C++ 扇区对齐读取 [英] C++ sector aligned read

问题描述

我问这个是因为我在读取原始设备时无法理解扇区对齐读取.

Im asking this because i am having trouble understanding sector aligned reading when we are reading raw device.

假设在 Windows 机器中，我们使用 ReadFile() C 函数从设备读取 x 个字节.

Lets assume whe are in a Windows machined, and we are using the ReadFile() C function to read x bytes from a device.

我知道我们只能读取扇区对齐的数据，但最近我发现了 SetFilePointer() 函数，它允许我们将指针放在我们之前用 CreateFile().

I know we can only read sector aligned data, but recently i discovered the SetFilePointer() function, that allow us to put a pointer in x bytes of the device we have previously opened with CreateFile().

我的问题是，如果我们需要读取扇区对齐的数据，如果我们使用 SetFilePointer() 例如像这样:

My question is, if we need to read sector aligned data, if we use SetFilePointer() for example like this:

SetFilePointer(device, 12, NULL, FILE_BEGIN);

(设备是现有设备的 HANDLE，为了这个例子，我们假设它是一个 USB 笔式驱动器)，在那个例子中，我们设置了一个指向第 12 个字节的指针，从FILE_BEGIN.

(device is a HANDLE to an existing device,for the sake of this example lets assume its a USB pen drive), in that example we set a pointer thatis pointing to the 12th byte starting from FILE_BEGIN.

如果我要从第 12 个字节开始读取相当于一个扇区(512 个字节)的内容，我是否需要像这样设置读取功能:

If i were to read the equivalent of one sector (512 bytes) starting from that 12th byte, would i need to make my read fucntion like this:

ReadFile(device, sector, (512 - 12), &bytesRead, NULL)

或者像这样:

ReadFile(device, sector, 512, &bytesRead, NULL)

不管怎样，谢谢！

推荐答案

我的问题是，如果我们需要读取扇区对齐的数据，如果我们使用 SetFilePointer() 例如像这样:

SetFilePointer(device, 12, NULL, FILE_BEGIN);

... 那么您将不再读取扇区对齐的数据，并且您将在 ReadFile 调用中收到错误 87.读取扇区对齐的数据不仅意味着您必须读取扇区大小的块，而且您必须始终读取从扇区边界开始的块.

... then you are no longer reading sector-aligned data, and you'll get error 87 in the ReadFile call. Reading sector-aligned data doesn't just mean that you have to read in sector-sized blocks, but you must always read blocks that start on sector boundaries.

您必须寻找包含您感兴趣的字节的扇区(因此，position/sector_size*sector_size)，读取整个扇区并从您读取的数据中提取您感兴趣的字节.

You have to seek to the sector containing the bytes of your interest (so, position/sector_size*sector_size), read the whole sector and extract the bytes of your interest from the data you read.

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