Why Does Valgrind Report \"Conditional Jump or Move Depends on Uninitialized Value(s)\" and How Can I Track Its Origin?

Pinpointing "Conditional Jump or Move Depends on Uninitialized Value(s)" Valgrind Message

Valgrind's enigmatic "conditional jump or move depends on uninitialized value(s)" message can be a puzzling puzzle. While it identifies the point where an uninitialized value is utilized, it fails to reveal its origin.

To tackle this issue, you can leverage the valgrind option --track-origins=yes. This enables Valgrind to trace the genesis of uninitialized values but comes at the cost of increased memory usage and execution time.

Understanding the Reporting Behavior

It's crucial to note that Valgrind does not report uninitialized values immediately upon their first use. Instead, it waits until the data is employed in a way that could potentially affect the program's observable behavior. This is explained in the Valgrind manual:

It is important to understand that your program can copy around junk (uninitialized) data as much as it likes. Memcheck observes this and keeps track of the data, but does not complain. A complaint is issued only when your program attempts to make use of uninitialized data in a way that might affect your program's externally-visible behavior.

Additional Insights from Valgrind

The Valgrind FAQ further clarifies:

As for eager reporting of copies of uninitialised memory values, this has been suggested multiple times. Unfortunately, almost all programs legitimately copy uninitialised memory values around (because compilers pad structs to preserve alignment) and eager checking leads to hundreds of false positives. Therefore Memcheck does not support eager checking at this time.

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