Part 3: Structs, Unions, Preprocessor & Operators

1. Structs — layout and size

• Fields in declaration order; compiler may pad to alignment.

struct A { char c; int i; };
printf("%zu\n", sizeof(struct A));   // >= 5, usually 8 (padding)

• Packing: #pragma pack(1) (or attribute) removes padding — smaller/slower, used for wire formats.
• Alignment: struct offset must be multiple of member alignment.

struct A { char c; int i; };  // offset c=0, i=4 once padded

2. Unions

• All members share the same memory; size = largest member.
• Writing one member reads the other — type-punning territory (UB beyond the last written member in C++ strict-aliasing terms; in C it’s the trick).

union U { int i; float f; };
printf("%zu\n", sizeof(union U));  // 4

3. Bit fields

• Fields in bits directly: struct Flags { unsigned a : 3; unsigned b : 5; };.
• Wraps at the declared width; packing depends on layout; portability limited. Use when you need flag compression.

4. Preprocessor & macros

• #define NAME value — textual substitution, no type.
• Function-like macros run before the compiler; no runtime cost.

#define SQUARE(x) ((x) * (x))
SQUARE(1 + 2) // ((1+2)*(1+2))

• Always parenthesise params and the whole body.
• # stringify: #x; ## concatenation:

#define TO_STR(x) #x        // argument to string
#define CAT(a,b) a##b

• Precedence trap: 2 * SQUARE(3) parenthesised is fine; unparenthesised isn’t.
• #ifdef, #ifndef, #elif, #endif conditional compilation; #pragma once+include guards.

5. Bitwise operators (interview pack)

Op	Meaning
&, |, ^, ~	and, or, xor, not
<<, >>	shift left / right

Patterns: set a bit x |= (1 << n); clear x &= ~(1 << n); flip x ^= (1 << n); test x & (1 << n).

• Extracting bits: (x >> start) & mask.
• Be careful with signed right shift (implementation-defined arithmetic vs logical shift).

6. Operator precedence — the scoreless overview

High → low (memory aide):

• Postfix () [] . -> ++/—
• Unary + - ! ~ * & (type) ++/--
• * / %
• + -
• << >>
• < > <= >=
• == !=
• &, ^, |
• Logical &&, ||
• Ternary ?:
• Assignment and += etc.
• Comma

Interview golden rule: write parentheses whenever precedence is non-obvious — the compiler isn’t your reviewer.

7. Interview checkpoint

• Struct padding & pragma pack.
• Unions sharing memory.
• Macro parameter parens; stringify/paste.
• Bitwise ops for flags.
• Precedence — especially << and assignment mixing.

Junior fluency bonus: know that sizeof and & have higher precedence than everything else binary.