Explanation of C language file operation function

introduction

We may want to store specific data relatively stably in the computer , And don't want the program to be destroyed at the end . This involves the problem of data persistence , 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 .

Want to achieve this function , File manipulation functions must be mastered , It can conveniently and quickly process the file accordingly .

file name

A file should 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 convenience , The file ID is often referred to as the file name .

The file pointer

First , We have to accept a concept , That is the operation of the program on the file （ That is, the operation of hardware ）, Not directly , Instead, create a file type structure , Operate the members in this structure to realize the operation of files .

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

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

Opening and closing of files

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

// Open file 
FILE * fopen ( const char * filename, const char * mode );
// Close file 
int fclose ( FILE * stream );

How files are used	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 , Suggest a new 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

Sequential reading and writing of files

The following functions should be used for 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

Let's look at a piece of code ：

int main()
{
	// Open file 
	FILE* pFile = fopen("myfile.txt", "w");

	// File operations 
	if (pFile != NULL)
	{
		fputs("fopen example\n", pFile);
		fputs("fopen example\n", pFile);
		fputs("fopen example\n", pFile);
		// Close file 
		fclose(pFile);
	}
	return 0;
}

FILE* pFile = fopen("myfile.txt", "w");

The first step is to open the file , Return an address to the file pointer pFile, Of course , It may open successfully or fail . If opening fails ,fopen Function will return NULL, So we have to judge

if (pFile != NULL) Only the return value is not NULL, The program will continue to operate on the file , Or it's over .

When the program successfully opens the file ,fputs("fopen example\n", pFile); Is to output a line of string to the file ,"fopen example" It's the content ,pFile Point to the corresponding file .

Finally close the file , Let's see the final result ：

  We can know , If you want to wrap , You need to add ‘\n’.

On the basis of the original, let's take a look fgets The use of ：

  The first is the way to open the file , It's read-only （"r"）,fgets(arr, 4, pFile); among arr Is the pointer to the array used to store the read string ,4 Is the maximum number of characters read （ It includes '\0'）, So only three characters are printed ,pFile It's a file pointer . however ,fgets At most one line can be read at a time .

Although the maximum number of read characters is set to 90, But what is read in the end is still the content before the newline character .

 fprintf and fscanf

fprintf Is to output formatted data to the corresponding stream ：

 fscanf Is to convert the formatted data from the stream into assigned variables ：

Random reading and writing of documents  

fseek

int fseek ( FILE * stream, long int offset, int origin );

Locate the file pointer according to its position and offset .

ftell

long int ftell ( FILE * stream );

Returns the offset of the file pointer from its starting position .

rewind

void rewind ( FILE * stream );

Return the file pointer to the beginning of the file