Comment by jandrese
1 day ago
> Not ignore the compilation warnings – this code most likely threw a warning in the original code that was either ignored or disabled!
What compiler error would you expect here? Maybe not checking the return value from scanf to make sure it matches the number of parameters? Otherwise this seems like a data file error that the compiler would have no clue about.
Trying g++ version 11.4, there's no warning by default if you don't check the return value of sscanf. Even `g++ -Wall -Wextra -Wunused-result` produces no warnings for a small example.
Undefined behavior to access the uninitialized memory. A sanitizer would have flagged that.
The compiler has no way of knowing that the memory would be undefined, not unless it somehow can verify the data file. The most I think it can do is flag the program for not checking the return value of scanf, but even that is unlikely to be true since the program probably was checking for end of file which is also in the return value. It was failing to check the number of matched parameters. This is the kind of error that is easy to miss given the semantics of scanf.
> The compiler has no way of knowing that the memory would be undefined
Yes it would. -fsanitize=address does a bunch of instrumentation - it allocates shadow memory to keep track of what main memory is defined, and it checks every read and write address against the shadow memory. It is a combination of compile-time instrumentation and run-time checking. And yes, it is expensive, so it should be used for debugging and not the final release.
https://clang.llvm.org/docs/AddressSanitizer.html , https://learn.microsoft.com/en-us/cpp/sanitizers/asan?view=m...
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Uninitialized variables are a really common case.
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Yeah, the debugging here is great, but the actual cause is super mild.
Good point. When reading, I kind of just assumed the "use of initialised memory" warning would pick this up.
But because the whole line is parsed in a single sscanf call, the compiler's static analysis is forced to assume they have now initialised. There doesn't seem to be any generic static analysis approach that can catch this bug.
Though... you could make a specialised warning just for scanf that forced you to either pass in pre-initilized values or check the return result.