[C Advanced] file operation (2)

Catalog

1、 Random reading and writing of documents

（1）fseek

（2）ftell

（3）rewind

2、 Text files and binaries

3、 Determination of the end of file reading

（1） Misused feof

4、 File buffer

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1、 Random reading and writing of documents

（1）fseek

Create a file test.txt, There is... In it abcdef Such a string , We were reading the document before , The initial file pointer is to a Of , Read a character pointing to b, Read another point c…… At this time, it is read in sequence , This is the sequential reading and writing of files . Let's review ：

#include<stdio.h>
int main()
{
	// Open file 
	FILE* pf = fopen("test.txt", "r");
	if (pf == NULL)
	{
		perror("fopen");
		return 1;
	}
	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	ch = fgetc(pf);
	printf("%c\n", ch); //b
	ch = fgetc(pf);
	printf("%c\n", ch); //c
	// Close file 
	fclose(pf);
	pf = NULL;
	return 0;
}

Can we read in the way we want , For example, read directly d, At this point, a function of file pointer positioning is involved ：fseek

• fseek： Move a file pointer to a specific location

Locate the file pointer according to its position and offset .

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

• Understand the three parameters ：

1. stream： For which stream
2. offset： Offset
3. origin： The starting position

• There are three options for starting position ：

• SEEK_CUR： 

1、 Shift the file forward from its current location 1 Bytes ：

	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	// Adjust file pointer 
	fseek(pf, -1, SEEK_CUR);
	ch = fgetc(pf);
	printf("%c\n", ch); //a
	ch = fgetc(pf);
	printf("%c\n", ch); //b

2、 Let files from The current position Offset backward 2 Bytes ：

	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	// Adjust file pointer 
	fseek(pf, 2, SEEK_CUR);
	ch = fgetc(pf);
	printf("%c\n", ch); //d
	ch = fgetc(pf);
	printf("%c\n", ch); //e

• SEEK_END：

From file At the end of Start to shift forward 2 byte  

	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	// Adjust file pointer 
	fseek(pf, -2, SEEK_END);
	ch = fgetc(pf);
	printf("%c\n", ch); //e
	ch = fgetc(pf);
	printf("%c\n", ch); //f

• SEEK_SET：

From file The starting position Start to shift back 3 byte

	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	// Adjust file pointer 
	fseek(pf, 3, SEEK_SET);
	ch = fgetc(pf);
	printf("%c\n", ch); //d
	ch = fgetc(pf);
	printf("%c\n", ch); //e

（2）ftell

Returns the offset of the file pointer from its starting position

long int ftell ( FILE * stream );

• for example ：

	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	// Adjust file pointer 
	fseek(pf, 3, SEEK_SET);
	ch = fgetc(pf);
	printf("%c\n", ch); //d
	int ret = ftell(pf);
	printf("%d\n", ret);//4

（3）rewind

Return the file pointer to the beginning of the file

void rewind ( FILE * stream );

• for example ：

	// Read the file 
	int ch = fgetc(pf);
	printf("%c\n", ch); //a
	// Adjust file pointer 
	fseek(pf, -1, SEEK_END);
	ch = fgetc(pf);
	printf("%c\n", ch); //f
	// Return the file to its starting position 
	rewind(pf);
	ch = getc(pf);
	printf("%c\n", ch); //a

2、 Text files and binaries

According to the organization of data , Data files are called text file perhaps Binary .

Data is stored in memory in binary form , If the output without conversion is to external memory , Namely Binary .

If it's required to use ASCII In the form of code , You need to convert before storing . With ASCII The file stored in the form of characters is text file .

• How is a data stored in memory ？

All characters are written in ASCII stored , Numerical data can be used either ASCII stored , It can also be stored in binary form .
If there are integers 10000, If the ASCII Code output to disk , The disk is occupied by 5 Bytes （ One byte per character ）, And binary output , On the disk 4 Bytes （VS2013 test ）.

• test ：（ Store in binary form ） 

#include <stdio.h>
int main()
{
	int a = 10000;
	FILE* pf = fopen("test.txt", "wb");
	if (pf == NULL)
	{
		perror("fopen");
		return 1;
	}
	fwrite(&a, sizeof(int), 1, pf);// The binary form is written to the file 
	fclose(pf);
	pf = NULL;
	return 0;
}

We put test.txt Put the contents of the document in VS2022 Look inside the compiler ：

• 10000 Binary system ：0000 0000 0000 0000 0010 0111 0001 0000
• Convert it to hexadecimal ：00 00 27 10

3、 Determination of the end of file reading

（1） Misused feof

Keep in mind ： During file reading , Out-of-service feof The return value of the function is directly used to determine whether the end of the file .

feof It is applied when the file reading ends , The judgment is that the read failed and ended , Or end of file .

1. Whether the reading of text file is finished , Determine whether the return value is EOF （ fgetc ）, perhaps NULL （ fgets ）

• for example ：

1. fgetc Judge whether it is EOF .
2. fgets Determine whether the return value is NULL .

2. Judgment of reading end of binary file , Judge whether the return value is less than the actual number to be read .

• for example ：

fread Judge whether the return value is less than the actual number to be read .

•   Example ：

Suppose we put a piece of code in test.txt In file

  Now write the code test.txt A copy of the document , Generate test2.txt file

#include<stdio.h>
int main()
{
	FILE* pfread = fopen("test.txt", "r");
	if (pfread == NULL)
	{
		return 1;
	}
	FILE* pfwrite = fopen("test2.txt", "w");
	if (pfwrite == NULL)
	{
		fclose(pfread);
		return 1;
	}
	// File opened successfully 
	// Read and write files 
	int ch = 0;
	while ((ch = fgetc(pfread)) != EOF)
	{
		// Writing documents 
		fputc(ch, pfwrite);
	}
	// Judge why it ended 
	if (feof(pfread))
	{
		printf(" End of file flag encountered , The file ends normally \n");
	}
	else if (ferror(pfread))
	{
		printf(" File read failed end \n");
	}
	// Close file 
	fclose(pfread);
	pfread = NULL;
	fclose(pfwrite);
	pfwrite = NULL;
	return 0;
}

And test.txt The contents of the document are exactly the same  

4、 File buffer

It means that the system automatically opens up a block in memory for each file being used in the program “ File buffer ”. Data output from memory to disk is first sent to a buffer in memory , After the buffer is filled, it is sent to the disk together . If you read data from disk to computer , Then read the data from the disk file and input it into the memory buffer （ Fill the buffer ）, And then send the data from the buffer to the program data area one by one （ Program variables, etc ）. The size of the buffer depends on C The compiler system decides .

•   Purpose ： Improve the efficiency of the operating system .
• Example ：

#include <stdio.h>
#include <windows.h>
int main()
{
	FILE* pf = fopen("test.txt", "w");
	fputs("abcdef", pf);// Put the code in the output buffer first 
	printf(" sleep 10 second - The data has been written , open test.txt file , Found no content in the file \n");
	Sleep(10000);
	printf(" Refresh buffer \n");
	fflush(pf);// When the buffer is flushed , Write the data in the output buffer to a file （ disk ）
	// notes ：fflush  In high version VS It can't be used on 
	printf(" Sleep again 10 second - here , Open again test.txt file , There's something in the file \n");
	Sleep(10000);
	fclose(pf);
	// notes ：fclose When closing a file , It also flushes the buffer 
	pf = NULL;
	return 0;
}

 

•   Conclusion ：

Because there is a buffer ,C Language when operating files , You need to flush the buffer or close the file at the end of the file operation . If you don't do , May cause problems in reading and writing files .