C language functions and pointers

function

An independent function module

2、 Why use functions

2.1 Make the program modular

2.2 Improve code reuse rate

3、 Classification of functions

3.1 Library function

3.1.1、 Import header file

#include <stdio.h>   #include <string.h>

3.1.2、 Call function

printf（）;

Be careful ： How many parameters are there , What is the data type , Return value

Function name （ List of actual parameters ）;

3.2 Custom function

3.2.1 Function definition

Storage type   data type   Function name （ List of formal parameters ）

{

       The body of the function ;

       Return value ;

}

Storage type ：auto、register、extern、static

data type ： The return value of the function （ The return value of the function is not written, and the default is int）

Function name ： See the name and know the meaning

List of formal parameters ： Parameters needed to realize the function , Need to call the incoming

The body of the function ： Implementation of specific functions

Return value ： If there is no return value , This sentence can be omitted , If there is a return value , There is only one return value

3.2.2 Function call

Function name （ List of actual parameters ）

Be careful ：

1. You need to pass the value of the argument to the formal parameter , The number and data type of arguments must be consistent with formal parameters
2. Arguments can be variables 、 Constant 、 It can also be an expression , It must be a definite number
3. Arguments and formal parameters are two independent spaces
4. Parameter passing is actually copying the value of the argument to the formal parameter
5. Parameter is a local variable , Defined when a function is called , Free space at the end of the function call

a、 Invocation time

b、 Call end

c、 Function declaration

If not in main Function , It needs to be in main Function is declared before

Statement ： Is to copy the function header directly and add a semicolon

Before making a statement ：

Add the statement ：

The role of declaration ： Help compiler do syntax check

4、 The pointer

1、 Concept

A pointer is a data type , Is a data type that saves addresses

int ： It's a data type , Is a data type that holds integers 3  4  5

float： It's a data type , Is a data type that holds floating-point numbers 1.23  3.24

char： It's a data type , Is a data type that stores characters ‘a’  , ‘b’

2、 What is an address

The minimum unit of memory allocation is bytes , Each byte has a number , This number is called address

The essence of address ： Number of memory unit

The pointer ： The pointer is the address

The nature of the pointer ： Number of memory unit

3、 Pointer to the variable

int a;// Integer variables

float b;// Floating point variables

Pointer to the variable ： Dedicated to storing addresses （ Number of memory unit ） The variable of

4、 Definition of pointer

Storage type   data type * Pointer variable name ;

int *p;

Storage type ：auto 、register、static、extern

data type ： The data type pointed to by the pointer ( Get rid of * And variable names are the data types pointed to by the pointer )  //int

The data type of the pointer : data type *    //int *

& and * They are inverses of each other

5、 Assignment of pointer

Be careful ：（1） The direction of the pointer can be changed

     （2） When assigning a value to a pointer , Pay attention to type matching

reflection ？

1. What is the pointer
2. What's the address
3. What are pointer variables
4. How to define pointer variables
5. How to assign values to pointer variables
6. What can I do after pointer variable assignment

Be careful ： stay 32OS All the pointers in 4 Bytes

         stay 64OS All the pointers in 8 Bytes

6、 Null pointer

No pointer to （ The value is 0 The pointer to , It is considered that the pointer does not point to ）

Be careful ：0 No. address prohibits operation

To operate, you need to change the direction

7、 Wild pointer

I don't know where the pointer points

When local variables are not initialized , The value is a random value

Local pointer variables are wild pointers if they are not initialized

How to avoid the appearance of wild pointer ？ Initialize to NULL

int *p = NULL;

8、 Value passed

Realize the exchange of two numbers in the form of sub functions

9、 Address delivery