Memory Sanitizer

Question

I am playing around with Memory Sanitizer with Clang 3.7.0 on Ubuntu 14.04. The following code does work perfectly:

#include <cstdio>

int main() {
  double ans;
  printf("Hello World: %f\n", ans);

  return 0;
}

when compiled with

 clang++ -g -O1 -fsanitize=memory -fsanitize-memory-track-origins=2 -fomit-frame-pointer sanitize.cpp -o sanitize

I was expecting an error. Doesn't Memory Sanitizer catch the fact that ans was not initialized?

Thanks for your help.

Answer 1

From the clang santitizer documentation it is clear that it only deals with unitialized memory reads from dynamically allocated memory. Automatic memory is not part of sanitizer checks.

Answer 2

Valgrind memcheck could be an option to detect the uninitialized stack values.

Valgrind documentation:

For uninitialised values originating from a heap block, Memcheck shows where the block was allocated. For uninitialised values originating from a stack allocation, Memcheck can tell you which function allocated the value, but no more than that -- typically it shows you the source location of the opening brace of the function. So you should carefully check that all of the function's local variables are initialised properly.

Reference: http://valgrind.org/docs/manual/mc-manual.html

Answer 3

You don't need any Sanitizer to catch this error. The compiler can figure out this error in compile time (sanitizers and valgrind work at run time). In fact, all of GCC Clang and ICC will all give a warning for this code if you switch on the warnings. This particular warning is controlled with -Wuninitialized flag. In general, it is a good practice to always use high warning level. I would recommend the following combination of warning flags, especially while learning the language:

-Wall -Wextra -pedantic

If you get some false positives, only after rigorously checking that they are really false, you can disable specific warnings. There is no reason not to use warning flags. Some projects even use -Werror flag, turning all the warnings into errors.