C Structs & Unions Cheat Sheet
Explains defining and using C structs and unions, including member access, typedef, padding, bit-fields, and memory layout differences.
2 PagesBeginnerMar 22, 2026
Struct Basics
Declaring, initializing, and accessing struct members.
c
struct Point { int x; int y;};struct Point p1 = {10, 20}; // Aggregate initializationp1.x = 30; // Access member with the dot operatorstruct Point *ptr = &p1;ptr->y = 40; // Access member through a pointer with ->
typedef and Designated Initializers
Common patterns for naming and initializing structs.
c
typedef struct { char name[32]; int age;} Person;Person alice = {.name = "Alice", .age = 30}; // Designated initializers (C99)Person people[3]; // Array of structspeople[0] = alice;
Unions
Multiple members sharing the same storage.
c
union Value { int i; float f; char bytes[4];};union Value v;v.i = 65; // Only one member is valid at a timeprintf("%d\n", v.i);v.f = 3.14f; // Overwrites v.i; the same memory is reusedprintf("%f\n", v.f);
Struct vs Union
Key differences in layout and sizing.
- struct- Each member has its own storage; total size >= sum of member sizes plus padding
- union- All members share the same storage; size equals the size of the largest member
- Padding- The compiler may insert padding bytes between members for alignment
- sizeof- Use sizeof(struct T) to get the actual allocated size, including padding
- Self-referential structs- A struct can contain a pointer to its own type, enabling linked lists and trees
- Bit-fields- struct Flags { unsigned int a: 1; unsigned int b: 3; }; packs fields into fewer bits
- Anonymous unions- C11 allows a union to be embedded in a struct without a member name
Pro Tip
When passing a struct to a function, the entire struct is copied by value — pass a pointer (const struct Point *) for large structs to avoid the copy overhead.
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