C language - Introduction - Foundation - syntax - [main function, header file] (II)

Generally speaking ,C A program of language , It is composed of header file and main function .

The header file

The following code is the header file of our program .

#include <stdio.h>

This is a preprocessing instruction , Tell the compiler to do some preprocessing before running . Generally speaking, it means stdio.h The contents of are imported into the first line intact .
If we want to realize some basic functions , for instance printf() Output , Then we must write this code at the beginning , Otherwise the program will go wrong . There is more than one header file

such as ：

• ctype.h Define character processing functions , For example, judge whether the character is a blank character 、 Character case conversion ;
• math.h Define data functions , For example, calculate the value of trigonometric function 、 exponential 、 logarithm 、 The absolute value 、 square 、 integer 、 To search for redundancy, etc ;
• stdio.h Define various inputs and outputs , Including standard input and output 、 File read and write 、 Format input and output, etc ;
• stdlib.h Define some general functions , For example, memory allocation and release 、 String and number types are converted to each other 、 Random function 、 Sort 、 Find functions, etc ;
• string.h Define string handler , For example, find the length of the string 、 String copy 、 String comparison 、 String lookup, etc ;
• time.h Define time and date processing functions , For example, get the current time .

notes ： The above are commonly used standard library header files , Other follow-up time-consuming introductions .

#include The difference between the two ways of writing instructions

• #incldue <stdio.h> Represents the address defined by the standard library in the system path stdio.h file ;
  -#include "stdio.h" Means to search for customized... Under the current program directory first stdio.h The header file , If you can't find it , Then go to the system path to find ;
• If you write your own header file , Use double quotes "", If it is a header file defined by a standard library or system , Use Angle brackets <>.

What is the main function ？

First , The main function main, A standard main function is as follows ：

int main()  //  This is the main function 
{
    
    return 0;   //  The return value of the main function 
}

main Translation into Chinese is the main 、 The most important meaning , And in the C The language represents a main function .（ Later we will discuss what is a function in computer language , And the main function in C The meaning of existence in language programs .） After the double slash is the comment , Make a more vivid analogy , The notes are similar to the teacher's notes on the exercise book .

Generally speaking , Comments are used to mark the purpose of this code or explain ideas . Because comments are not compiled as code , So no matter what comments are added , Will not have any impact on the actual operation of the code .

In the code above , Annotate where the main function is , Where is the return value of the function .

What is a function ？

Let's first talk about what is a function ？ In people's perception , Function is a noun in the field of Mathematics , It may be as shown in the figure below ：

however , This is just a function in the field of Mathematics , It is completely different from the functions in the programming language .

In programming languages , You can think of a function as a box , This box has the following features ：

• At the beginning of execution , Function can be input with some values
• In the process of execution , Functions can do some things
• After execution , Function can return some values

Let's take a look at our main function , Here 3 Features , What did they do .


among ,int Indicates the return value type of the function ,int yes integer（ Integers ） Abbreviation .main Is the function name ,main The brackets that follow () Inside is the input parameter , It is currently empty .return Followed by the return value of the function , by 0. and 0 It's an integer , And before the function name int Corresponding .
Let's summarize the formula of function ：

 Function return value type   Function name ( Function input parameter value )  
{
    
     Do something 
    return  Function return value ;
}

Write your own function

We might as well strike while the iron is hot , According to the above formula of function writing , Write a function that adds two integers . This function needs to do ： Enter two integers , Return the result of their addition . Since this function is used to calculate addition , We name the function add. Of course, the function name of the custom function can be written according to your preferences , Even if it's written as aaaaa It's OK . however , For function names to have semantics , It is convenient for people to read and understand , We usually use English as the function name .

//  This piece of code is called add Function definition of function 
int add(int a, int b)
{
    
    return a+b;
}

well , Then we finished writing a add Function . This piece of code is called add Function definition of function .

The main function is C The entrance to language programs

Above, we have defined one by ourselves add function , How do we use it ？add Functions can run directly ？, The answer is No .

be-all C Language codes have a starting entry , And this entry is the main function main. After entering the main function , To call other functions through the main function .

It also means that , Every C The language code , There can only be one main function . Let's modify the code a little , Now the code is as follows .

#include <stdio.h>
int add(int a, int b)  
{
    
    return a + b;
}

int main()
{
    
    int result;
    result = add(2, 3);
    printf("%d", result);
    return 0;
}

When the program is running , First, we will enter the main function main. Then call what we just wrote add Function . We passed on 2 The values are integers 2 and 3 Of add function . The definition of function stipulates , Pass on 2 Parameters , When we call , It must also be passed on 2 individual , And the type also needs to be consistent , Otherwise the compilation will report an error .

So naturally , We will think , Who called the main function ？ The return value of the main function must be int Do you ？ The main function is called automatically at the beginning of the program , There is no need to actively call the main function in the program . The return value of the main function will be returned to the program calling this program .C The language standard stipulates that the main function has a return value and must be int. If the program ends normally , Generally, the return value is set to 0.

Call function , You must know the function first

Let's see , What does the compiler mean add Of this identifier . The compiler starts with code , Read the code from top to bottom . The compiler first sees the definition of a function , Describes a man called add Function of . next , stay main Required in add, Because the compiler already knows add The definition of , Therefore, the compiler can compile normally .

however , What if the function definition and function call are reversed ？ First , The compiler sees add identifier , The compiler will be confused ,add What is it? ？ The compiler cannot understand add What is it . therefore , The compiler will report an error , And stop compiling .

Let's see how it works in software

summary ： That is to say c All functions in must be declared in Main Function

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