I want to get the memory address of a value type of constant in Swift. For example with the variables it looks like:
My code:
var value = 10
withUnsafePointer(to: &value) {
print(" str value \(value) has address: \($0)")
}
There is a variant of withUnsafePointer(to:,_:) that accepts a non-inout argument as its first input argument.
let immutableValue = 1
withUnsafePointer(to: immutableValue, { pointer -> Void in
print(pointer)
})
As MartinR pointed out, this pointer though is valid only for the invocation of the closure.
It is also important to note that with values known at compile time, the compiler might substitute the value in place of the variable, so the variable might not even exist in memory at runtime.
After reading:
https://github.com/rodionovd/SWRoute/wiki/Function-hooking-in-Swift
https://github.com/rodionovd/SWRoute/blob/master/SWRoute/rd_get_func_impl.c
I understood that Swift function pointer is wrapped by swift_func_wrapper and swift_func_object (according to the article in 2014).
I guess this still works in Swift 3, but I couldn't find which file in https://github.com/apple/swift best describes these structs.
Can anyone help me?
I believe these details are mainly part of the implementation of Swift's IRGen – I don't think you'll find any friendly structs in the source showing you the full structure of various Swift function values. Therefore if you want to do some digging into this, I would recommend examining the IR emitted by the compiler.
You can do this by running the command:
xcrun swiftc -emit-ir main.swift | xcrun swift-demangle > main.irgen
which will emit the IR (with demangled symbols) for a -Onone build. You can find the documentation for LLVM IR here.
The following is some interesting stuff that I've been able to learn from going through the IR myself in a Swift 3.1 build. Note that this is all subject to change in future Swift versions (at least until Swift is ABI stable). It goes without saying that the code examples given below are only for demonstration purposes; and shouldn't ever be used in actual production code.
Thick function values
At a very basic level, function values in Swift are simple things – they're defined in the IR as:
%swift.function = type { i8*, %swift.refcounted* }
which is the raw function pointer i8*, along with a pointer to its context %swift.refcounted*, where %swift.refcounted is defined as:
%swift.refcounted = type { %swift.type*, i32, i32 }
which is the structure of a simple reference-counted object, containing a pointer to the object's metadata, along with two 32 bit values.
These two 32 bit values are used for the reference count of the object. Together , they can either represent (as of Swift 4):
The strong and unowned reference count of the object + some flags, including whether the object uses native Swift reference counting (as opposed to Obj-C reference counting), and whether the object has a side table.
or
A pointer to a side table, which contains the above, plus the weak reference count of the object (on forming a weak reference to an object, if it doesn't already have a side table, one will be created).
For further reading on the internals of Swift reference counting, Mike Ash has a great blog post on the subject.
The context of a function usually adds extra values onto the end of this %swift.refcounted structure. These values are dynamic things that the function needs upon being called (such as any values that it has captured, or any parameters that it has been partially applied with). In quite a few cases, function values won't need a context, so the pointer to the context will simply be nil.
When the function comes to be called, Swift will simply pass in the context as the last parameter. If the function doesn't have a context parameter, the calling convention appears to allow it to be safely passed anyway.
The storing of the function pointer along with the context pointer is called a thick function value, and is how Swift usually stores function values of known type (as opposed to a thin function value which is just the function pointer).
So, this explains why MemoryLayout<(Int) -> Int>.size returns 16 bytes – because it's made up of two pointers (each being a word in length, i.e 8 bytes on a 64 bit platform).
When thick function values are passed into function parameters (where those parameters are of non-generic type), Swift appears to pass the raw function pointer and context as separate parameters.
Capturing values
When a closure captures a value, this value will be put into a heap-allocated box (although the value itself can get stack-promoted in the case of a non-escaping closure – see later section). This box will be available to the function through the context object (the relevant IR).
For a closure that just captures a single value, Swift just makes the box itself the context of the function (no need for extra indirection). So you'll have a function value which looks like a ThickFunction<Box<T>> from the following structures:
// The structure of a %swift.function.
struct ThickFunction<Context> {
// the raw function pointer
var ptr: UnsafeRawPointer
// the context of the function value – can be nil to indicate
// that the function has no context.
var context: UnsafePointer<Context>?
}
// The structure of a %swift.refcounted.
struct RefCounted {
// pointer to the metadata of the object
var type: UnsafeRawPointer
// the reference counting bits.
var refCountingA: UInt32
var refCountingB: UInt32
}
// The structure of a %swift.refcounted, with a value tacked onto the end.
// This is what captured values get wrapped in (on the heap).
struct Box<T> {
var ref: RefCounted
var value: T
}
In fact, we can actually verify this for ourselves by running the following:
// this wrapper is necessary so that the function doesn't get put through a reabstraction
// thunk when getting typed as a generic type T (such as with .initialize(to:))
struct VoidVoidFunction {
var f: () -> Void
}
func makeClosure() -> () -> Void {
var i = 5
return { i += 2 }
}
let f = VoidVoidFunction(f: makeClosure())
let ptr = UnsafeMutablePointer<VoidVoidFunction>.allocate(capacity: 1)
ptr.initialize(to: f)
let ctx = ptr.withMemoryRebound(to: ThickFunction<Box<Int>>.self, capacity: 1) {
$0.pointee.context! // force unwrap as we know the function has a context object.
}
print(ctx.pointee)
// Box<Int>(ref:
// RefCounted(type: 0x00000001002b86d0, refCountingA: 2, refCountingB: 2),
// value: 5
// )
f.f() // call the closure – increment the captured value.
print(ctx.pointee)
// Box<Int>(ref:
// RefCounted(type: 0x00000001002b86d0, refCountingA: 2, refCountingB: 2),
// value: 7
// )
ptr.deinitialize()
ptr.deallocate(capacity: 1)
We can see that by calling the function between printing out the value of the context object, we can observe the changing in value of the captured variable i.
For multiple captured values, we need extra indirection, as the boxes cannot be stored directly as the given function's context, and may be captured by other closures. This is done by adding pointers to the boxes to the end of a %swift.refcounted.
For example:
struct TwoCaptureContext<T, U> {
// reference counting header
var ref: RefCounted
// pointers to boxes with captured values...
var first: UnsafePointer<Box<T>>
var second: UnsafePointer<Box<U>>
}
func makeClosure() -> () -> Void {
var i = 5
var j = "foo"
return { i += 2; j += "b" }
}
let f = VoidVoidFunction(f: makeClosure())
let ptr = UnsafeMutablePointer<VoidVoidFunction>.allocate(capacity: 1)
ptr.initialize(to: f)
let ctx = ptr.withMemoryRebound(to:
ThickFunction<TwoCaptureContext<Int, String>>.self, capacity: 1) {
$0.pointee.context!.pointee
}
print(ctx.first.pointee.value, ctx.second.pointee.value) // 5 foo
f.f() // call the closure – mutate the captured values.
print(ctx.first.pointee.value, ctx.second.pointee.value) // 7 foob
ptr.deinitialize()
ptr.deallocate(capacity: 1)
Passing functions into parameters of generic type
You'll note that in the previous examples, we used a VoidVoidFunction wrapper for our function values. This is because otherwise, when being passed into a parameter of generic type (such as UnsafeMutablePointer's initialize(to:) method), Swift will put a function value through some reabstraction thunks in order to unify its calling convention to one where the arguments and return are passed by reference, rather than value (the relevant IR).
But now our function value has a pointer to the thunk, rather than the actual function we want to call. So how does the thunk know which function to call? The answer is simple – Swift puts the function that we want to the thunk to call in the context itself, which will therefore look like this:
// the context object for a reabstraction thunk – contains an actual function to call.
struct ReabstractionThunkContext<Context> {
// the standard reference counting header
var ref: RefCounted
// the thick function value for the thunk to call
var function: ThickFunction<Context>
}
The first thunk that we go through has 3 parameters:
A pointer to where the return value should be stored
A pointer to where the arguments for the function are located
The context object which contains the actual thick function value to call (such as shown above)
This first thunk just extracts the function value from the context, and then calls a second thunk, with 4 parameters:
A pointer to where the return value should be stored
A pointer to where the arguments for the function are located
The raw function pointer to call
The pointer to the context of the function to call
This thunk now retrieves the arguments (if any) from the argument pointer, then calls the given function pointer with these arguments, along with its context. It then stores the return value (if any) at the address of the return pointer.
Like in the previous examples, we can test this like so:
func makeClosure() -> () -> Void {
var i = 5
return { i += 2 }
}
func printSingleCapturedValue<T>(t: T) {
let ptr = UnsafeMutablePointer<T>.allocate(capacity: 1)
ptr.initialize(to: t)
let ctx = ptr.withMemoryRebound(to:
ThickFunction<ReabstractionThunkContext<Box<Int>>>.self, capacity: 1) {
// get the context from the thunk function value, which we can
// then get the actual function value from, and therefore the actual
// context object.
$0.pointee.context!.pointee.function.context!
}
// print out captured value in the context object
print(ctx.pointee.value)
ptr.deinitialize()
ptr.deallocate(capacity: 1)
}
let closure = makeClosure()
printSingleCapturedValue(t: closure) // 5
closure()
printSingleCapturedValue(t: closure) // 7
Escaping vs. non-escaping capture
When the compiler can determine that the capture of a given local variable doesn't escape the lifetime of the function it's declared in, it can optimise by promoting the value of that variable from the heap-allocated box to the stack (this is a guaranteed optimisation, and occurs in even -Onone). Then, the function's context object need only store a pointer to the given captured value on the stack, as it is guaranteed not to be needed after the function exits.
This can therefore be done when the closure(s) capturing the variable are known not to escape the lifetime of the function.
Generally, an escaping closure is one that either:
Is stored in a non-local variable (including being returned from the function).
Is captured by another escaping closure.
Is passed as an argument to a function where that parameter is either marked as #escaping, or is not of function type (note this includes composite types, such as optional function types).
So, the following are examples where the capture of a given variable can be considered not to escape the lifetime of the function:
// the parameter is non-escaping, as is of function type and is not marked #escaping.
func nonEscaping(_ f: () -> Void) {
f()
}
func bar() -> String {
var str = ""
// c doesn't escape the lifetime of bar().
let c = {
str += "c called; "
}
c();
// immediately-evaluated closure obviously doesn't escape.
{ str += "immediately-evaluated closure called; " }()
// closure passed to non-escaping function parameter, so doesn't escape.
nonEscaping {
str += "closure passed to non-escaping parameter called."
}
return str
}
In this example, because str is only ever captured by closures that are known not to escape the lifetime of the function bar(), the compiler can optimise by storing the value of str on the stack, with the context objects storing only a pointer to it (the relevant IR).
So, the context objects for each of the closures1 will look like Box<UnsafePointer<String>>, with pointers to the string value on the stack. Although unfortunately, in a Schrödinger-like manner, attempting to observe this by allocating and re-binding a pointer (like before) triggers the compiler to treat the given closure as escaping – so we're once again looking at a Box<String> for the context.
In order to deal with the disparity between context objects that hold pointer(s) to the captured values rather than holding the values in their own heap-allocated boxes – Swift creates specialised implementations of the closures that take pointers to the captured values as arguments.
Then, a thunk is created for each closure that simply takes in a given context object, extracts the pointer(s) to the captured values from it, and passes this onto the specialised implementation of the closure. Now, we can just have a pointer to this thunk along with our context object as the thick function value.
For multiple captured values that don't escape, the additional pointers are simply added onto the end of the box, i.e
struct TwoNonEscapingCaptureContext<T, U> {
// reference counting header
var ref: RefCounted
// pointers to captured values (on the stack)...
var first: UnsafePointer<T>
var second: UnsafePointer<U>
}
This optimisation of promoting the captured values from the heap to the stack can be especially beneficial in this case, as we're no longer having to allocate separate boxes for each value – such as was the case previously.
Furthermore it's worth noting that lots of cases with non-escaping closure capture can be optimised much more aggressively in -O builds with inlining, which can result in context objects being optimised away entirely.
1. Immediately-evaluated closures actually don't use a context object, the pointer(s) to the captured values are just passed directly to it upon calling.
I'm trying to understand the use of pointers in Swift, in particular: Unsafe[Mutable]Pointer and UnsafeRaw[Mutable]Pointer. I have several questions on the subject.
Is UnsafePointer <T> equal to const T * Pointer in ? and UnsafeMutablePointer <T> is equal to T * Pointer in C?
What is the difference between Unsafe[Mutable]Pointer and UnsafeRaw[Mutable]Pointer?
Why does this compile
func receive(pointer: UnsafePointer<Int> ) {
print("param value is: \(pointer.pointee)")
}
var a: Int = 1
receive(pointer: &a) // prints 1
but this gives me an error?
var a: Int = 1
var pointer: UnsafePointer<Int> = &a // error : Cannot pass immutable value of type 'Int' as inout argument
Is UnsafePointer <T> equal to const T * Pointer in ? and UnsafeMutablePointer <T> is equal to T * Pointer in C?
Well, use a bridging header in a Swift app to see how the C pointers are bridged:
const int *myInt;
int *myOtherInt;
bridges to
var myInt: UnsafePointer<Int32>!
var myOtherInt: UnsafeMutablePointer<Int32>!
What is the difference between Unsafe[Mutable]Pointer and UnsafeRaw[Mutable]Pointer?
Swift 3 added a UnsafeRawPointer API to replace the Unsafe[Mutable]Pointer<Void> type. Conversion between pointers of a different type is no longer allowed in Swift. Instead, the API provides interfaces (.assumingMemoryBound(to:) or .bindMemory(to:capacity:)) to bind memory to a type.
With regard to question 3, the ampersand means that the variable is inout. I don't believe you can declare a variable as inout unless it is being used by a function that directly modifies the underlying memory, but I'll let the experts correct me. Instead, use withUnsafePointer.
Thanks to Martin's helpful comment, this syntax was never valid in Swift, and there is no safe way to create "free pointers" to Swift variables.
This is just an exercise in pointers in swift, but I was trying to write a function that would print the pointer to self, but I kept getting an error "cannot assign to immutable value of type C". Is this something that is even possible in Swift?
class C {
static var c = C()
var a = 1
func printMyPointer() {
printPointer(&self) //cannot assign to immutable value of type C
}
func printPointer(ptr:UnsafePointer<C>) {
print(ptr)
}
}
C.c.printMyPointer()
As already explained in the other answer, you cannot pass a constant
as the argument for an in-out parameter. As a workaround, you can
pass an array, as this will actually pass the address of the first
element:
func printMyPointer() {
printPointer([self])
}
But even simpler, use unsafeAddressOf():
func printMyPointer() {
print(unsafeAddressOf(self))
}
Update for Swift 3: As of Xcode 8 beta 6, unsafeAddressOf
does not exist anymore. You can convert self to a pointer:
print(Unmanaged.passUnretained(self).toOpaque())
or
print(unsafeBitCast(self, to: UnsafeRawPointer.self))
From the Swift Docs:
You can only pass a variable as the argument for an in-out parameter. You cannot pass a constant or a literal value as the argument, because constants and literals cannot be modified. You place an ampersand (&) directly before a variable’s name when you pass it as an argument to an inout parameter, to indicate that it can be modified by the function.
Since self is immutable, you can't pass it as an inout parameter. Even though the method signature dosen't use inout, Swift treats pointer parameters like inout, according to the Swift blog:
Swift allows pointer parameters to be treated like inout parameters, so you can pass a reference to a var as a pointer argument by using the same & syntax.
I'm using a type from a different module, let's call it OtherModule.MyType,
This code:
var a = [OtherModule.MyType]()
will produce an error invalid use of '()' to call a value of non-function type '[MyType.Type]'
This code won't:
var ax = [OtherModule.MyType]
But I believe ax is not an array any more, since this code
ax.append(OtherModule.MyType())
will cause an error Cannot invoke 'append' with an argument list of '(MyType)'
So I wonder what ax really is?
Besides, this code works fine:
var ay = Array<OtherModule.MyType>()
ay.append(OtherModule.MyType())
UPDATE:
I'm using swift 1.2 with Xcode 6.3
For some reason best known to the Swift team (modules are very scantly documented), Module.Thing behaves differently to Thing.
While Int is just a type name:
let i: Int = 1 // fine
// not fine, "expected member name or constructor call after type name"
let j = Int
Swift.Int can be both:
// used as a type name
let k: Swift.Int = 1
let t = Swift.Int.self
// but also used as a value
let x = Swift.Int
// equivalent to this
let y = Int.self
toString(x) == toString(y) // true
Under some uses it only wants to be a value, not a type name though. Hence this works:
// a will be of type [Int.Type], initialized with an array
// literal of 1 element, the Int metatype
let a = [Swift.Int]
But trying to use it as a type name in this context fails: [Swift.Int]() is no more valid than writing [1]() or let strs = ["fred"]; strs().
This behaviour seems a little arbitrary, and may even be a bug/unintentional.
Since the only way in which Swift.Int can be used in this context:
Array<Swift.Int>()
is as a type not a value (since only types can go between the angle brackets), it kind of makes sense that this works while the more ambiguous array literal syntax behaves differently.