Functional pointers in C

C supports pointers to functions. Functions are not variables but you can define a pointer to a function. The function name refers to the address of the function much like array name points to first element.

lets define a function , this function takes a integer as parameter and prints it

int printInt(int a){

return printf("%d",a);

}

you can define a pointer to this function as follows

int (*funcPtr)(int);

funcPtr=printInt;

The use of the functional pointer is very similar to how we call a function

(*funcPtr)(10);

The full program looks something like this

#include "stdio.h"

int printInt(int a){
 return printf("%d",a); 
}

int main(){
 int (*funcPtr)(int);
 funcPtr=printInt;
 (*funcPtr)(10);
}

We can also pass the functional pointers to other functions.Continuing with the previous example , let us assume that there is method which expects a functional pointer as a parameter to print its output

int printInt(int a){

return printf("%d",a);

}

void printSomething(int (*printFuncPtr)(int)){

 int a=10,b=20,c;

 c=a+b;

(*printFuncPtr)(c);

}


int main(){
 int (*funcPtr)(int);
 funcPtr=printInt;
 printSomething(funcPtr);
}

You can use typedef to define the functional pointer

#include "stdio.h"
typedef int (*ptr)(int);

int printInt(int a){
 return printf("%d",a); 
}

void frmtPrint(ptr ptr1){
 (*ptr1)(2);
}

int main(){
 int (*funcPtr)(int);
 funcPtr=printInt;
 frmtPrint(funcPtr);
}

Lets a real world example where we use functional pointers.

MQTT is publish/subscribe based messaging protocol.  PubSubClient is one of the library available for arduino.

To receive messages from broker , you need a set a callback function, So your code will be invoked by the library when there is new message on the subscribed channels. The following snippet is from PubSubClinet example mqtt_basic

client.setCallback(callback);

here callback is function name

void callback(char* topic, byte* payload, unsigned int length) {
Serial.print("Message arrived [");
Serial.print(topic);
Serial.print("] ");
for (int i=0;i<length;i++) {
Serial.print((char)payload[i]);
}
Serial.println();
}

The function signature should match with that of expected by the library

#define MQTT_CALLBACK_SIGNATURE void (*callback)(char*, uint8_t*, unsigned int)