type that does not bind the type of its Pointee. Such a new pointer type would allow inferred and implicit conversion from typed to untyped pointers, which is the common, safe use case for UnsafePointer conversion. More importantly it would provide an ideal foundation for an API that allows byte-wise pointer arithmetic and a legal, well-defined means to access an untyped region of memory (safe type punning). No alternative currently exists for manual memory layout and direct access to untyped memory. Not only is this a legitimate use case that Swift should support, but the lack a proper API has already lead to dangerous overuse of UnsafePointer.
As motivation for such an API, consider that an UnsafePointer<Void> or OpaquePointer may be currently be obtained from an external API. However, the developer may know the memory layout and may want to read or write elements whose types are compatible with that layout. This a reasonable use case, but unless the developer can guarantee that all accesses to the same memory location have the same type, then they cannot use UnsafePointer to access the memory without risking undefined behavior.
An example based on the proposed new UnsafeBytePointer API is included below.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#proposed-solution>Proposed solution
This proposal introduces UnsafeBytePointer and UnsafeMutableBytePointer types, along with an API for obtaining an UnsafeMutableBytePointer value at a relative byte offset and loading and storing arbitrary types at that location.
UnsafePointer<T> to UnsafeBytePointer conversion will be allowed via an unlabeled initializer and in some cases may be implicit. However, inferred UnsafePointer<T> conversion will now be statically prohibited. Converting arbitrary UnsafePointer<T> types will instead require a labeled initializer that accepts the destination type:
struct S {
let ptr : UnsafePointer<T>
}
let p = UnsafePointer<U>(...)
S(ptr: UnsafePointer(p, to: T))
Just as with unsafeBitCast, although the destination of the cast can usually be inferred, we want the developer to explicitly state the intended destination type, both because type inferrence can be surprising, and because it's important to the reader for code comprehension.
While the new UnsafeBytePointer type supports the removal of dangerous UnsafePointer conversion, it's API also meets multiple requirements:
An untyped pointer to memory
Type-unsafe access to memory (legal type punning)
Pointer arithmetic within byte-addressable memory
UnsafeMutableBytePointer will replace UnsafeMutablePointer<Void> and UnsafeBytePointer will replace UnsafePointer<Void> as the standard representations for untyped memory. The Swift imported type for void* and const void* will be UnsafeMutableBytePointer and UnsafeBytePointer respectively.
Note: For API clarity we could consider a typealias for VoidPointer. A separate VoidPointer type would not be very useful--there's no danger that UnsafeBytePointer will be casually dereferenced, and no danger in allowing pointer arithmetic since the only reasonable interpretation is that of a byte-addressable memory.
Loading from and storing to memory via an Unsafe[Mutable]BytePointer is safe independent of the type of value being loaded or stored and independent of the memory's allocated type as long as layout guarantees are met (per the ABI). This allows legal type punning within Swift and allows Swift code to access a common region of memory that may be shared across an external interface that does not provide type safety guarantees. Accessing type punned memory directly through a designated Unsafe[Mutable]BytePointer type provides sound basis for compiler implementation of strict aliasing. This is in contrast with the approach of simply providing a special unsafe pointer cast operation for bypassing type safety, which cannot be reliably implemented.
Providing an API for type-unsafe memory access would not serve much purpose without the ability to compute byte offsets. Of course, we could require users to convert back and forth using bitPatterns, but that would be awkward and only obscure the purpose of the UnsafeBytePointer type.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#detailed-design>Detailed design
The public API is shown here. For details, see the unsafeptr_convert branch <https://github.com/atrick/swift/commits/unsafeptr_convert>.
struct UnsafeMutableBytePointer : Hashable, _Pointer {
let _rawValue: Builtin.RawPointer
var hashValue: Int {...}
init<T>(_ : UnsafePointer<T>)
init<T>(_ : UnsafeMutablePointer<T>)
init?<T>(_ : UnsafePointer<T>?)
init?<T>(_ : UnsafeMutablePointer<T>?)
init<T>(_ : OpaquePointer<T>)
init?<T>(_ : OpaquePointer<T>?)
init?(bitPattern: Int)
init?(bitPattern: UInt)
/// Load a single `T` value from memory.
///
/// - Precondition: The underlying pointer is properly aligned for
/// accessing `T`.
///
/// - Note: The converse of loading a value, storing a value `T` into
/// initialized memory requires the user to know the previously initialized
/// value's type. Full 'store' semantics can be achieved with:
/// `deinitialize(PreviousType)`
/// `initialize(NewType, with: value)`
/// If the previosly initialized type cannot reference any managed objects,
/// then the `deinitialize` call can be skipped.
func load<T>(_ : T.Type) -> T
/// Load a `T` value at the specified `index` from `self` as if it
/// contains at least `index` + 1 contiguous values of type `T`.
///
/// - Precondition: The underlying pointer is properly aligned for
/// accessing `T`.
func load<T>(asArrayOf _: T.Type, at index: Int) -> T
/// Initialize this memory location with `count` consecutive copies
/// of `newValue`
///
/// Returns a `UnsafeBytePointer` to memory one byte past the last
/// initialized value.
///
/// - Precondition: The memory is not initialized.
///
/// - Precondition: The underlying pointer is properly aligned for
/// accessing `T`.
///
/// - Precondition: `count` is non-negative.
///
/// - Postcondition: The memory is initialized; the value should eventually
/// be destroyed or moved from to avoid leaks.
func initialize<T>(_: T.Type, with newValue: T, count: Int = 1)
-> UnsafeBytePointer
/// Initialize the memory location at `index` with `newValue` as if `self`
/// holds at least `index` + 1 contiguous values of type `T`.
///
/// Returns a `UnsafeBytePointer` to memory one byte past the
/// initialized value.
///
/// - Precondition: The memory at `index` is not initialized.
///
/// - Precondition: The underlying pointer is properly aligned for
/// accessing `T`.
///
/// - Postcondition: The memory is initialized; the value should eventually
/// be destroyed or moved from to avoid leaks.
func initialize<T>(asArrayOf _: T.Type, initialValue: T, at index: Int)
-> UnsafeBytePointer
/// De-initialize the `count` `T`s starting at `self`, returning
/// their memory to an uninitialized state.
///
/// - Precondition: The `T`s at `self..<self + count` are initialized.
///
/// - Postcondition: The memory is uninitialized.
func deinitialize<T>(_ : T.Type, count: Int = 1)
/// De-initialize `T` at the memory location `index` as if `self` holds at
/// least `index` + 1 contiguous values of type `T`, returning
/// the memory to an uninitialized state.
///
/// - Precondition: The `T` value at `index` is initialized.
///
/// - Postcondition: The memory at `index` is uninitialized.
func deinitialize<T>(asArrayOf _: T.Type, at index: Int)
/// Allocate and point at uninitialized memory for `size` bytes with
/// `alignedTo` alignment.
///
/// - Postcondition: The memory is allocated, but not initialized.
@warn_unused_result
init(allocatingBytes size: Int, alignedTo: Int)
/// Allocate and point at uninitialized memory for `count` values of `T`.
///
/// - Postcondition: The memory is allocated, but not initialized.
@warn_unused_result
init<T>(allocatingCapacity count: Int, of: T.Type)
/// Deallocate uninitialized memory allocated for `size` bytes with
/// `alignedTo` alignment.
///
/// - Precondition: The memory is not initialized.
///
/// - Postcondition: The memory has been deallocated.
func deallocateBytes(_ size: Int, alignedTo: Int)
/// Deallocate uninitialized memory allocated for `count` values of `T`.
///
/// - Precondition: The memory is not initialized.
///
/// - Postcondition: The memory has been deallocated.
func deallocateCapacity<T>(_ num: Int, of: T.Type)
/// Omitting comments for the following convenient variations on
/// intitialize...
func initialize<T>(from: UnsafePointer<T>, count: Int) -> UnsafeBytePointer
func initializeBackward<T>(from source: UnsafePointer<T>, count: Int)
func moveInitialize<T>(from source: UnsafeMutablePointer<T>, count: Int) {
func moveInitializeBackward<T>(from source: UnsafeMutablePointer<T>,
count: Int)
}
extension OpaquePointer {
init(_ : UnsafeBytePointer)
}
extension Int {
init(bitPattern: UnsafeBytePointer)
}
extension UInt {
init(bitPattern: UnsafeBytePointer)
}
extension UnsafeBytePointer : Strideable {
func distance(to : UnsafeBytePointer) -> Int
func advanced(by : Int) -> UnsafeBytePointer
}
func == (lhs: UnsafeBytePointer, rhs: UnsafeBytePointer) -> Bool
func < (lhs: UnsafeBytePointer, rhs: UnsafeBytePointer) -> Bool
func + (lhs: Int, rhs: UnsafeBytePointer) -> UnsafeBytePointer
func - (lhs: UnsafeBytePointer, rhs: UnsafeBytePointer) -> Int
UnsafeBytePointer provides the same interface as UnsafeMutablePointer except for store, variations on initialize, and deinitialize.
Occasionally, we need to convert from a UnsafeBytePointer to an UnsafePointer. This should only be done in very rare circumstances when the author understands the compiler's strict type rules for UnsafePointer. Although this could be done by casting through an OpaquePointer, an explicit, designated unsafe pointer cast API would makes the risks more obvious and self-documenting. For example:
extension UnsafePointer {
init(_ from: UnsafeBytePointer, to: Pointee.type)
}
extension UnsafeMutablePointer {
init(_ from: UnsafeBytePointer, to: Pointee.type)
}
Similarly, conversion between UnsafePointer types must now be spelled with an explicitly Pointee type:
extension UnsafePointer {
init<U>(_ from: UnsafePointer<U>, to: Pointee.Type)
init<U>(_ from: UnsafeMutablePointer<U>, to: Pointee.Type)
}
extension UnsafeMutablePointer {
init<U>(_ from: UnsafeMutablePointer<U>, to: Pointee.Type)
}
Some existing conversions between UnsafePointer types do not convert Pointee types but instead coerce an UnsafePointer to an UnsafeMutablePointer. This is no longer an inferred conversion, but must be explicitly requested:
extension UnsafeMutablePointer {
init(mutating from: UnsafePointer<Pointee>)
}
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#impact-on-existing-code>Impact on existing code
The largest impact of this change is that void* and const void* are imported as UnsafeMutableBytePointer and UnsafeBytePointer. This impacts many public APIs, but with implicit argument conversion should not affect typical uses of those APIs.
Any Swift projects that rely on type inference to convert between UnsafePointer types will need to take action. The developer needs to determine whether type punning is necessary. If so, they must migrate to the UnsafeBytePointer API. Otherwise, they can work around the new restriction by using a to: Pointee, or mutating label.
Disallowing inferred UnsafePointer direct conversion requires some standard library code to use an explicit to: Pointee label for unsafe conversions that may violate strict aliasing.
All occurrences of Unsafe[Mutable]Pointer<Void> in the standard library are converted to UnsafeBytePointer. e.g. unsafeAddress() now returns UnsafeBytePointer, not UnsafePointer<Void>.
Some occurrences of Unsafe[Mutable]Pointer<Pointee> in the standard library are replaced with UnsafeBytePointer, either because the code was playing too loosely with strict aliasing rules, or because the code actually wanted to perform pointer arithmetic on byte-addresses.
StringCore.baseAddress changes from OpaquePointer to UnsafeBytePointer because it is computing byte offsets and accessing the memory. OpaquePointer is meant for bridging, but should be truly opaque; that is, nondereferenceable and not involved in address computation.
The StringCore implementation does a considerable amount of casting between different views of the String storage. The current implementation already demonstrates some awareness of strict aliasing rules. The rules are generally followed by ensuring that the StringBuffer only be accessed using the appropriate CodeUnit within Swift code. For interoperability and optimization, String buffers frequently need to be cast to and from CChar. This is valid as long access to the buffer from Swift is guarded by dynamic checks of the encoding type. These unsafe, but dynamically legal conversion points will now be labeled with to: Pointee.
CoreAudio utilities now use an UnsafeBytePointer.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#implementation-status>Implementation status
On my unsafeptr_convert branch <https://github.com/atrick/swift/commits/unsafeptr_convert>, I've made most of the necessary changes to support the addition of UnsafeBytePointerand the removal of inferred UnsafePointer conversion.
There are a several things going on here in order to make it possible to build the standard library with the changes:
A new UnsafeBytePointer type is defined.
The type system imports void* as UnsafeBytePointer.
The type system handles implicit conversions to UnsafeBytePointer.
UnsafeBytePointer replaces both UnsafePointer<Void> and UnsafeMutablePointer<Void> (Recent feedback suggestes that UnsafeMutablePointer should also be introduced).
The standard library was relying on inferred UnsafePointer conversion in over 100 places. Most of these conversions now either take an explicit label, such as to: Pointee, mutating. Some have been rewritten.
Several places in the standard library that were playing loosely with strict aliasing or doing bytewise pointer arithmetic now use UnsafeBytePointer instead.
Explicit labeled Unsafe[Mutable]Pointer initializers are added.
The inferred Unsafe[Mutable]Pointer conversion is removed.
TODO:
Once this proposal is accepted, and the rules for casting between pointers types have been decided, we need to finish implementing the type system support. The current implementation (intentionally) breaks a few tests in pointer_conversion.swift. We also need to ensure that interoperability requirements are met. Currently, many argument casts to be explicitly labeled. The current implementation also makes it easy for users to hit an "ambiguous use of 'init'" error when relying on implicit argument conversion.
Additionally:
UnsafeMutableBytePointer needs to be introduced, and we need to distinguish between void* and const void*import types.
A name mangled abbreviation needs to be created for UnsafeBytePointer.
The StringAPI tests should probably be rewritten with UnsafeBytePointer.
The NSStringAPI utilities and tests may need to be ported to UnsafeBytePointer
The CoreAudio utilities and tests may need to be ported to UnsafeBytePointer.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#alternatives-considered>Alternatives considered
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#existing-workaround>Existing workaround
In some cases, developers can safely reinterpret values to achieve the same effect as type punning:
let ptrI32 = UnsafeMutablePointer<Int32>(allocatingCapacity: 1)
ptrI32[0] = Int32()
let u = unsafeBitCast(ptrI32[0], to: UInt32.self)
Note that all access to the underlying memory is performed with the same element type. This is perfectly legitimate, but simply isn't a complete solution. It also does not eliminate the inherent danger in declaring a typed pointer and expecting it to point to values of a different type.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#discarded-alternatives>Discarded alternatives
We considered adding a typePunnedMemory property to the existing Unsafe[Mutabale]Pointer API. This would provide a legal way to access a potentially type punned Unsafe[Mutabale]Pointer. However, it would certainly cause confusion without doing much to reduce likelihood of programmer error. Furthermore, there are no good use cases for such a property evident in the standard library.
The opaque _RawByte struct is a technique that allows for byte-addressable buffers while hiding the dangerous side effects of type punning (a _RawByte could be loaded but it's value cannot be directly inspected). UnsafePointer<_RawByte> is a clever alternative to UnsafeBytePointer. However, it doesn't do enough to prevent undefined behavior. The loaded _RawByte would naturally be accessed via unsafeBitCast, which would mislead the author into thinking that they have legally bypassed the type system. In actuality, this API blatantly violates strict aliasing. It theoretically results in undefined behavior as it stands, and may actually exhibit undefined behavior if the user recovers the loaded value.
To solve the safety problem with UnsafePointer<_RawByte>, the compiler could associate special semantics with a UnsafePointer bound to this concrete generic parameter type. Statically enforcing casting rules would be difficult if not impossible without new language features. It would also be impossible to distinguish between typed and untyped pointer APIs. For example, UnsafePointer<T>.load<U> would be a nonsensical vestige.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#alternate-proposal-for-void-type>Alternate proposal for void* type
Changing the imported type for void* will be somewhat disruptive. Furthermore, this proposal currently drops the distinction between void* and const void*--an obvious loss of API information.
We could continue to import void* as UnsafeMutablePointer<Void> and const void* as UnsafePointer<Void>, which will continue to serve as an "opaque" untyped pointer. Converting to UnsafeBytePointer would be necesarry to perform pointer arithmetic or to conservatively handle possible type punning.
This alternative is much less disruptive, but we are left with two forms of untyped pointer, one of which (UnsafePointer) the type system somewhat conflates with typed pointers.
Given the current restrictions of the language, it's not clear how to statically enforce the necessary rules for castingUnsafePointer<Void> once general UnsafePointer<T> conversions are disallowed. The following conversions should be inferred, and implied for function arguments (ignoring mutability):
UnsafePointer<T> to UnsafePointer<Void>
UnsafePointer<Void> to UnsafeBytePointer
I did not implement this simpler design because my primary goal was to enforce legal pointer conversion and rid Swift code of undefined behavior. I can't do that while allowing UnsafePointer conversions.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#future-improvements>Future improvements
UnsafeBytePointer should eventually support unaligned memory access. I believe that we will eventually have a modifier that allows "packed" struct members. At that time we may also want to add a "packed" flag to UnsafeBytePointer's load and initialize methods.
<https://github.com/atrick/swift-evolution/blob/voidpointer/proposals/XXXX-unsafebytepointer.md#unsafebytepointer-example>UnsafeBytePointer example
/// An example of using UnsafeMutableBytePointer to implement manual memory
/// layout.
/// A Buffer for reading and writing basic types at a fixed address.
/// Indirection allows the buffer to refer to mutable state elsewhere.
struct MessageBuffer {
let ptr: UnsafeMutableBytePointer
enum IndirectFlag { case Direct, Indirect }
private func getPointer(atOffset n: Int, _ isIndirect: IndirectFlag)
-> UnsafeMutableBytePointer {
switch isIndirect {
case .Indirect:
return (ptr + n).load(UnsafeMutableBytePointer.self)
case .Direct:
return ptr + n
}
}
func readUInt32(atOffset n: Int, _ isIndirect: IndirectFlag) -> UInt32 {
return getPointer(atOffset: n, isIndirect).load(UInt32.self)
}
func readFloat32(atOffset n: Int, _ isIndirect: IndirectFlag) -> Float32 {
return getPointer(atOffset: n, isIndirect).load(Float32.self)
}
func writeUInt32(_ val: UInt32, atOffset n: Int) {
getPointer(atOffset: n, .Direct).initialize(with: val)
}
func writeFloat32(_ val: Float32, atOffset n: Int) {
getPointer(atOffset: n, .Direct).initialize(with: val)
}
func writeIndirect(_ ptr: UnsafeMutableBytePointer, atOffset n: Int) {
getPointer(atOffset: n, .Direct).initialize(with: ptr)
}
}
/// Encoded message format.
struct MessageFormat : Sequence, IteratorProtocol {
typealias Element = MessageFormat
private static let maxFormatFields = 32 / 4
static let maxBufferBytes = maxFormatFields * strideof(UInt)
var formatCode: UInt32 = 0
var elementCode: UInt32 = 0
var offset: Int = 0
init(bitPattern: UInt32) {
formatCode = bitPattern
}
enum Kind {
case None, Reserved, UInt32, Float32, IndirectUInt32, IndirectFloat32
}
/// The first field's kind.
var kind : Kind {
get {
switch elementCode {
case 0x0: return Kind.None
case 0x2: return Kind.UInt32
case 0x3: return Kind.Float32
case 0x6: return Kind.IndirectUInt32
case 0x7: return Kind.IndirectFloat32
default: return Kind.Reserved
}
}
}
func elementStride() -> Int {
return (elementCode & 0x4) != 0 ? strideof(UInt) : 4
}
/// Get the format for the next element.
mutating func next() -> Element? {
if elementCode != 0 {
offset += elementStride()
}
elementCode = formatCode & 0xF
formatCode >>= 4
if kind == .None {
return nil
}
// align to the next element size
let offsetMask = elementStride() - 1
offset = (offset + offsetMask) & ~offsetMask
return self
}
}
func createBuffer() -> MessageBuffer {
return MessageBuffer(ptr: UnsafeMutableBytePointer(
allocatingBytes: MessageFormat.maxBufferBytes, alignedTo: strideof(UInt)))
}
func destroy(buffer: MessageBuffer) {
buffer.ptr.deallocateBytes(MessageFormat.maxBufferBytes,
alignedTo: strideof(UInt))
}
var sharedInt: UInt32 = 42
var sharedFloat: Float32 = 16.25
func generateMessage(inBuffer mb: MessageBuffer) -> MessageFormat {
let mf = MessageFormat(bitPattern: 0x06727632)
for field in mf {
switch field.kind {
case .UInt32:
mb.writeUInt32(66, atOffset: field.offset)
case .Float32:
mb.writeFloat32(41.625, atOffset: field.offset)
case .IndirectUInt32:
mb.writeIndirect(&sharedInt, atOffset: field.offset)
case .IndirectFloat32:
mb.writeIndirect(&sharedFloat, atOffset: field.offset)
case .None:
fallthrough
case .Reserved:
return MessageFormat(bitPattern: 0)
}
}
return mf
}
func handleMessage(buffer mb: MessageBuffer, format: MessageFormat) -> Bool {
for field in format {
switch field.kind {
case .UInt32:
print(mb.readUInt32(atOffset: field.offset, .Direct))
case .Float32:
print(mb.readFloat32(atOffset: field.offset, .Direct))
case .IndirectUInt32:
print(mb.readUInt32(atOffset: field.offset, .Indirect))
case .IndirectFloat32:
print(mb.readFloat32(atOffset: field.offset, .Indirect))
case .None:
fallthrough
case .Reserved:
return false
}
}
return true
}
func runProgram() {
let mb = createBuffer()
let mf = generateMessage(inBuffer: mb)
if handleMessage(buffer: mb, format: mf) {
print("Done")
}
destroy(buffer: mb)
}
runProgram()
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