[LLVMdev] On DataExtractor isValidOffset* interface (original) (raw)

Timur Iskhodzhanov timurrrr at google.com
Thu Oct 23 12:52:12 PDT 2014

Hi Ben,

Me and David were discussing ways to use DataExtractor recently and I found one thing in the comments that seems wrong.

From include/llvm/Support/DataExtractor.h: 336 /// Test the validity of \a offset. 337 /// 338 /// @return 339 /// \b true if \a offset is a valid offset into the data in this 340 /// object, \b false otherwise. 341 bool isValidOffset(uint32_t offset) const { return Data.size() > offset; } 342 343 /// Test the availability of \a length bytes of data from \a offset. 344 /// 345 /// @return 346 /// \b true if \a offset is a valid offset and there are \a 347 /// length bytes available at that offset, \b false otherwise. 348 bool isValidOffsetForDataOfSize(uint32_t offset, uint32_t length) const { 349 return offset + length >= offset && isValidOffset(offset + length - 1); 350 }

The "if \a offset is a valid offset" part of the comment for isValidOffsetForDataOfSize doesn't seem right, as it doesn't actually call isValidOffset(offset). Specifically, "isValidOffsetForDataOfSize(x, 0)" is equivalent to "isValidOffset(x - 1)" rather than "isValidOffset(x)", which is surprising (at least if you don't look at the implementation).

The other thing we've discussed is how to check DE for "everything we've read so far was at valid offsets". The first solution that comes to mind is "isValidOffset(NextByte - 1)"... but I wonder if there are any typical usage patterns that should be addressed in a more natural way. Like "hasEverReadPastEndOfBuffer()" flag maybe?

–– Thanks, Tim

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