Page History

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| BB. Definitions#indeterminate value]" \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] (see also [undefined behavior 10 | CC. Undefined Behavior#ub_10] of Annex J). In the common case, on architectures [implementations| BB. Definitions#implementation] 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. On architecturesimplementations that include [trap representations|BB. Definitions#trap representation], reading an uninitialized object of any type other than {{unsigned char}} (i.e., including {{int}}) may trigger a trap (see [undefined behavior 11 | CC. Undefined Behavior#ub_11] of Annex J). Consequently, uninitialized memory can cause a program to behave in an unpredictable or unplanned manner, lead to [undefined behavior | BB. Definitions#undefined behavior], and may provide an avenue for attack.