typedef in C

typedef is a keyword in C that is used to create an **alias** (alternative name) for an existing data type.

It does **not** create a new type — it only gives a new meaningful name to an already existing type, making code more readable, maintainable, and portable.

• Improves code readability
• Makes complex declarations simple
• Very commonly used with structures, pointers, arrays, and function pointers
• Does NOT allocate memory — purely a compile-time feature

1. Basic Syntax

              
C

          General syntax
        
typedef existing_type new_name;

2. Common and Useful Examples

Basic Types

              
C

            typedef unsigned long long ull;

ull population = 1420000000ULL;

// Now ull is alias for unsigned long long

With Structures (Most Common Use)

              
C

            typedef struct {
    char name[50];
    int roll;
    float marks;
} Student;

// Now you can write:
Student s1, s2[30];

Pointer Types

              
C

            typedef int* IntPtr;
IntPtr ptr1, ptr2;          // both are int*

// More readable for function pointers
typedef void (*FuncPtr)(int, float);
FuncPtr callback;

Arrays

              
C

            typedef int Marks[5];
Marks student1 = {85, 92, 78, 88, 95};

Enums

              
C

            typedef enum { RED, GREEN, BLUE } Color;
Color bg = GREEN;

3. typedef vs #define – Key Differences

Feature	typedef	#define
Type	Creates type alias	Text replacement (macro)
Scope	Follows C scoping rules	Global, preprocessor level
Can be used with pointers/arrays	Yes (correctly)	No (needs extra care)
Error checking	Compiler checks types	No type checking
Example	typedef int* IntPtr;	#define IntPtr int*
Common mistake	Safe	IntPtr a, b; → int* a, b; (b is int!)

4. Best Practices & Naming Conventions

Use meaningful names: Student, Node, Point, CallbackFn, etc.
Common suffixes/prefixes:
- Ptr for pointers: IntPtr, NodePtr
- Fn for function pointers: CompareFn, SortFn
- No suffix for structures: Student, Book, Employee
Use typedef with anonymous structs (very common):
typedef struct { ... } Student;

5. Real-World & Standard Library Examples

size_t — typedef unsigned long/long long (platform dependent)
FILE — typedef struct _iobuf FILE;
wchar_t — typedef short/unsigned int/long (compiler dependent)
clock_t, time_t, ptrdiff_t — all typedefs

Note: typedef makes code portable across compilers and platforms