Pointer of C language (1)

One 、 The pointer

1、 The pointer

 Equivalent to indirect access to data ;
 The pointer variable size is 4 byte ;

2、 Value passing and reference passing

 Reference passing is also value passing , But it's the address ;
 The essence of function call is value passing ;

#include <stdio.h>

// Value passed 
void demo01(int a) {
    
    a = 100;

}

// reference : Delivery address 
void demo02(int* b) {
    

    *b = 100;
}

int main()
{
    
    int i;
    i = 10;

    printf(" Original value :%d\n",i);
    demo01(i);
    printf(" Value passed :%d\n",i);
    demo02(&i);
    printf(" reference %d\n",i);

    return 0;
}

result ：

reason ：
（1）、 Check the memory address ：

（2）、

Variable i Memory address of ：

Variable a Memory address of ：
Before the change ：

After modification ：

You can see the value passing , The value passed in the subfunction , Different memory addresses , Is the memory address of the subfunction on the stack ; Therefore, when modifying the value passed by value transfer , The value of the main function will not be modified

The following figure , You'll find variables b Memory addresses and variables a In the same , This is because demo01 After the function is executed , Free memory , So continue to implement demo02 I used demo01 The memory address after the function is released .
Variable b Memory address of ：

See the variables b The value stored in the memory address of is , Variable i Memory address of , When using b It's equivalent to visiting a Memory address of , And change the value to 100, Just modify the original data .*

The execution process of function in memory ：

When executed main function , Will open up a memory space in the stack ; When executing to a sub function demo01 when , Will open up a new piece of memory on the stack , Used to execute sub functions , The actual parameters will be copied to the sub function and copied to a Variable ; When the sub function is completed , Will reclaim this memory , So it's done demo01 after , Free memory , When executed demo02 when , It will open up new memory space , The actual parameters will be copied to the sub function and copied to b Variables are used to execute demo02 function ,b It's a pointer variable , The address is stored in .