📚 CS61C Lecture Notes: Introduction to C & Pointers¶

Instructor: Stephan Kaminsky | Date: Jun 2019

📌 Key Concepts in C Programming¶

1. Compilation in C¶

• Compiled Language: Converts C code directly to machine-specific instructions (0s and 1s).
• 🚀 Advantages: Faster execution than Java/Python (no bytecode/JVM).
• ⚠️ Disadvantages: Platform-dependent executables; slower edit-compile-run cycle.

2. Variable Types & Declarations¶

• Typed Variables: Must declare type before use.
• Example:
  
  C

  int x = 5;          // Integer
  float y = 1.618;    // Floating point
  char z = 'A';       // Character
  

• Type Sizes: Machine-dependent (e.g., int = 4/8 bytes).
• Special Keywords: short, long, unsigned.

3. Characters & ASCII¶

• ASCII Encoding: Characters stored as numbers (e.g., 'a' = 97).
  
  C

  char c = 'a';     // Same as char c = 97;
  

• Size: 1 byte (8 bits).

4. Type Casting¶

• Weak Typing: Explicitly cast between types.
• Example:
  
  C

  int i = -1;
  if ((unsigned int)i < 0) { ... }  // False
  

• ⚠️ Caution: Risky casts (e.g., casting structs to integers).

5. Functions¶

• Prototypes & Definitions:
  
  C

  int add(int a, int b);   // Prototype
  int add(int a, int b) { return a + b; }  // Definition
  

• Return Types: Must declare return type (void for no return).

6. Structs & Unions¶

• Structs: Group related variables.
  
  C

  typedef struct {
    int length;
    int year;
  } Song;
  Song s = {213, 1994}; 
  

• Unions: Overlapping memory for different types.
  
  C

  union Data {
    int i;
    float f;
  };
  

🧮 Example: Struct Padding¶

struct foo {
  int a;     // 4 bytes
  char b;    // 1 byte (+3 padding)
  struct foo* c;  // 4 bytes
};  
// Total size = 12 bytes (32-bit architecture)

📊 C vs Java Comparison¶

🎯 Pointers: Address vs Value¶

• Pointer Syntax:
  
  C

  int y = 5;
  int *p = &y;   // p stores address of y
  int z = *p;    // z = value at address p (5)
  

• Pointer Types:
• int*, char*, void* (generic pointer).
• ⚠️ Dangling Pointers: Uninitialized pointers → undefined behavior!

🔍 Example: Pointer Parameter Passing¶

void addOne(int *p) { (*p)++; }  
int main() {
  int y = 3;
  addOne(&y);    // y becomes 4
  return 0;
}

❓ Quiz: 4-Bit Number Representations¶

Given x = 0b1010 (4 bits), which value does NOT represent x?

Options:
(A) -4  (B) -6 (C) 10 (D) -2

Analysis:
- Unsigned: \(2^3 + 2^1 = 10\) ✔️ (C)
- Sign & Magnitude: \(-2^1 = -2\) ✔️ (D)
- Biased (Bias=7): \(10 - 7 = 3\) → Not an option.
- Two’s Complement: \(-6\) ✔️ (B)
- One’s Complement: \(-5\) → Not an option.

Answer: (A) -4 ❌

🛑 Common Pointer Bugs¶

• Uninitialized Pointers:
  
  C

  int *p;     // p points to garbage!
  *p = 5;     // Crash/undefined behavior
  

• Memory Leaks: Forgetting to free() after malloc().

📖 Key Takeaways¶

• C offers low-level control but requires careful memory management.
• Pointers = powerful but error-prone. Always initialize!
• Structs/unions organize data; padding affects memory layout.

🔗 Resources:
- K&R Book ("The C Programming Language")
- C99 Standard