Chapter 16: C and the Memory Model

Vol 3: Computer Core Expedition · Chapter 16

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Metadata Card

Attribute	Value
Keywords	Pointers, Stack vs Heap, Buffer Overflow, Undefined Behavior, Memory Safety

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Your Progress

"C gives you unparalleled control over memory — and unparalleled ability to shoot yourself in the foot."

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Encounter 1: Stack vs Heap

	Stack	Heap
Allocation	Automatic (function calls)	Manual (malloc, free)
Size	Small (MB)	Large (GB)
Lifetime	Function scope	Until freed
Speed	Very fast	Slower
Fragmentation	None	External fragmentation

Encounter 2: Pointers and Arrays

int arr[10];     // arr is a pointer to the first element
int *p = arr;    // p points to arr[0]
*(p + 3) = 42;   // same as arr[3] = 42

Encounter 3: Buffer Overflow

Writing beyond the bounds of an array or buffer. Classic vulnerability:

void vulnerable(char *input) {
    char buf[64];
    strcpy(buf, input);  // No bounds check!
}

If input is longer than 63 characters, it overwrites the return address on the stack — enabling code injection attacks.

Encounter 4: Undefined Behavior (UB)

In C, certain operations have UNDEFINED behavior:

• Signed integer overflow
• Use-after-free
• Buffer overflow
• Dereferencing NULL
• Data race

The compiler may assume UB never happens and generate unexpected code.

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Verification Checklist

• [ ] Can explain stack vs heap allocation
• [ ] Can identify a buffer overflow vulnerability
• [ ] Can list common C undefined behaviors
• [ ] Can explain what happens during a function call (stack frame)

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Chapter 17: Assembly Basics and Calling Conventions