How to check for errors from SetFilePointer

Raymond Chen

The SetFilePointer function reports an error in two different ways, depending on whether you passed NULL as the lpDistanceToMoveHigh parameter. The documentation in MSDN is correct, but I’ve discovered that people prefer when I restate the same facts in a different way, so here comes the tabular version of the documentation.

I’d show some sample code, but the documentation in MSDN already contains sample code both for the lpDistancetoMoveHigh == NULL case as well as the lpDistancetoMoveHigh != NULL case. A common mistake is calling GetLastError even if the return value is not INVALID_SET_FILE_POINTER. In other words, people ignore the whole retVal == INVALID_SET_FILE_POINTER part of the “did the function succeed or fail?” test. Just because GetLastError() returned an error code doesn’t mean that the SetFilePointer function failed. The return value must also have been INVALID_SET_FILE_POINTER. I will admit that the documentation in MSDN could be clearer on this point, but the sample code hopefully resolves any lingering ambiguity. But why does SetFilePointer use such a wacky way of reporting errors when lpDistanceToMoveHigh is non-NULL? The MSDN documentation also explains this detail: If the file size is greater than 4GB, then INVALID_SET_FILE_POINTER is a valid value for the low-order 32 bits of the file position. For example, if you moved the pointer to position 0x00000001`FFFFFFFF, then *lpDistanceToMoveHigh will be set to the high-order 32 bits of the result (1), and the return value is the low-order 32 bits of the result (0xFFFFFFFF, which happens to be the numerical value of INVALID_SET_FILE_POINTER). In that case (and only in that case) does the system need to use SetLastError(ERROR_SUCCESS) to tell you, “No, that value is perfectly fine. It’s just a coincidence that it happens to be equal to INVALID_SET_FILE_POINTER“. Why not call SetLastError(ERROR_SUCCESS) on all success paths, and not just the ones where the low-order 32 bits of the result happen to be 0xFFFFFFFF? That’s just a general convention of Win32: If a function succeeds, it is not required to call SetLastError(ERROR_SUCESS). The success return value tells you that the function succeeded. The exception to this convention is if the return value is ambiguous, as we have here when the low-order 32 bits of the result happen to be 0xFFFFFFFF. You might argue that this was a stupid convention, But what’s done is done and until time travel has been perfected, you just have to live with the past. (Mind you, UNIX uses the same convention with the errno variable. Only if the previous function call failed is the value of errno defined.) Looking back on it, the designers of SetFilePointer were being a bit too clever. They tried to merge 32-bit and 64-bit file management into a single function. “It’s generic!” The problem with this is that you have to check for errors in two different ways depending on whether you were using the 32-bit variation or the 64-bit variation. Fortunately, the kernel folks realized that their cleverness backfired and they came up with a new function, SetFilePointerEx. That function produces a 64-bit value directly, and the return value is a simple BOOL, which makes checking for success or failure a snap.

Exercise: What’s the deal with the GetFileSize function?

Raymond Chen

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