Core · Module 4 · Lesson 2 of 9

Unions, Bitfields, and Type Punning

Master shared memory overlays with unions, pack hardware register flags using bitfields, and inspect object representations without violating aliasing rules.

ProAbout 16 minutes

By the end you can

Diagram shared memory layout and calculate allocation sizing for unions
Inspect raw binary bit patterns using `memcpy` and implementation-defined union techniques
Pack boolean flags and registers into discrete bit widths using bitfield syntax
Identify portability traps caused by hardware endianness and strict aliasing rules

Builds on Structures, Alignment, and Padding.

01 · A question first

If you cast a float to an unsigned int, C converts its mathematical value, turning 3.14 into 3. How can you inspect the exact raw IEEE-754 binary bits of that float without triggering undefined behavior from strict aliasing violations?

Make a guess before you read on.

02 · The idea

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What's inside

Start with the problem
How the idea works
Why it is correct
What it costs
Pitfalls and variations
Where it is used