Tutorial :Are there known false positives issues with Valgrind?


Are there any known false positives with Valgrind? I get a 'Conditional jump or move depends on uninitialised value(s)' with the fmemopen function, writing in C and compiling with GCC. Can I be sure it's real?

EDIT: Are there known issues that are not in the suppression files? Are there some things one can do in a program, that are not really errors but Valgrind will say they are? If there are known issues, a list would be nice.


Yes, there are false positives with Valgrind, that's why it has suppression files for particular glibc and gcc versions, for example. The false positives may arise if you are using older valgrind with newer gcc and glibc, i.e., valgrind 3.3 with glibc 2.9.

Having said that, you still have to look into issue and find out if it is really a false positive (if that turns out to be the case, you can write a suppression for it yourself) or is it a real bug in your program.

There is no quick and easy way to say what is going on here, but in this case I'd suspect that you are passing uninitialized value from your code to library code. Try Valgrind option --track-origins=yes. It will show where the uninitialized value came from. If it is your code, probably you should initialize it. If it's inside library, it could be the false positive or, still, bad values of library call arguments might be causing it, so check those.


Valgrind comes with some default error suppression, but they are by no means covering all libraries.

The error-checking tools detect numerous problems in the base libraries, such as the GNU C library, and the X11 client libraries, which come pre-installed on your GNU/Linux system. You can't easily fix these, but you don't want to see these errors (and yes, there are many!) So Valgrind reads a list of errors to suppress at startup. A default suppression file is created by the ./configure script when the system is built.

You can create your own error suppressions that you know are irrelevant to your code.


Wasn't the Debian SSL thing motivated by fixing some false positives with Valgrind?

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