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Local, automatic variables can assume unexpected values if they are used before they are initialized. C99 specifies "If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate" \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]. In practice, this value defaults to whichever values are currently stored in stack memory. While uninitialized memory often contains zerozeroes, this is not guaranteed. Consequently, uninitialized memory can cause a program to behave in an unpredictable or unplanned manner and may provide an avenue for attack. |
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In this non-compliant code example, the set_flag() function is intended to set the variable sign to 1 if number is positive and -1 if number is negative. However, the programmer neglected to account for number being 0. If number is 0, then sign remains uninitialized. Because sign is uninitialized, it assumes whatever value is at that location in the program stack. This may lead to unexpected , or otherwise incorrect program behavior.
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