Buffer overflows, undefined and implementation-defined behavior, integer overflow, sequence points, aliasing, and debugging tools such as gdb and valgrind.
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- What is undefined behavior in C?
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Program behavior that is not defined by the C standard; the program may crash, produce incorrect output, or behave in any way whatsoever.- How can undefined behavior manifest at runtime?
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The program may crash, produce wrong results, or appear to work correctly. The standard does not constrain the outcome.- What is implementation-defined behavior in C?
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Behavior not specified by the standard, but a conforming compiler must document how it handles the situation.- Is a C compiler required to warn about undefined behavior?
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No. The compiler may silently permit undefined behavior. Avoiding it is the programmer's responsibility.- Name three common sources of undefined behavior in C.
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Buffer overflow, integer overflow, use-after-free, null pointer dereference, uninitialized variables, accessing out-of-bounds memory, or violating the strict aliasing rule.- What is a buffer overflow?
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Writing data beyond the boundary of an allocated buffer, corrupting adjacent memory.- Why is the gets() function dangerous?
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It reads input into a buffer without size checking, making buffer overflow inevitable when input exceeds the buffer size.- What is the vulnerability of strcpy()?
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It copies a string without checking whether the destination buffer is large enough, allowing buffer overflow.- What is an off-by-one error?
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An error in loop bounds or array indexing that causes access to one element beyond the intended range, often from confusion about inclusive vs exclusive bounds.- Can a stack buffer overflow corrupt heap memory?
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No. Stack and heap are separate regions. A stack overflow corrupts adjacent stack memory such as return addresses or saved registers.- What happens when a signed integer overflows in C?
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Undefined behavior. The result may wrap around, saturate, trap, or behave unpredictably depending on the compiler and platform.- How does unsigned integer overflow differ from signed?
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Unsigned overflow wraps modulo 2^n and is well-defined. Signed overflow is undefined behavior.- How can you detect integer overflow before it occurs?
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Check operand bounds before the operation. For addition, verify that a <= INT_MAX - b before computing a + b.- What is integer underflow?
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The result of an operation is smaller than the minimum representable value. For unsigned types it wraps; for signed types it is undefined.- Name one defense against integer overflow.
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Check operands before the operation, use wider types for intermediate results, or use safe arithmetic libraries.
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