Master C Function Pointers and Callbacks: Concepts, Syntax, and Real‑World Examples

1. What Is a Function Pointer?

A function pointer is a variable that stores the address of a function, allowing the function to be called indirectly. Its declaration follows the pattern return_type (*pointer_name)(parameter_types); where the parentheses around the pointer name are mandatory to distinguish it from a regular function declaration.

2. Declaring Function Pointers with typedef

Using typedef simplifies the syntax: typedef int (*Fun1)(int); Examples:

typedef int (*Fun1)(int);

typedef int (*Fun2)(int, int);

typedef void (*Fun3)(void);

typedef void* (*Fun4)(void*);

3. Calling a Function Through a Pointer

Assign the function’s name (or its address) to the pointer and invoke it:

int Func(int x);

int (*p)(int);

p = Func;   // or p = &Func

int result = (*p)(5);   // can also be written as p(5)

A complete example demonstrates reading two integers, assigning Max to a pointer, and calling it via the pointer.

4. Using Function Pointers as Parameters

Define a function‑pointer type and pass a function to another function:

typedef void (*FunType)(int);

void callFun(FunType fp, int x) { fp(x); }

void myFun(int x) { printf("myFun: %d
", x); }

int main() { callFun(myFun, 100); return 0; }

5. Function Pointers as Return Types

A function can return a pointer to another function:

void (*func5(int, int, float))(int, int) { /* ... */ }

The returned pointer can then be used to invoke the target function.

6. Arrays of Function Pointers

Since a function pointer is a pointer, it can be stored in an array:

void (*func_array_1[5])(int, int, float);

typedef void (*p_func_array)(int, int, float);

p_func_array func_array_2[5];

7. Summary of Function Pointer Rules

Function‑pointer constant: Max ; variable: p . Both (*myFun)(10) and myFun(10) are valid; the latter is preferred for readability. Function pointers can be stored in arrays, e.g., int (*fArray[10])(int);

8. What Is a Callback Function?

A callback is a function invoked through a function pointer passed as an argument to another function. It enables the caller to remain agnostic of the concrete implementation, only requiring a matching signature.

9. Why Use Callbacks?

Callbacks decouple the caller from the callee, allowing flexible substitution of behavior without changing the caller’s code.

10. Implementing Callbacks in C

Define several callback functions and a handler that receives a function pointer:

int Callback_1(int a) { printf("Hello, Callback_1: a = %d
", a); return 0; }

int Handle(int x, int (*Callback)(int)) { Callback(x); return 0; }

int main() { Handle(4, Callback_1); /* … */ return 0; }

This demonstrates that passing the function name supplies its address, making callbacks a specific use case of function pointers.

11. Callback Example: Arithmetic Operations

A struct holds pointers to basic arithmetic functions, which can be called directly or via a generic library function:

typedef struct { float (*p_add)(float,float); /* … */ } OP;

float ADD(float a,float b){ return a+b; } /* SUB, MUL, DIV */

void init_op(OP *op){ op->p_add = ADD; /* … */ }

float add_sub_mul_div(float a,float b,float (*op_func)(float,float)){ return (*op_func)(a,b); }

The program prints results using both direct pointer calls and the generic callback function.

12. Real‑World Callback Use: State‑Machine for GPRS Module

A state‑machine array stores status codes and corresponding handler functions. The engine iterates the array, finds the matching status, and invokes the associated callback, illustrating how callbacks simplify complex workflows.