[swift-users] UnsafeMutableRawPointer to UnsafeMutablePointer: EXC_BAD_ACCESS on pointee

[nyg nyg via swift-users]

Hello all,

I'm trying to get an UnsafeMutablePointer from an
UnsafeMutableRawPointer obtained using the Unmanaged structure:

class C { var foo = 42, bar = "bar" }
let c = C()

let rawPointer = Unmanaged.passUnretained(c).toOpaque()

let pointer = rawPointer.bindMemory(to: C.self, capacity: 1)
let pointee = pointer.pointee
print(pointee.foo) // EXC_BAD_ACCESS

Here's some LLDB output, which looks strange to me as everything seems
alright in pointer until I ask for its pointee:

(lldb) frame variable -L c
scalar: (memtest2.C) c = 0x0000000101d00030 {
0x0000000101d00040: foo = 42
0x0000000101d00048: bar = "bar"
}
(lldb) frame variable -L rawPointer
0x00000001005e2e08: (UnsafeMutableRawPointer) rawPointer = {
scalar: _rawValue = 0x0000000101d00030 {
0x0000000101d00040: foo = 42
0x0000000101d00048: bar = "bar"
}
}
(lldb) frame variable -L pointer
0x00000001005e2e10: (UnsafeMutablePointer<memtest2.C>)
                                            pointer = 0x0000000101d00030
(lldb) frame variable -L pointer._rawValue
scalar: (memtest2.C) pointer._rawValue = 0x0000000101d00030 {
0x0000000101d00040: foo = 42
0x0000000101d00048: bar = "bar"
}
(lldb) frame variable -L pointee
0x00000001005e2e18: (memtest2.C) pointee = 0x00000001005b65d8 {
0x00000001005b65e8: foo = 140736790071664
0x00000001005b65f0: bar = ""
}

I've also tried assumingMemoryBound(to:) or simply doing:

let pointer = UnsafePointer<C>(bitPattern: Int(bitPattern: rawPointer))!
print(pointer.pointee.foo) // EXC_BAD_ACCESS

But I always get this EXC_BAD_ACCESS error. What is going on here?


Thanks for your help,
Nick
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