Complete the C language pointer sixth article

A pointer to another pointer

First, the concept of the needle:

As early as in the second part of this series, I have expounded the essence of the pointer. Today we are going to learn a pointer to the address of another pointer. Let's look at the concept of pointers first!

When we program the following declaration variables:

short int i;

Char A;

short int * PI;

The program opens up space for variables in an address space on memory, as shown in the following illustration.

Memory address →6 7 8 9 10 11 12 13 14 15

------------------- ------------------------------------------------------------------

... |　　|　　|　　|　　|　　|　　|　　|　　| |

--------------------------------------- ----------------------------------------------

|short int i |char a| |short int * pi|

As you can see in the figure:

The i variable occupies two bytes in the location of memory address 5.

The A variable occupies a byte at the location of memory address 7.

The pi variable occupies two bytes at the location of memory address 9. (Note: Pi is the pointer, my pointer here is only two bytes wide, 32-bit system is four bytes)

Next, assign the following values:

i=50;

pi=&i;

After the two sentences are assigned, the memory image of the variable is as follows:

Memory address →6 7 8 9 10 11 12 13 14 15

----- ---------------------------------------------------------------------------------

... |　　50 |　　|　　　 |　　6 |　　|　　| |

----------- ---------------------------------------------------------------------------

|short int i |char a| |short int * pi|

See no: The value of the short integer pointer variable pi is 6, which is the memory start address of the I variable. So, when we read and write to *pi, it is actually read and write to the I variable. Such as:

*pi=5//is equivalent to i=5;

You can look back at the second article in this series, where there are more detailed explanations.

Second, the pointer's address and a pointer to another pointer address

In the previous section, we saw that the pointer variable itself is also in a memory address like any other variable, such as PI's memory start address is 10. Similarly, we may have a pointer to this address.

Look at the following code:

Short int * * PPI;//This is a pointer to the pointer, note that there are two *

ppi=π

The first sentence: the 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 that is to take PI address, ppi=π is the PI address assigned to the PPI. The address value 10 is assigned to PPI. The following figure:

Memory address →6 7 8 9 10 11 12 13 14 15

------------------------------------------------------------------------ ------------

... |　　50 |　　|　　|　　6 |　　 10 | |

---------------------------------------------------------------------------------- --

|short int I|char a| |short int * Pi|short int * * ppi|

As you see from the diagram, the content of the pointer variable PPI is the starting address of the pointer variable pi. So......

How much is the PPI value? --10.

What is the value of *ppi? --6, which is the value of pi.

What is the value of **ppi? --50, which is the value of I, is also the value of *PI.

Oh! Needless to say too much, I believe you should understand this kind of pointer!