sanitisers-guide

use-of-uninitialized-value

Hopefully it’s clear that use-of-uninitialized-value occurs when you try to use uninitialised values.

This can happen when you declare a variable/allocate memory but never assign a value to it before using it for something.

Note: This error only appears if compiled with MSan (which is usually disabled in favour of ASan for most tasks). See how to switch here.

Reading the error message

This is a typical use-of-uninitialized-value message:

Here is the useful information you can get from this error message:

Error Location

The first stack trace describes where (which line/function/file) the error occurred.

Origin of Uninitialised Value

The other stack traces tell you where the uninitialised value originated.

Note: Origins are only shown if compiled with -fsanitize-memory-track-origins

Created by…

This stack trace shows where (which line/function/file) the memory/variable was initially created.

If this line says created by a heap allocation, then the stack trace points to a specific heap memory allocation (i.e. a call to malloc() or calloc()) that was never initialised.

If this line says created by an allocation of 'var' in the stack frame of function 'func', then the stack trace will only point to the line that the function func is on. However, this simply means that some local variable var inside of the function func was not initialised.

Stored to memory at…

There may be one or more stack traces that track each place (line/function/file) the uninitialised value was stored into a variable before it was finally used.