swift - 在 Swift 3 中使用 withMemoryRebound 的问题

Question

我有一个以前使用 UnsafeMutablePointer 调用 C 函数的应用程序，如下所示:

    var size     = HOST_BASIC_INFO_COUNT
    let hostInfo = host_basic_info_t.allocate(capacity: 1)
    let result = host_info(machHost, HOST_BASIC_INFO,
                                     UnsafeMutablePointer(hostInfo), &size)

自从迁移到 Swift 3、Xcode Beta 6 后，系统会提示您使用 withMemoryRebound。问题是我不明白在这种情况下如何使用它，而且还没有它的文档或示例代码。我的做法:

    var size     = HOST_BASIC_INFO_COUNT
    let hostInfo = host_basic_info_t.allocate(capacity: 1)
    let temp = hostInfo.withMemoryRebound(to: host_info_t!.self, capacity: Int(size)) {
        UnsafeBufferPointer(start: $0, count: Int(size))
    }
    let result = host_statistics(machHost,
                                 HOST_BASIC_INFO,
                                 temp.baseAddress?.pointee,
                                 &size)

简单地因 Bad Access 而崩溃。 withMemoryRebound 的正确使用方法是什么？

Answer 1

hostInfo是 UnsafeMutablePointer<host_basic_info> ，那个指针必须“反弹”到指向 HOST_BASIC_INFO_COUNT 的指针项目的 integer_t ，正如 hostInfo() 所预期的那样功能:

let HOST_BASIC_INFO_COUNT = MemoryLayout<host_basic_info>.stride/MemoryLayout<integer_t>.stride
var size = mach_msg_type_number_t(HOST_BASIC_INFO_COUNT)
let hostInfo = host_basic_info_t.allocate(capacity: 1)
let result = hostInfo.withMemoryRebound(to: integer_t.self, capacity: HOST_BASIC_INFO_COUNT) {
    host_info(mach_host_self(), HOST_BASIC_INFO, $0, &size)
}

print(result, hostInfo.pointee)
hostInfo.deallocate(capacity: 1)

而不是 allocate/deallocate你也可以使用本地变量并传递其地址:

let HOST_BASIC_INFO_COUNT = MemoryLayout<host_basic_info>.stride/MemoryLayout<integer_t>.stride
var size = mach_msg_type_number_t(HOST_BASIC_INFO_COUNT)
var hostInfo = host_basic_info()

let result = withUnsafeMutablePointer(to: &hostInfo) {
    $0.withMemoryRebound(to: integer_t.self, capacity: Int(size)) {
        host_info(mach_host_self(), Int32(HOST_BASIC_INFO), $0, &size)
    }
}
print(result, hostInfo)