How to apply for a second-level pointer in the C language?

In the C programming language, a double pointer refers to a pointer that points to another pointer. It is commonly used to reference pointer arrays or arrays of pointers. Double pointers make it easier to manipulate multiple levels of pointers, such as passing the address of a pointer in a function.

Here is a simple example demonstrating how to use double pointers.

#include <stdio.h>

void modifyValue(int **ptr) {
    **ptr = 20;
}

int main() {
    int value = 10;
    int *ptr = &value;
    int **ptr_ptr = &ptr;

    printf("Value before modification: %d\n", *ptr);
    
    modifyValue(ptr_ptr); // 通过二级指针传递指针的地址

    printf("Value after modification: %d\n", *ptr);

    return 0;
}

In the example above, an integer variable named value and a pointer named ptr pointing to value are first defined. Then, a double pointer ptr_ptr, pointing to the address of ptr, is defined. By calling the modifyValue function and passing ptr_ptr, the value of value can be modified.

In conclusion, double pointers in C language can be used to easily manipulate pointers to pointers, effectively passing the address of pointers, allowing for more flexible pointer operations.