[C language series] - detailed explanation of file operation (Part 1)

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*  Blog column ：【C All living things 】*

Introduction to this article ： Detailed introduction C File operation part in language ！

Get to know the author ： Aspire to be a great programmer , Xiaobai, who is currently a sophomore in College .

Inspirational terms ： The tediousness of programming , Let's study together and become interesting ！

    Text begins

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Catalog

   Why use files ？

  What is a document ？

* Program files

* Data files

* file name

The file pointer

  Opening and closing of files

Sequential reading and writing of files

*  Output and input

*  fputc

*  fgetc

*  fgets And fputs

*  fprintf

 *  fscanf

 *  fwrite

 *  fread

Conclusion  

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   Why use files ？

We know that data is stored in memory , When the program exits , Data naturally does not exist , To achieve data persistence , It is necessary to store the data in disk files or databases .

Using files, we can store data directly on the hard disk of the computer , Data persistence .

  What is a document ？

A file on disk is a file .

But in programming , There are two kinds of documents ：1. Program files    2. Data files （ Classify from the perspective of file function ）.

* Program files

Source program 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 the file of output content .

* file name

A file must have a unique file id , So that users can identify and reference .

The filename contains 3 part ： File path + File name trunk ++ file extension

for example ： Code E:\C Advanced language \c-language-advanced-code\test_7_24\test.c

For convenience , The file ID is often referred to as the file name

The file pointer

Buffer file system , The key concept is “ File type pointer ”, abbreviation “ The file pointer ”.

Each used file has a corresponding file information area in memory , Used to store relevant information of files （ Such as the name of the document , The status of the file and the current location of the file ）. This information is stored in a structure variable , The structure type is system declared , named FILE.

for example ： stay VS2022 In the header file stdio.h There are the following file type declarations in ：

#ifndef _FILE_DEFINED
    #define _FILE_DEFINED
    typedef struct _iobuf
    {
        void* _Placeholder;
    } FILE;
#endif

  But as the VS The continuous upgrading of this compiler ,FILE The content contained in the type is more and more simplified , But they are similar .

Every time you open a file , The system will automatically create a FILE Structural variables , And fill in the information , Users don't have to care about details .

It's usually through a FILE To maintain this FILE Structural variables , This is more convenient to use .

Let's create a FILE* Pointer variable for ：

FILE* pf;// File pointer variable 

Definition pf It's a point FILE Pointer variable of type data . You can make pf Point to the file information area of a file （ It's a structure ）. The file can be accessed through the information in the file information area . in other words , Through the file pointer variable, you can find the file associated with it .

such as ：

  Opening and closing of files

The file should be opened before reading and writing , The file should be closed after use .

In programming , While opening the file , Will return to one FILE*   A pointer variable to the file , It is also equivalent to establishing the relationship between pointer and file .

ANSIC（ standard C） To prescribe the use of fopen Function to open a file ,fclose To close the file .

// Open file 
FILE* fopen(const char* filename, const char* mode);//（ file name , Open method ）
// Close file 
int fclose(FILE* stream);// File pointer variable name 

What are the opening methods ？ Look at these ：

Meaning of document use	meaning	If the specified file does not exist
“r”（ read-only ）	To enter data , Open an existing text file	error
“w”（ Just write ）	To output data , Open a text file	Create a new file
“a”（ Additional ）	Add data to the end of the text file	Create a new file
“rb”（ read-only ）	To enter data , Open a binary file	error
“wb”（ Just write ）	To output data , Open a binary file	Create a new file
“ab”（ Additional ）	Add data to the end of a binary file	error
“r+”（ Reading and writing ）	For reading and writing , Open a text file	error
“w+”（ Reading and writing ）	For reading and writing , Create a text file	Create a new file
“a+”（ Reading and writing ）	Open a file , Read and write at the end of the file	Create a new file
“rb+”（ Reading and writing ）	Open a binary file for reading and writing	error
“wb+”（ Reading and writing ）	For reading and writing , Create a new binary file	Create a new file
“ab+”（ Reading and writing ）	Open a binary file , Read and write at the end of the file	Create a new file

Let's write an example code ：

#include<stdio.h>
int main()
{
	FILE* pf = fopen("test.txt", "w");// Open in writing 
	if (pf == NULL)
	{
		ferror("File Fail");// Print error messages 
		return 1;// Erroneous return 
	}
	//  Read and write files 
	// ......
	
	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Sequential reading and writing of files

function	Function name	Apply to
Character input function	fgetc	All input streams
Character output function	fputc	All output streams
Text line input function	fgets	All input streams
Text line output function	fputs	All output streams
Format input function	fscanf	All input streams
Format output function	fprintf	All output streams
Binary input	fread	file
Binary output	fwrite	file

*  Input and output

*  Output and input

What is input ？ What is output ？ Enter where , Where does the output go ？ I believe many Lao tie will be confused , It doesn't matter. , Let's take a look at a picture , I understand. .

  good , At a glance , good , Next, we will explain the various functions of these functions ！

---

*  fputc

Let's first look at the function of this function ：

Function function instance 1：

#include<stdio.h>

int main()
{
	FILE* pf = fopen("test.txt", "w");
	if (pf == NULL)
	{
		ferror("fopen fail");
		return 1;
	}
	// Output the file 
	fputc('c', pf);

	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Run the output results ：

  We can see that this character is successfully written into the file .

---

*  fgetc

  Function function instance 2：

#include<stdio.h>

int main()
{
	FILE* pf = fopen("test.txt", "r");
	if (pf == NULL)
	{
		ferror("fopen fail");
		return 1;
	}
	// Input the file 
	char c = fgetc(pf);
	printf("%c\n", c);

	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Run the output results ：

  Print out the characters in the file on the screen .

---

*  fgets And fputs

I have not explained these two functions much here , They just changed the characters to be able to input and output strings , The key points are the following functions .

---

*  fprintf

Let's first look at the function of this function ：

  Function function instance code 3：

// Specify the format output function 
#include<stdio.h>
struct S
{
	char name[10];
	int age;
	float score;
};
int main()
{
	struct S s = { "tangcheng",20,99.9f };
	FILE* pf = fopen("test.txt", "w");
	if (pf == NULL)
	{
		perror("fopen");
		return 1;
	}
	//fprintf
	fprintf(pf, "%s %d %f", s.name, s.age, s.score);

	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Run the output results ：

Format one （ It's the structure ） The data of is output to the file .

---

 *  fscanf

First look at the function ：

  Function function code example 4：

#include<stdio.h>
struct S
{
	char name[10];
	int age;
	float score;
};
int main()
{
	struct S s = { 0 };
	FILE* pf = fopen("test.txt", "r");
	if (pf == NULL)
	{
		perror("fopen");
		return 1;
	}
	//fscanf
	fscanf(pf, "%s %d %f", s.name, &(s.age), &(s.score));
	// Print 
	fprintf(stdout,"%s %d %f\n", s.name, s.age, s.score);//stdout Is the standard output stream - The screen 

	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Running results （ Input the formatted data in the file into the screen ）：

---

 *  fwrite

The function of this function ：

function

fwrite                               // Where to put it           How many bytes               How many?             The file pointer

size_t fwrite ( const void * ptr, size_t size, size_t count, FILE * stream );

Write block of data to stream  

The sample code 5：

#include<stdio.h>
struct S
{
	char name[20];
	int age;
	float score;
};
int main()
{
	struct S s = { "zhangsan",20,50.5f };
	// Open file    Output to file in binary mode 
	FILE* pf = fopen("test.txt", "wb");
	if (pf == NULL)
	{
		perror("fopen");
		return 1;
	}
	//  Write in binary 
	fwrite(&s, sizeof(struct S), 1, pf);

	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

The result of running ：

Is it a little hard to understand , Because this is binary data , We don't need to understand , The compiler can understand it . How to enter it on the screen , Don't worry. , Let's introduce fread This function .

---

 *  fread

The functionality ：

function

fread                // Where to put it             How many bytes           Number                   The file pointer

size_t fread ( void * ptr, size_t size, size_t count, FILE * stream );

Read block of data from stream

The sample code 6：

#include<stdio.h>
struct S
{
	char name[20];
	int age;
	float score;
};
int main()
{
	struct S s = { 0 };
	// Open file   
	FILE* pf = fopen("test.txt", "rb");
	if (pf == NULL)
	{
		perror("fopen");
		return 1;
	}
	//  Read in binary 
	fread(&s, sizeof(struct S), 1, pf);
	// A print 
	printf("%s %d %f", s.name, s.age, s.score);

	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Running results ：

？？？ It's not a binary file , It shows that the compiler automatically recognizes that binary data is converted into formatted data that we can understand .

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Conclusion  

Family , There are too many details about file operation , It can only be divided into （ On ） and （ Next ） There are two parts to write ！！！