C language from entry to proficiency on the 18th day (combination of pointers and functions)

A first-class pointer as a formal parameter of a function

When the formal parameter of the function is an array, we define the function as follows:

grammar:

数据类型 函数名(数据类型 数组名)

For example :

void func(int a[],int a){
语句;
}

But in actual use, we usually define it in the form of a pointer , where the pointer a indicates that the data is an array type, for example:

void func(int *a,int n){
语句;
}

code show as below:

// 将数组的地址赋值给指针变量
int func(int *a,int n){
    
    
    printf("wo%d",a[1]);
    return 0;
}

int main(){
    
    
    int b[] = {
    
    1,2,3,3};
    int c = 10;
    func(b,c);

    return 0;
}

When we call a function, we need to pass a string , which is how we can set the parameter type in the function tochar *变量

code show as below:

// 将传过来的字符串变量的地址赋值给指针变量p
int func1(char *p){
    
    
    printf("%c\n",p[0]);
    return 0;
}

int main(){
    
    
    func1("hello");
    return 0;
}

Tip : If the formal parameter of the function is a pointer, we generally judge the value of the pointer first to determine whether the value of the pointer is NULL.

secondary pointer

Use a pointer variable to save the address of a first-level pointer variable, which we call a second-level pointer .

grammar:

数据类型 **变量名

Examples are as follows:

int a = 10;
int *p = &a;  // p为一级指针保存变量a的地址
int **pp = &p;   // pp为二级指针，保存一级指针p的地址

Here you can think of it as an array of pointers, put all pointer variables (first-level pointers) in an array, and then assign the pointer variables in the array to another pointer variable (secondary pointer) in turn.

code show as below:

int main(){
    
    

    int a = 10;
    int *p = &a;
    int **pp = &p;   // 指针pp保存了指针p的地址

    printf("%d\n",*p);
    printf("%d\n",**pp);
    printf("%p\n",*pp); // 二级指针pp保存的地址
    printf("%p\n",p);  // 一级指针p的地址

    // 这里就相当于
    // &a == p == *pp
    // a == *p == **pp

    return 0;
}