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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 zero, this is not guaranteed. Consequently, unitializeduninitialized memory can cause a program to behave in an unpredictable or unplanned manner and may provide an avenue for attack. Most compilers warn about uninitialized variables, but these can be ignored by the programmer.