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Local, automatic variables can assume _unexpected_ values if they are used before they are initialized. The current C specification states "If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate." \[ISO/IEC 9899, 6.7.8 Initialization\] In practice, this value is what ever is on the stack. This may cause variables to take on unintended values. Consequently, a program this can cause a program to behave in an unpredictable or unplanned manner, and may provide an avenue for attack. Some compilers warn about unitialized variables, but the warning is inconsistent and can be ignored by the programmer. As a result, it is necessary to guarantee that all local variables are initialized with a default value. The value assigned should be documented as the "default value" for that variable in the comments associated with that variable's declaration. |
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In this example, two functions are called one after another. The first function, func1(...) is passed an integer entered by a user. That integer is stored in variable: i for the duration of the function. The second function func2() declares a local integer variable: j. j is not initialized before being checked against a constant value, CONDITION_CHECK. Since j was not initialized, it assumes whatever value is at that location in the stack, in this case the value of i from func1(). Thus is if the user entered 42, the condition the statement if (j == CONDITION_CHECK) succeeds.
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