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I'm pretty new to this so any clarification would be appreciated. When using the function ReadFile, how does the type of the lpBuffer interact with the parameter of "number of bytes to read"?

For instance what if you had an unsigned short MyShort[5] as lpBuffer, and then you set bytes to read as 2. Will all data be stored in MyShort[0]? Or would the first byte go into MyShort[0] and the second byte go into MyShort[1]? What happens when you set bytes to read is increase say to 9? Will 16bits go into MyShort[0] and then 16 more into MyShort[1] etc...?

Thanks

MrEricSir
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TMa
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  • The requested number of bytes is sequentially stored in the buffer. The [documentation](https://msdn.microsoft.com/en-us/library/windows/desktop/aa365467(v=vs.85).aspx) is also pretty clear about that. –  Feb 13 '18 at 17:18
  • The *lpBuffer* argument is of type `void*`. The `ReadFile` call cannot know the type you passed, so how should it respond to different types? – IInspectable Feb 14 '18 at 09:21
  • @IInspectable couldn't lpBuffer be a type char or an int? I was just trying to figure out the interaction of 8 bits verses 16bits when 1 or 2 bytes are specified for number of bytes to read. – TMa Feb 14 '18 at 14:58
  • It doesn't matter, what type *your* buffer is. Once you pass it into `ReadFile`, all type information is lost. The [function signature](https://msdn.microsoft.com/en-us/library/windows/desktop/aa365467.aspx) wants it that way. – IInspectable Feb 14 '18 at 23:18

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lpBuffer is always treated as a pointer to an array of specified amount of bytes (nNumberOfBytesToRead). The amount of bytes actually read will be stored in the variable pointed to by lpNumberOfBytesRead parameter or as async (overlapped) result later. So in your case if you request to read 2 bytes it may either read two bytes storing both of them in MyShort[0], or just a single byte stored in lower half of MyShort[0] or nothing at all. If you request to read 9 bytes then it will ready up to 9 bytes storing 2 + 2 + 2 + 2 + 1 bytes sequentially.

user7860670
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