我有一台服务器通过 TCP 向我发送消息，其中前 4 个字节决定消息其余部分的长度。所以我需要

1) 将 4 个字节读入 UInt32(有效)并将其存储到 bytes_expected

2) 将 bytes_expected 字节读入消息

现在我的代码是这样的:

private let inputStreamAccessQueue  = DispatchQueue(label: "SynchronizedInputStreamAccess")

func inputStreamHandler(_ event: Stream.Event) {
    switch event {
        case Stream.Event.hasBytesAvailable:
            self.handleInput()

        ...
    }
}

func handleInput() {
    // **QUESTION: Do I use this barrier wrong?**
    self.inputStreamAccessQueue.sync(flags: .barrier) {            
        guard let istr = self.inputStream else {
            log.error(self.buildLogMessage("InputStream is nil"))
            return
        }

        guard istr.hasBytesAvailable else {
            log.error(self.buildLogMessage("handleInput() called when inputstream has no bytes available"))
            return
        }

        let lengthbuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: 4)
        defer { lengthbuffer.deallocate(capacity: 4) }
        let lenbytes_read = istr.read(lengthbuffer, maxLength: 4)

        guard lenbytes_read == 4 else {
            self.errorHandler(NetworkingError.InputError("Input Stream received \(lenbytes_read) (!=4) bytes"))
            return
        }

        let bytes_expected = Int(UnsafeRawPointer(lengthbuffer).load(as: UInt32.self).bigEndian)
        log.info(self.buildLogMessage("expect \(bytes_expected) bytes"))

        print("::DEBUG", call, "bytes_expected", bytes_expected)

        var message = ""
        var bytes_missing = bytes_expected
        while bytes_missing > 0 {
            //print("::DEBUG", call, "bytes_missing", bytes_missing)
            let buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: bytes_missing)
            let bytes_read = istr.read(buffer, maxLength: bytes_missing)

            print("::DEBUG", call, "bytes_read", bytes_read)

            guard bytes_read > 0 else {
                print("bytes_read not > 0: \(bytes_read)")
                return
            }

            guard bytes_read <= bytes_missing else {
                print("Read more bytes than expected. missing=\(bytes_missing), read=\(bytes_read)")
                return
            }

            guard let partial_message = String(bytesNoCopy: buffer, length: bytes_read, encoding: .utf8, freeWhenDone: true) else {
                log.error("ERROR WHEN READING")
                return
            }

            message = message + partial_message
            bytes_missing -= bytes_read
        }

        self.handleMessage(message)
    }
}

我的问题是 istr.read(buffer, maxLength: bytes_missing) 有时不会一次读取所有消息，所以我循环直到我读完所有我想要的。但我仍然看到我的应用程序崩溃(很少)，因为 handleInput() 被再次调用，而对该方法的另一个调用仍在运行。在这种情况下，bytes_expected 包含随机值，应用程序由于非法内存分配而崩溃。

我想我可以通过使用屏障来避免这种情况。但这似乎行不通……我是不是用错了屏障？

最佳答案

我的建议是不要反对网络 I/O 的异步性质。 每当 Stream.Event.hasBytesAvailable 事件发生时，读取并收集缓冲区中的数据 发出信号。如果缓冲区包含足够的数据(4 个长度字节加上 预期的消息长度)然后处理数据并将其删除。否则什么也不做 并等待更多数据。

以下(未经测试的)代码仅作为演示。 它仅显示与该特定问题相关的部分。 为简洁起见，省略了初始化、事件处理程序等。

class MessageReader {

    var buffer = Data(count: 1024) // Must be large enough for largest message + 4
    var bytesRead = 0 // Number of bytes read so far

    // Called from `handleInput` with a complete message.
    func processMessage(message: Data) {
        // ...
    }

    // Called from event handler if `Stream.Event.hasBytesAvailable` is signalled.
    func handleInput(istr: InputStream) {
        assert(bytesRead < buffer.count)

        // Read from input stream, appending to previously read data:
        let maxRead = buffer.count - bytesRead
        let amount = buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
            istr.read(p + bytesRead, maxLength: maxRead)
        }
        guard amount > 0 else {
            // handle EOF or read error ...
            fatalError()
        }
        bytesRead += amount

        while bytesRead >= 4 {
            // Read message size:
            let messageSize = buffer.withUnsafeBytes { (p: UnsafePointer<UInt32>) in
                Int(UInt32(bigEndian: p.pointee))
            }
            let totalSize = 4 + messageSize
            guard totalSize <= buffer.count else {
                // Handle buffer too small for message situation ...
                fatalError()
            }

            if bytesRead < totalSize {
                break // Not enough data to read message.
            }

            // Buffer contains complete message now. Process it ...
            processMessage(message: buffer[4 ..< totalSize])

            // ... and remove it from the buffer:
            if totalSize < bytesRead {
                // Move remaining data to the front:
                buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
                    _ = memmove(p, p + totalSize, bytesRead - totalSize)
                }
            }
            bytesRead -= totalSize
        }
    }
}

关于ios - 在 Swift 4 中从 InputStream 中准确读取 n 个字节，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48340728/