Value transmission and address transmission of C language, pointer of pointer

When the function is called ： Value and address

The secondary pointer ：

int a=5;

int* p=&a; 

int** p1=&p;

printf("a=%d, *p=%d, **p1=%d", a, *p, **p1);  The result is 5,5,5

*p: p The value of is a The address of ,p Point to a,*p Just look at p The variable value on this address ,*p Yes. p The variable value on the stored address , Not to see p The value of this variable （p The variable value of is the address , see p The variable value of should be to see p Value , Not through *p The way ,*p Is to look at pointer variables p The value of the variable on the address pointed to ！）

**p1 That is to say *（*p1）,*p1 Just look at p1 The variable value on the stored address ,p1 The value of is p The address of , therefore *p1 Namely p Value , therefore **p1 Namely *p=a.

* Address = The value of the variable occupying this address ≠ The value of the address

**p1：*（*p1）,p1 Put pointer variables p The address of , therefore *p1 It's a pointer variable p

* Whose address = Whose value

c The value and address of language - Baidu library c

Pass value ： Do not change the value of the real parameter in the main function

Byref ： Pass the address of the variable in the main function directly to other functions for use , Changed the actual value of the real parameter in the main function .

The difference is this ： Passing values only works inside the function , If it's an address , After a functional process , The arguments in the main function can be actually changed .

For arrays , Because arrays usually occupy a large amount of memory , The system will not copy another piece of memory in the memory space of the function for value transfer like value transfer , Instead, send the address directly , Directly change the actual parameter value of the array .

The system is quite strict with the memory of the array .

It also involves the pointer of the pointer ：

Byref ： You want to actually change the value of which argument through the function , Just pass in the address of this argument .

The pointer of the pointer ：

If the pointer is main An argument in the main function , And want to change the value of the pointer , What should be passed to the function parameter is main Address of argument pointer in function .

The variable of which argument you want to actually change , Just pass in the address of the argument , Byref .

malloc- The pointer of the pointer - Address transmission or value transmission ： Be careful ： If you want to change the value of an argument through a function call , The address passed in should be the address of the argument , Instead of copying a copy of memory in the called function , This memory will disappear with the end of the called function .

malloc The allocated memory needs to be free In order to release , Apply inside the function malloc, Even if the function ends , It will not release this memory .

The pointer of the pointer ( Simple and easy to understand )_ShenMingYi_ The blog of -CSDN Blog _ The pointer of the pointer

Function internal use malloc, Should I send address or value ？_ Big mulberry security team blog -CSDN Blog