Pointer function of C language

Variable parameter function

Variable parameter functions require specific header files , There's a specific format , Please look at the following example ：

#include <stdio.h>
#include <stdarg.h>// First, reference a specific header file 
// Write a sum function 
int sum(int n,...);// Statement & Definition  n Indicates the number of parameters  ... Indicates that the parameters are indefinite 
int sum(int n,...)
{
    
	int i,sum=0;
	va_list vap;// Load parameter list 
	va_start(vap,n);// Open the parameter list 
	for(i=0;i<n;i++)
	{
    
		sum+=va_arg(vap,int);// Using parameter 
	}
	va_end(vap);// Close the parameter list 
	return sum;// Return value 
}

Pointer function

A pointer function is a function that returns a pointer
for instance ：

char* xxx(int a)
{
    
	if(a==1)
		return "just do it!";// The address of the first character of the string is returned 
		...
}

Be careful ： Cannot return pointer to local variable

A function pointer

Is a pointer to a function , It is similar to the relationship between pointer array and array pointer

int square(int a)
{
    
	return a*a;
}
int (* xxx)(int x);// Define a function pointer xxx Point to one, there is one int Function of parameter 
xxx = □// to xxx initialization 
(*xxx)(2);// Call function through function pointer , Or you could write it as xxx(2), But in order to distinguish from the function name , Bracketed and *

Function pointer as function parameter

int add(int num1,int num2)
{
    
	return (num1+num2);
}
int a(int (*b)(int , int),int num1,int num2)// Define a function , Three parameters （ Look at the comma ）, One parameter is the function pointer , The return value of the function pointed to is int, There are two int Parameters , The other two parameters are int
{
    
	return (*b)(num1,num2);
}
a(add,1,3);// Then call directly 

Take the function pointer as the return value

int (*select(char))(int num1,int num2);

according to C Language From left to right, from inside to outside Analyze this definition in the order of ：select(char) First of all, it is confirmed that this is a named select Function of , There is one char Parameters , The rest is int (*)(int num1,int num2) Return value ： A function pointer , There are two points int Function of parameter