Structures, Unions & Pointers in C: quick revision, 15 bites

Struct vs Union: One Memory Rule Changes Everything

Same syntax, totally different memory.

A **struct** gives every member its own memory; a **union** makes all members share one block sized to the largest member.

Takeaway. Struct = separate boxes; union = one box shared by many labels.

Structure: A Named Bundle of Mixed Types

One name, many data types.

Takeaway. A struct is a blueprint; each variable of that type gets its own copy of all members.

Union: One Memory Block, Many Faces

All members, one address.

Takeaway. A union saves memory but only one member holds a valid value at any moment.

Pointer: A Variable That Holds an Address

Pointers don't store data — they store where.

Takeaway. & gives an address; * follows it to the value.

Accessing Struct Members: Dot vs Arrow

Dot for values, arrow for pointers.

Takeaway. Arrow operator = dereference + member access in one step.

Union in Action: Only One Value Survives

Write one member, lose the others.

Writing `d.f` overwrote the bytes that `d.i` was using — so reading `d.i` gives meaningless data.

Takeaway. Never read a union member other than the one last written.

Reading the Wrong Union Member

This bug is silent and deadly.

Takeaway. Unions demand discipline: know which member is live.

Forgetting to Initialize a Pointer

Wild pointers crash programs.

Takeaway. An uninitialized pointer points to nowhere safe.

When to Choose Struct vs Union

Pick by memory, not by habit.

Takeaway. Struct for records, union for memory-efficient variants.

sizeof Tells the Truth About Memory

One operator reveals the layout.

Takeaway. Use `sizeof` to verify memory usage — never assume.

Passing a Struct to a Function via Pointer

Pointers avoid copying big structs.

Takeaway. Passing by pointer is faster and lets the function modify the original.

Where Unions Shine: Embedded Systems

Every byte counts in firmware.

Takeaway. Unions are a memory-saving tool, not a general-purpose container.

Pointers + Structs = Linked Lists

Self-referencing structs build data structures.

A struct containing a pointer to its own type is the foundation of linked lists, trees, and graphs.

Takeaway. Self-referential structs are the building blocks of dynamic data structures.

Exam Favourite: Size of a Union

This question appears every year.

Takeaway. Union size = largest member; struct size = sum + padding.

Remember: Struct = Stack, Union = Share

Two S-words, two rules.

**S**truct = **S**tack of separate boxes. **U**nion = **U**nited (shared) memory. The first letter tells you the memory rule.

Takeaway. Struct stacks members; union unites them in one block.