Uninitialized Variable

In computing, an uninitialized variable is a variable that is declared but is not set to a definite known value before it is used. It will have some value, but not a predictable one. As such, it is a programming error and a common source of bugs in software.

A common assumption made by novice programmers is that all variables are set to a known value, such as zero, when they are declared. While this is true for many languages, it is not true for all of them, and so the potential for error is there. Languages such as C use stack space for variables, and the collection of variables allocated for a subroutine is known as a stack frame. While the computer will set aside the appropriate amount of space for the stack frame, it usually does so simply by adjusting the value of the stack pointer, and does not set the memory itself to any new state (typically out of efficiency concerns). Therefore, whatever contents of that memory at the time will appear as initial values of the variables which occupy those addresses.

Here's a simple example in C:

What is the final value of k? The fact is, it is impossible to tell. The answer that it must be 10 assumes that it started at zero, which may or may not be true. Note that in the example, the variable i is initialized to zero by the first clause of the for statement.

In C, variables with static storage duration that are not initialized explicitly are initialized to zero (or null, for pointers).

Not only are uninitialized variables a frequent cause of bugs, but this kind of bug is particularly serious because it may not be reproducible; in some cases, programs with uninitialized variables may even pass software tests.