Why does the documentation for ReadFile say that the lpNumberOfBytesRead parameter is optional when it is sometimes mandatory?

Raymond Chen

The documentation for the Read­File function says

  _In_ HANDLE hFile,
  _Out_ LPVOID lpBuffer,
  _In_ DWORD nNumberOfBytesToRead,
  _Out_opt_ LPDWORD lpNumberOfBytesRead,
  _Inout_opt_ LPOVERLAPPED lpOverlapped

The lpNumber­Of­Bytes­Read parameter is declared as _Out_opt_. The _Out_ part means that the function writes to the pointed-to value. The _opt_ part means that the parameter is optional (may be null). And yet, if you call Read­File and pass nullptr for the fourth parameter, it crashes. What’s going on here?

What’s going on is in the fine print:

lpNumberOfBytesRead [out, optional]

A pointer to the variable that receives the number of bytes read when using a synchronous hFile parameter. Read­File sets this value to zero before doing any work or error checking. Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.

This parameter can be NULL only when the lpOverlapped parameter is not NULL.

For more information, see the Remarks section.

Note that second paragraph.

The deal is that the fourth parameter is sometimes optional and sometimes mandatory. The simplified annotation language used by this function prototype cannot express this sort of conditional mandatory state, so it takes the most generous position of declaring the parameter as optional, so as to avoid raising false positives in static analysis tools.

A more precise annotation for that parameter would be something like _When_(lpOverlapped == NULL, _Out_opt_) _When_(lpOverlapped != NULL, _Out_).


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