Pointers and Memory

systems memory Pointers are variables that store memory addresses rather than values. They are the primary tool for Systems Programming in C, allowing direct manipulation of memory, arrays, and hardware.

Basic Syntax

• int *p; : Declares a pointer p that points to an integer.
• &x : Address-of Operator. Returns the memory address of variable x.
• *p : Dereference Operator. Accesses the value stored at the address p.

int x = 10;
int *p = &x;  // p holds the address of x
int y = *p;   // y gets the value 10 (dereferencing p)
*p = 20;      // x is now 20

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Pointer Arithmetic

Adding 1 to a pointer increases the address by sizeof(type).

• char *c = 0x1000; c++; → 0x1001 (1 byte)
• int *i = 0x1000; i++; → 0x1004 (4 bytes on 32-bit/64-bit usually)

This is how Arrays work in C. arr[i] is syntactic sugar for *(arr + i).

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The Generic Pointer (void*)

A void* is a pointer to something, but we don’t know the type.

• Pros: Can hold any pointer type (used in malloc, qsort, generic data structures).
• Cons: Cannot be dereferenced directly (must cast first). Cannot perform arithmetic (size unknown).

void* generic_ptr = &x;
int* int_ptr = (int*)generic_ptr; // Cast back to int* to use

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Common Pitfalls

DANGER

1. Segmentation Fault (Segfault): Accessing memory you don’t own (e.g., dereferencing NULL or a dangling pointer).
2. Memory Leaks: Allocating memory with malloc (see Manual Memory Management) and losing the pointer before calling free.
3. Dangling Pointers: Using a pointer after the memory it points to has been freed.

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