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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 practicethe common case, on architectures that make use of a program stack, this value defaults to whichever values are currently stored in stack memory. While uninitialized memory often contains zeroes, 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 and again assuming that the architecture makes use of a program stack, it uses whatever value is at that location in the program stack. This may lead to unexpected or otherwise incorrect program behavior.
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In this non-compliant code example, the programmer mistakenly fails to set the local variable {{error_log}} to the {{msg}} argument in the {{report_error()}} function \[[mercy 06|AA. C References#mercy 06]\]. Because {{error_log}} has not been initialized, on architectures making use of a program stack, it assumes the value already on the stack at this location, which is a pointer to the stack memory allocated to the {{password}} array. The {{sprintf()}} call copies data in {{password}} until a NULL byte is reached. If the length of the string stored in the {{password}} array is greater than the size of the {{buffer}} array, then a buffer overflow occurs. |
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