← C theorytheory 0/4 · 0%

level 1

Curious · 0 XP

streak

perfect

badges

100 XP to Apprentice

Systems · hard

3. Structs, Padding, and Memory Alignment

Byte packing, alignment, and cache-line boundaries.

1 min read2 questionsup to 50 XPtopic 3/4

Read in another language

AI translates the explanation and quiz text while preserving code and answer scoring.

Compilers insert padding bytes into structs so fields align with hardware word boundaries (e.g. 4-byte or 8-byte boundaries). Struct packing can be controlled via compiler directives (`#pragma pack(1)`).

c
struct Unpadded {
    char a;     // 1 byte
    // 3 bytes padding
    int b;      // 4 bytes
    char c;     // 1 byte
    // 3 bytes padding
}; // Total: 12 bytes

How to study this topic

1. Read the explanation once end to end, skipping the code.
2. Re-read the code snippets and say out loud what each line does.
3. Answer the quiz without scrolling back — that is the real check.
4. Read every explanation, even for the answers you got right.

Scoring

· Up to 30 XP from quiz accuracy.
· +15 XP for a perfect first attempt.
· +5 XP while a 3-day streak is running.
· 75% or higher marks the topic complete.

Check your understanding

1. Why does sizeof(struct { char a; int b; }) equal 8 bytes instead of 5 on 32/64-bit systems?
Compiler adds 3 padding bytes to align the int on a 4-byte boundaryCompiler bugint is 7 bytesC forbids odd byte counts
2. What directive removes padding bytes between struct fields?
#pragma pack(1) / __attribute__((packed))#pragma optimizeinlineextern

Badges

First light

Pass your first topic

Sharp shooter

3 perfect first-try quizzes

On a roll

Study 3 days in a row

Week warrior

Study 7 days in a row

Grinder

Earn 500 XP

Deep diver

Earn 1500 XP