Personal home page ： Welcome to ——> Populus euphratica under the desert

  Ladies and gentlemen , It's beautiful

  If you think the article is helpful to you

  You can support bloggers with one key and three links

  Your every care is the driving force of my persistence

 

 ： This issue focuses on ： Knowledge of file operation

   I hope everyone will study and work happily every day . 

What is a file ？ Why do you need documents ？ What is the filename ？

The files on disk are files , But in programming , There are two kinds of documents we usually talk about ： Program files 、 Data files （ Classified from the perspective of file function ）.

Program files ：

Include source files （ The suffix is .c ） , Target file （ windows Environment suffix is .obj ） , Executable program （ windows Environment suffix is .exe ）.

Data files ：

The content of the file is not necessarily a program , It's the data that the program reads and writes when it runs , For example, the file from which the program needs to read data , Or output content file

This is called file .

Our general methods of data persistence are , Store the data in a disk file 、 Stored in the database . Using files, we can store data directly on the hard disk of the computer , Data persistence is achieved , That's why we use files .

A file should have a unique file ID , So that users can identify and reference , The document ID contains 3 part ： File path + File name trunk + file extension , Generally, we also call the file ID file name .

  The concept of flow ：

Flow is a channel formed between a system and a program . The two are one-to-one , Flow expresses the ability to interact between the two . So-called “ANSI C When the program is running, the system must provide three streams ,stdin,stdout,stderr”, In other words, the system must provide these three “ passageway ”, namely

1, Program and standard input （ Default keyboard ） The passage between        stdin

2, Program and standard output （ The default display ） The passage between    stdout

3, Program and terminal （ The default is also the display ） The passage between    stderr

And we need to Today's focus is on file flow , use FILE* To point to Memory for subsequent operations .

Opening and closing of files

Files should be read and written first Open file , After use, you should Close file . In programming , While opening the file , Will return to one FILE* A pointer variable to the file , It is also equivalent to establishing a finger Relationship between needle and document . ANSIC To prescribe the use of fopen Function to open a file ,fclose To close the file .

  fopen The front is the file name , You need some specific strings later , and fclose Just close the file pointer variable .

File read and write operations

We can't just know how to open and close files , We should also know how to read and write files .

The above is the commonly used file operation function

Now let's practice ：

Put the numbers 0~9 write file ：

int main()
{
	FILE *p = fopen(" Populus euphratica .dat", "w");
	if (NULL == p)
	{
		perror("main:");
		exit(-1);
	}

	for (char i = '0'; i <= '9'; i++)
	{
		fputc(i,p);
	}

	fclose(p);
	p = NULL;
	return 0;
}

 

The open file is in write only form "w" , The writing character uses fputs function , Then I used a loop to 0~9 Each character is inserted into the file separately .

  Read the contents of the file

int main()
{
	FILE *p = fopen(" Populus euphratica .dat", "r");
	if (NULL == p)
	{
		perror("main:");
		exit(-1);
	}

	char a[15] = { 0 };

	fgets(a, 11, p);

	puts(a);

	fclose(p);
	p = NULL;
	return 0;
}

  Here's a small problem , Why? fgets Function to use 11 Well ？ Altogether 10 A digital , Read 11 Time ？

Because the default copy '\0' , Is to copy the end of string flag , You can see through debugging .

  It can be seen that , If it's a copy 10 Time , Then just copy to 8 It stopped , That is, the last one will copy by default '\0'

So use fgets when , More elements than the actual copy 1, in other words , You want to copy 10 Elements , that fgets It should be written in 11.

Read and write in binary

We want to write some confidential documents in the file , You can write... In binary in a file .

Let's take the structure as an example , To facilitate observation ：

typedef struct stu
{
	char name[20];
	int age;
	int ID[10];
}stu;

int main()
{
	FILE *p = fopen(" Under the tree .dat", "wb");
	if (NULL == p)
	{
		perror("main:");
		exit(-1);
	}
	
	stu s1 = { " Under Populus euphratica ", 18, { 1, 2, 3, 4, 5, 6, 7, 8, 9 } };

	fwrite(&s1, sizeof(s1), 1, p);
	

	fclose(p);
	return 0;
}

  Binary write , We don't understand , But the file knows .

Let's use it Binary on Of .

int main()
{
	FILE *p = fopen(" Under the tree .dat", "rb");
	if (NULL == p)
	{
		perror("main:");
		exit(-1);
	}

	stu s2 = { 0 };

	fread(&s2, sizeof(s2), 1, p);

	fclose(p);
	return 0;
}

  So that you can get through , Binary readout , The file is to remember .

matters needing attention

1. First of all, we need to know that when a file is operated , First of all, consider Open mode , Is it read-only or write only or in other ways .

2. Judge Whether the file was opened successfully .

3. To determine how to write a file , Whether to enter one by one or by line , Or output , These functions are different .

4. At the end of the day Close file .

Next up ：

The next issue will go on to explain the knowledge of document series .

The next issue is wonderful ~！~！~！