Question

我有两个结构：

struct pcap_hdr_s {
    UInt32 magic_number;
    UInt16 version_major;
    UInt16 version_minor;
    int32_t thiszone;
    UInt32 sigfigs;
    UInt32 snaplen;
    UInt32 network;
};
//packet header
struct pcaprec_hdr_s {
    UInt32 ts_sec;
    UInt32 ts_usec;
    UInt32 incl_len;
    UInt32 orig_len;
};

初始化如下（例如）：

    let pcapHeader : pcap_hdr_s = pcap_hdr_s(magic_number: 0xa1b2c3d4,
                                            version_major: 2, 
                                            version_minor: 4, 
                                            thiszone: 0, 
                                            sigfigs: 0,
                                            snaplen: 
                                            pcap_record_size, 
                                            network: LINKTYPE_ETHERNET)

    let pcapRecHeader : pcaprec_hdr_s = pcaprec_hdr_s(ts_sec: UInt32(ts.tv_sec),
                                        ts_usec: UInt32(ts.tv_nsec), 
                                        incl_len: plen, 
                                        orig_len: length)

我尝试像这样创建结构的Data / NSData对象：

            //write pcap header
            let pcapHeaderData : NSData = NSData(bytes: pcapHeader, length: sizeofValue(pcapHeader))
            //write pcaprec header
            let pcapRecHeaderData : NSData = NSData(bytes: pcapRecHeader, length: sizeofValue(pcapRecHeader))

但我总是在每一行都收到此错误：

"Connot convert value if type 'pcap_hdr_s' to expected arguemnt type 'UsafeRawPointer?'"

我在Swift中查看了UnsafeRawPointers的文档，但是我现在还不能从结构中创建NSData对象。我是在正确的方式还是有更好的方式来实现我的意图？

如果此数据初始化可行，我接下来的步骤将是

将pcapRecHeaderData附加到pcapHeaderData
使用提供的Data / NSData函数以原子方式将pcapHeaderData写入文件/ url

编辑：

//packet ethernet header
struct ethernet_hdr_s {
    let dhost : [UInt8]
    let shost : [UInt8]
    let type : UInt16
};

let src_mac : [UInt8] = [0x66, 0x77, 0x88, 0x99, 0xAA, 0xBB]
let dest_mac : [UInt8] = [0x00, 0x11, 0x22, 0x33, 0x44, 0x55]

let ethernetHeader : ethernet_hdr_s = ethernet_hdr_s(dhost: dest_mac, shost: src_mac, type: 0x0800)

编辑2：

let payloadSize = packet.payload.count
            let plen = (payloadSize < Int(pcap_record_size) ? payloadSize : Int(pcap_record_size));

            bytesWritten = withUnsafePointer(to: &(packet.payload)) {
                $0.withMemoryRebound(to: UInt8.self, capacity: Int(plen)) {
                    ostream.write($0, maxLength: Int(plen))
                }
            }
            if bytesWritten != (Int(plen)) {
                // Could not write all bytes, report error ...
                NSLog("error in Writting packet payload, not all Bytes written: bytesWritten: %d|plen: %d", bytesWritten, Int(plen))
            }

Answer 1

您可以在不创建任何数据的情况下将任意数据写入InputStream 先(NS)Data个对象。 “挑战”是如何将指针转换为 UInt8方法所期望的write指针的结构：

let ostream = OutputStream(url: url, append: false)! // Add error checking here!
ostream.open()

var pcapHeader = pcap_hdr_s(...)
let headerSize = MemoryLayout.size(ofValue: pcapHeader)

let bytesWritten = withUnsafePointer(to: &pcapHeader) {
        $0.withMemoryRebound(to: UInt8.self, capacity: headerSize) {
        ostream.write($0, maxLength: headerSize)
    }
}
if bytesWritten != headerSize {
    // Could not write all bytes, report error ...
}

以同样的方式，您可以从InputStream中读取数据：

let istream = InputStream(url: url)! // Add error checking here!
istream.open()

let bytesRead = withUnsafeMutablePointer(to: &pcapHeader) {
    $0.withMemoryRebound(to: UInt8.self, capacity: headerSize) {
        istream.read($0, maxLength: headerSize)
    }
}
if bytesRead != headerSize {
    // Could not read all bytes, report error ...
}

如果文件可能是在另一个平台上创建的不同的字节顺序然后你可以检查“魔术”和交换字节如有必要（如https://wiki.wireshark.org/Development/LibpcapFileFormat所述）：

switch pcapHeader.magic_number {
case 0xa1b2c3d4:
    break // Already in host byte order
case 0xd4c3b2a1:
    pcapHeader.version_major = pcapHeader.version_major.byteSwapped
    pcapHeader.version_minor = pcapHeader.version_minor.byteSwapped
    // ...
default:
    // Unknown magic, report error ...
}

为了简化编写和读取结构的任务，可以定义自定义扩展方法，例如

extension OutputStream {

    enum ValueWriteError: Error {
        case incompleteWrite
        case unknownError
    }

    func write<T>(value: T) throws {
        var value = value
        let size = MemoryLayout.size(ofValue: value)
        let bytesWritten = withUnsafePointer(to: &value) {
            $0.withMemoryRebound(to: UInt8.self, capacity: size) {
                write($0, maxLength: size)
            }
        }
        if bytesWritten == -1 {
            throw streamError ?? ValueWriteError.unknownError
        } else if bytesWritten != size {
            throw ValueWriteError.incompleteWrite
        }
    }
} 

extension InputStream {

    enum ValueReadError: Error {
        case incompleteRead
        case unknownError
    }

    func read<T>(value: inout T) throws {
        let size = MemoryLayout.size(ofValue: value)
        let bytesRead = withUnsafeMutablePointer(to: &value) {
            $0.withMemoryRebound(to: UInt8.self, capacity: size) {
                read($0, maxLength: size)
            }
        }
        if bytesRead == -1 {
            throw streamError ?? ValueReadError.unknownError
        } else if bytesRead != size {
            throw ValueReadError.incompleteRead
        }
    }
}

现在您可以使用

进行简单的书写和阅读

try ostream.write(value: pcapHeader)
try istream.read(value: &pcapHeader)

当然，这只适用于像你这样的“自包含”结构 pcap_hdr_s和pcaprec_hdr_s。

Answer 2

您可以使用

在Swift 3中将pcap_hdr_s转换为Data，反之亦然

pcap_hdr_s - ＆gt; Data

var pcapHeader : pcap_hdr_s = pcap_hdr_s(magic_number ...

let data = withUnsafePointer(to: &pcapHeader) {
     Data(bytes: UnsafePointer($0), count: MemoryLayout.size(ofValue: pcapHeader))
}

Data - ＆gt; pcap_hdr_s

let header: pcap_hdr_s = data.withUnsafeBytes { $0.pointee }

参考：round trip Swift number types to/from Data

将结构写入文件的最佳方法是什么？

2 个答案: