Pointer array and pointer to pointer

Today I finally figured out the difference between a pointer array and a pointer pointing to a pointer.

 

1 # include <stdio. h>
2 # include <string. h>
3 void main ()
4 {
5
6 char a [] = "1234567890 MMMMMMM ";
7 char * p =;
8 char ** argv = & p;
9 printf ("% s \ n", * (argv + 0 ));
10 // char * argv [] = {"12345", "67890", "MMMMMMM "};
11 // for (I = 0; I <3; I ++)
12 //{
13 // printf ("% s \ n", argv [I]);
14 //}
15} I. pointer concept:
When our program declares the following variables:
Short int I;
Char;
Short int * pi;
The program will open up space for each variable in an address space in the memory, as shown in.
Memory Address → 6 7 8 9 10 11 12 13 14 15
Bytes -------------------------------------------------------------------------------------
... |
Bytes -------------------------------------------------------------------------------------
| Short int I | char a | short int * pi |
As shown in the figure, we can see:
The I variable occupies two bytes at the memory address 5.
A variable occupies one byte at the location of memory address 7.
The pi variable occupies two bytes at the memory address 9. (Note: pi is a pointer. Here, the pointer width is only two bytes, and the 32-bit system is four bytes)
Next, assign values as follows:
I = 50;
Pi = & I;
After the assignment in the preceding two sentences, the memory image of the variable is as follows:
Memory Address → 6 7 8 9 10 11 12 13 14 15
Bytes --------------------------------------------------------------------------------------
... | 50 | 6 |
Bytes --------------------------------------------------------------------------------------
| Short int I | char a | short int * pi |
No: the pi value of the Short integer pointer variable is 6, which is the memory start address of the I variable. Therefore, when we perform read/write operations on * pi, it is actually a read/write operation on the I variable. For example:
* Pi = 5; // It is equivalent to I = 5;

2. pointer address and pointer pointing to another pointer address
We can see that the pointer variable itself andOthersVariables are also in a memory address, for example, pi's memory start address is 10. Similarly, we may also point a pointer to this address.
See the following code:
Short int ** ppi; // This is a pointer to the pointer. Note that there are two * numbers.
Ppi = pi
First sentence: short int ** ppi; -- declares a pointer variable ppi, which is used to store (or point to) the address of a short int * type pointer variable.
The second sentence: & pi is the pi address, and ppi = pi is the address assigned to the ppi. Assign the address value 10 to the ppi. For example:
Memory Address → 6 7 8 9 10 11 12 13 14 15
Bytes ------------------------------------------------------------------------------------
... | 50 | 6 | 10 |
Bytes ------------------------------------------------------------------------------------
| Short int I | char a | short int * pi | short int ** ppi |
We can see that the starting address of the pointer variable pi is the content of the pointer variable ppi. So ......
What is the ppi value? -- 10.
* What is the ppi value? -- 6, that is, the value of pi.
** What is the ppi value? -- 50, that is, the value of I, is also the value of * pi.