Why Does Valgrind Report \"Uninitialized Value\" Errors Only When the Value is Used?

Pinpointing the Source of Uninitialized Value Errors

Valgrind's "conditional jump or move depends on uninitialized value(s)" message can be a puzzling one, as it only indicates where the uninitialized value is used, not its origin. To address this issue, the following options are available:

• --track-origins=yes: This option enables Valgrind to track the origin of uninitialized values. While it slows down execution and requires more memory, it can be invaluable for identifying the source of such values.
• Understanding Valgrind's Reporting Behavior: Valgrind's manual clarifies that uninitialized data can be copied without triggering complaints until it is used in a manner that may affect the program's behavior. This explains why some uses of uninitialized values may not generate an error immediately.

The Valgrind FAQ further explains that eager reporting of all copies of uninitialized values has been dismissed due to the prevalence of false positives in legitimate programs.

Example

Consider the code snippet:

movespeed = stat.speedfactor * speedfac * currentbendfactor.val;

If speedfac is an uninitialized float, Valgrind may not report an error at this point. However, when the value is used for a printout, the error will be raised. This behavior reflects Valgrind's reporting policy outlined above.

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