Introduction to C language function parameter passing mode

一、引言
函数是Ｃ源程序的基本模块,Specific functions can be implemented by calling function modules.在定义函数的时候,Function parameters are almost mandatory options.To be proficient in using function calls,The passing mode of function parameters must be clear.Functions can be divided into calling functions and called functions,The main calling function refers to calling other functions inside the function,and by other functions（包括主函数）The called function is called the called function.It is because of the calling function and the called function,There is also a classification of function parameters——形参和实参.当然了,Functions can be further divided into functions with parameters and functions without parameters.The following will describe the function parameter classification and parameter transmission methods respectively.
二、函数参数的分类
函数的参数可以分为实参（实际参数）和形参（形式参数）.
A formal parameter is a function definition（声明）parameters that appear in parentheses after the function name.
Parameters that appear in parentheses after the function name when the function is called are called actual parameters.
需要注意的是,Function parameters are not required.
Form parameter example：
For example, there is the following function declaration：

int max( int a[], int n );

此时,max称为函数名（Also known as an interface）,inside parenthesesa[]和n称为形参,因为此时数组a和变量nThere is no actual data,Hence it is called a formal parameter.
Practical example：
For example, there is the following code：

int fun(  )
{
    
int a[] = {
     1, 2, 3, 4, 9, 5 };
int n = 6;
int var = max( a, n );
......
}

此时函数funcalled the calling function,函数maxcalled the called function.函数maxarray in parenthesesa和变量n称为实参,Because the parameters at this time have already been assigned.
三、The passing mode of function parameters
Actual parameters to formal parameters“单向传递”：实参—>形参;
“值”传递.That is, when calling a function,Pass the value of the actual parameter to the formal parameter one-way.This is also done to protect the actual parameters,Avoid the actual parameter being taken away by the formal parameter.

Demo 1：Find the maximum value of an array of integers.
参考代码：

#include"stdio.h" 
int max( int a[], int n );
int main()
{
    
	int a[] = {
     1, 2, 3, 4, 9, 5 };
	int n = 6;
	int maxV = max( a, n );//a和n为实参 
	printf( " max Value = %d\n", maxV );//Output an array to the screena的最大值
	printf( " n = %d\n", n );//Output arguments to the screenn的值
	return 0;
}
//Find an array of integersa中的最大值,a和n均是形参 
int max( int a[], int n ){
    
	int maxV, i;
	maxV = a[0];
	for( i = 1; i < n; i++ ){
    
		if( a[i] > maxV ){
    
			maxV = a[i];
		}
	}
	n = 10; //Attempt to change parametersn的值 
	return maxV;
}

输出结果：

The first of this example21行n = 10;是在函数maxInternally changes the value of the formal parameter,But through no9The line statement gets the output but it isn=6,This shows that the parameters of the function are passed in one way.
Such a function parameter passing mechanism,There are troubles when it comes to actual programming,For example, if you want to get the maximum and minimum values ​​of an integer array at the same time,Returning two values ​​at the same time through the pattern of function return values ​​is not acceptable.解决的方法有很多,其中常用的有：
①　返回数组,The maximum and minimum values ​​are stored in the array;
②　返回结构体变量,The members are the maximum and minimum values;
③　Add pointer-type function parameters.CThe language adds a two-way pass-by-value mechanism for function parameters,That is to achieve bidirectional transfer of data through pointer variables：实参<—>形参.此时,The function parameter passed from the actual parameter to the actual parameter is the address of the pointer variable（Because at this time the value of the pointer is the address,仍旧是“值”传递）,As long as the address of the pointer is not changed inside the function,You can change anything else,其实,You can also change the data value in the address.（This article does not discuss the method of changing the pointer address,In fact, it is a double pointer problem,依然是“值”传递）.

Demo 2：Find the maximum and minimum values ​​of an array of integers.
方法一：返回数组
参考代码：

#include"stdio.h" 
#include"malloc.h"
int *max( int a[], int n );
int main()
{
    
	int a[] = {
     1, 2, 3, 4, 9, 5 };
	int n = 6;
	int *mV = max( a, n );//a和n为实参 
	printf( " max Value = %d\n min Value = %d\n", mV[0], mV[1] );
	return 0;
}
//Find an array of integersa中的最大值,a和n均是形参 
int *max( int a[], int n ){
    
	int maxV, minV, i;
	maxV = a[0];
	minV = a[0];
	for( i = 1; i < n; i++ ){
    
		if( a[i] > maxV ){
    
			maxV = a[i];
		}
		if( a[i] < minV ){
    
			minV = a[i];
		}
	}
	//动态分配空间,存储最大值和最小值 
	int *mv = ( int * )malloc( 2 * sizeof( int ) ) ;
	mv[0] = maxV;
	mv[1] = minV; 
	return mv;
}

方法二：返回结构体变量
参考代码：

#include"stdio.h" 
typedef struct
{
    
	int maxV;
	int minV;	
}MValue;
MValue max( int a[], int n );
int main()
{
    
	int a[] = {
     1, 2, 3, 4, 9, 5 };
	int n = 6;
	MValue mV = max( a, n );//a和n为实参 
	printf( " max Value = %d\n min Value = %d\n", mV.maxV, mV.minV );
	return 0;
}
//Find an array of integersa中的最大值,a和n均是形参 
MValue max( int a[], int n ){
    
	int maxV, minV, i;
	maxV = a[0];
	minV = a[0];
	for( i = 1; i < n; i++ ){
    
		if( a[i] > maxV ){
    
			maxV = a[i];
		}
		if( a[i] < minV ){
    
			minV = a[i];
		}
	}
	MValue mv;
	mv.maxV = maxV;
	mv.minV = minV; 
	return mv;
}

方法三：Add pointer-type function parameters
参考代码：

#include"stdio.h" 
#include"malloc.h"
void max( int a[], int n, int *maxV, int *minV );
int main()
{
    
	int a[] = {
     1, 2, 3, 4, 9, 5 };
	int n = 6;
	int *maxV = ( int * )malloc( sizeof( int ) );
	int *minV = ( int * )malloc( sizeof( int ) );
	max( a, n, maxV, minV );//a、n、maxV和minV为实参 
	printf( " max Value = %d\n min Value = %d\n", *maxV, *minV );
	free( maxV );
	free( minV );
	return 0;
}
/*Find an array of integersa中的最大值 a、n、maxV和minV均是形参 maxV和minVUsed to implement bidirectional value transfer */ 
void max( int a[], int n, int *maxV, int *minV ){
    
	int i;
	*maxV = a[0];
	*minV = a[0];
	for( i = 1; i < n; i++ ){
    
		if( a[i] > *maxV ){
    
			*maxV = a[i];
		}
		if( a[i] < *minV ){
    
			*minV = a[i];
		}
	}
}

补充说明：
①　上述三个方法中,Using structs is relatively simple;
②　在上述三个方法中,函数max的参数a是数组名,Therefore, bidirectional value transfer can also be achieved,例如Demo 3.
Demo 3：Sort an array of integers from smallest to largest.
参考代码：

#include"stdio.h" 
void sort( int a[], int n);
int main()
{
    
	int a[] = {
     1, 2, 3, 4, 9, 5 };
	int n = 6;
	sort( a, n );
	for( int i = 0; i < n; i++ )
		printf( "%5d", a[i]  );
	return 0;
}
//对数组aSort using selection sort
//参数a在函数sort被调用时,The elements passed in are the elements of the array,
//当函数sort调用结束后,aStored in is the sorted result 
void sort( int a[], int n){
    
	int i, j, t;
	for( i = 0; i < n - 1; i++ ){
    
		for( j = i + 1; j < n; j++ )
		if( a[j] < a[i] ){
    
			t = a[i];
			a[i] = a[j];
			a[j] = t;
		}
	}
}