Pointer parameter transfer in C language

Pointer parameter transfer in C language

1. How to change the value of the parameter

1. Common mistakes in writing:

The following function cannot change the value of a since the variable will be destroyed as the stack exits.

#include<stdio.h>
void f(int b){
    
      //参数b初始化为1
    b=3;//改变b的值为3
}//随着函数的结束，变量b被销毁，a的值并没有发生改变
int main(void){
    
    
    int a=1;
    f(a);//将a的值传入函数
    printf("a的值为：%d",a);
}

operation result:

a的值为：1

2. The solution is to use pointers

#include<stdio.h>
void f(int *ptr){
    
    //ptr的值等于变量a的地址
    *ptr=3;//对ptr间接访问，相当于变量a，这里把3赋值给变量a
}
int main(void){
    
    
    int a=1;
    f(&a);//将a的地址传入
    printf("a的值为：%d",a);
}

operation result:

a的值为：3

2. Compare with return

1. Use return to return the value:

#include<stdio.h>
int f(){
    
    
    int a=3;
    return a;
}
int main(void){
    
    
    int a=1;
    a=f();
    printf("a的值为：%d",a);
}

operation result:

a的值为：3

It can be seen that the value of a can also be changed using return, but unlike a pointer, it can only return one value.
Using a pointer, the values ​​of multiple variables can be changed at the same time, as follows:

2. Use pointers to change the value of multiple parameters

#include<stdio.h>
void f(int *ptr1,int *ptr2){
    
    
    *ptr1=3;
    *ptr2=3;
}
int main(void){
    
    
    int a=1;
    int b=1;
    f(&a,&b);
    printf("a的值为：%d\n",a);
    printf("b的值为：%d\n",b);
}

operation result:

a的值为：3
b的值为：3

3. How to change the value of the pointer?

pointers using pointers

#include<stdio.h>
void f(int **ptr){
    
    //ptr初始化为b的地址
    *ptr=NULL;//对ptr进行间接访问，得到的是指针b，把它赋值为NULL
}
int main(void){
    
    
    int a=1;
    int *b=&a;
    printf("a的地址为：%d\n",&a);//打印a的地址
    printf("b的值为：%d\n",b);//此时b指向a，打印的是a的地址
    f(&b);
    printf("b的值为：%d\n",b);//此时指针b的值已经改变了，为NULL
}

operation result:

a的地址为：6422264
b的值为：6422264
b的值为：0