What are the command keywords contained in files in C language?

The keyword of the file inclusion command is "include". The file inclusion command "#include" is a commonly used preprocessing command in C language programs. It is used to introduce the corresponding header file (".h" file), and its format is "#include "Header.h"".

The operating environment of this tutorial: windows7 system, c99 version, Dell G3 computer.

#include is called the file inclusion command , which is used to introduce the corresponding header file (.h file). #include is also a type of C language preprocessing command. The processing process of

#include is very simple, which is to insert the contents of the header file into the location of the command, thereby connecting the header file and the current source file into one source file, which is the same as copying Pasting has the same effect. There are two ways to use

#include, as follows:

#include <stdHeader.h>
#include "myHeader.h"

Use angle brackets and double quotes " "The difference between is that the search path for the header file is different:

• Use angle brackets , the compiler will search for the header in the system path File;

• When using double quotes " ", the compiler first searches for the header file in the current directory. If not found, it then searches for it in the system path.

In other words, using double quotes provides one more search path than using angle brackets, and its function is more powerful.

We have been using angle brackets to introduce standard header files, now we can also use double quotes, as shown below:

#include "stdio.h"
#include "stdlib.h"

stdio.h and stdlib.h are both standard header files , they are stored in the system path, so they can be successfully introduced using angle brackets and double quotes; and the header files we write ourselves are generally stored in the path of the current project, so angle brackets cannot be used and only double quotes can be used.

Of course, you can also add the directory where the current project is located to the system path, so that you can use angle brackets, but generally no one does this, it is purely unnecessary and thankless.

Notes on #include usage:

• A #include command can only contain one header file, multiple header files require multiple # include command.

• The same header file can be introduced multiple times. The effect of multiple introductions is the same as the effect of one introduction, because the header file has a mechanism at the code level to prevent repeated introduction.

• File inclusion allows nesting, which means that another file can be included in an included file.

Example:

##My.c contains code:

//计算从m加到n的和
int sum(int m, int n) {
    int i, sum = 0;
    for (i = m; i <= n; i++) {
        sum += i;
    }
    return sum;
}

The code contained in my.h:

//声明函数
int sum(int m, int n);

main.c The code contained in:

#include <stdio.h>
#include "my.h"
int main() {
    printf("%d\n", sum(1, 100));
    return 0;
}

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