Buffer Overflow Attack with Int

Question

char c[10];
int value = 1;

Why the value changes to 0 until I enter 12 chars? Why is 12 not 10 or 11? (I know the terminator and how it pushed to the next memory space)

Answer 1

Objects of a given type are typically placed at memory locations that are a multiple of the object size, ie the object is aligned to a boundary matching its size.

An int is typically 4 bytes in size. If c is placed before value in memory, and if c started at a 4 byte offset, then there will be two bytes of padding between them. This would explain why value doesn't get overwritten unless more than 12 bytes or more are written to c .

Note also that this is all undefined behavior as far as the C standard is concerned, however for the purpose of exploiting vulnerabilities it can be useful to examine what specific implementations do under certain conditions.