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Swift中的字节数组指针作用域

swift c

Nick · 技术社区 · 4 年前

我正在调用库中的一个C函数,它需要指向 uint8_t

import Foundation

struct CStruct {
    let val: UnsafeMutablePointer<UInt8>
    let count: UInt32
}

var structs = [CStruct]()
var datas = [[UInt8]]()

for _ in 0..<10 {
    // In the real application, Data contains data rather than being zero length
    let d = Data()
    var data = [UInt8](d)
    // I don't want the `data` buffers to be deallocated, so I store them in an array.
    datas.append(data)
    structs.append(CStruct(val: &data, count: UInt32(data.count)))
}

print("Structs: \(structs)")
// pass the structs to a C function which processes them

这就发出了警告

Inout表达式创建一个临时指针,但参数“val”应该是一个比调用“init(val:count:)”还长的指针

我不能用 data.withUnsafeMutableBufferPointer 因为它提供的指针超出了闭包的范围。我不能用 Unmanaged Array 是一个结构。我也不确定 data 数组到父作用域中的数组将延长其生存期(因为它是一个结构)。正确的方法是什么?

编辑:

import Foundation

struct CStruct {
    let val: UnsafeMutablePointer<UInt8>
    let count: UInt32
}

var structs = [CStruct]()
var dataStorage: [UnsafeMutablePointer<UInt8>] = []

for _ in 0..<10 {
    // In the real application, Data contains data rather than being zero length
    let d = Data()
    let ptr = UnsafeMutablePointer<UInt8>.allocate(capacity: d.count)
    d.withUnsafeBytes { (buff) -> Void in
        ptr.initialize(from: buff.bindMemory(to: UInt8.self).baseAddress!, count: d.count)
    }
    dataStorage.append(ptr)
    structs.append(CStruct(val: ptr, count: UInt32(d.count)))
}
defer {
    for ptr in dataStorage {
        ptr.deallocate()
    }
    dataStorage = []
}

print("Structs: \(structs)")
// pass the structs to a C function which processes them

也许有更好的方法来复制 Data ?

1 回复 | 直到 4 年前

bscothern 4 年前

有很多方法可以解决这个问题,但根本原因是需要分配缓冲区并将其传入。然后你需要有一个方法知道你什么时候清理它,这样你就不会泄漏内存。

解决这个问题的一个相当简单的方法是使用类类型来管理 CStruct

import Foundation

struct CStruct {
    let val: UnsafeMutablePointer<UInt8>
    let count: UInt32
}

final class CStructLifetimeManager {
    private(set) var cStruct: CStruct // This needs to be mutable so it can be deallocated later on.

    private init(count: Int) {
        var val = UnsafeMutablePointer<UInt8>.allocate(capacity: count)
        for i in 0..<10 {
            val[I] = 0
        }
        self.cStruct = CStruct(val: val, count: UInt32(count))
    }

    deinit {
        cStruct.val.deinitialize(count: 10) // Not needed if you really just have UInt8 types since they are trivial
        cStruct.val.deallocate()
    }

    static func createStruct(count: Int) -> CStructLifetimeManager {
        CStructLifetimeManager(count: count)
    }
}

var structs = [CStruct]()
var structLifetimeManagers = [CStructLifetimeManager]() // This now keeps the manager objects alive

for _ in 0..<10 {
    let cStructLifetimeManager = CStructLifetimeManager(count: 10)
    structs.append(cStructLifetimeManager.cStruct)
    structLifetimeManagers.append(cStructLifetimeManager)
}

print("Structs: \(structs)")

这种方法的问题是 structs 和 structLifetimeManagers 照搬 结构 结构生命时间管理器 一个 结构

所以最好删除第二个数组,让包装器对象管理生命周期。你只需要 结构 需要的时候。如果这是不可能的,你只需要非常小心,以确保事情没有超出范围之前,他们应该。