Variable storage unit and pointer

Test procedure

#include <rtthread.h>

int main(void)

 {

   char num  = '1'; // Defining character variables num, And assign characters ‘1’

   char *ptr = # // Defines a pointer to a character variable ptr, And point to num Variable 

   char **p_ptr = &ptr; // Defines a pointer to a character variable p_ptr, And point to ptr Variable 

   rt_kprintf(" The pointer ptr The value of the point %x\n",*ptr);

   rt_kprintf(" The pointer ptr Value %p\n",ptr);

   rt_kprintf(" Variable num The value of is output in hexadecimal format %x\n",num);

   rt_kprintf(" Variable num The value of is output in character format %c\n",num);

   rt_kprintf(" Variable num The address of %p\n",&num);

   rt_kprintf(" Point to ptr Pointer value of pointer variable %p\n",p_ptr);  

   return 0;

 }

After executing the program, the output is as follows ：

 The pointer ptr The value of the point 31

 The pointer ptr Value 20002d1c

 Variable num The value of is output in hexadecimal format 31

 Variable num The value of is output in character format 1

 Variable num The address of 20002d1c

 Point to ptr Pointer value of pointer variable 20002d18

analysis

There are three variables involved in the program , Namely num、ptr、p_ptr, Their values are hexadecimal 31、20002d1c、20002d18. They are all variables , Although the declared types are char Character , But for the 32 Bit address system , The hands are all 32bit, That accounts for 4 byte ; And character variables num Occupy 1 byte .ptr、p_ptr Is a special variable , Is the variable used to hold the pointer , It is conventionally called pointer variable , In fact, what is saved is that of the computer system “ Address ”.

Three variables num、ptr、p_ptr The location in the memory is shown in the figure below ：

When the computer compiles, it divides the memory 3 Space binding this 3 A variable , When writing a program , The values stored in the corresponding storage space are manipulated by variables . This system uses a small terminal to store data , That is, the low order address stores the data of low order bytes .

  Variables have three elements

1. data type （Type）： Decide how much space to allocate in memory

2. Variable name （Name） ： The name of the manipulation of the allocated space when the program is written

3. value （Value） ： Data stored in space

When debugging a program , You can usually view these elements ：

Common variables num Represents the value of the storage space ,“&num” For its address . For pointer variables ptr Come on ,ptr Save address ,“*ptr” Represents the value whose address points to the space ,“&ptr” Represents the address where the pointer space is saved . and “*num” Indicates that num The value is the content of the address , because num Only 1 byte , The compiler will consider an error and cannot generate an executable , so “*num” It makes no sense .