[advanced C language] file operation (II)

1、 Random reading and writing of files

1.1、fseek function

fseek Function function , Locate the file pointer according to its position and offset

Random read file

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main()
{
    
	FILE* pf = fopen("text.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
	}
	// Reading documents 
	int ch = fgetc(pf);
	printf("%c\n", ch);
	ch = fgetc(pf);
	printf("%c\n", ch);
	// Locate file pointer 
	//fseek(pf, 1, SEEK_CUR);// Three parameters ( flow 、 Offset 、 File pointer location )
	//fseek(pf, 3, SEEK_SET);
	fseek(pf, -1, SEEK_END);
	ch = fgetc(pf);
	printf("%c\n", ch);

	fclose(pf);
	pf = NULL;

	return 0;
}

Write files randomly

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main()
{
    
	FILE* pf = fopen("text.txt", "w");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
	}
	// Writing documents 
	int ch = 0;
	for (ch = 'a'; ch <= 'z'; ch++)
	{
    
		fputc(ch, pf);
	}
	// Locate file pointer 
	fseek(pf, -1, SEEK_END);// You can modify what you write in the specified location 
	fputc('#', pf);

	fclose(pf);
	pf = NULL;

	return 0;
}

1.2、ftell function

ftell The offset() function tells you the offset of the current file pointer （ Tell you the current file pointer position ）

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main()
{
    
	FILE* pf = fopen("text.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
	}
	// Reading documents 
	int ch = fgetc(pf);
	printf("%c\n", ch);
	ch = fgetc(pf);
	printf("%c\n", ch);

	int ret = ftell(pf);
	printf("%d\n", ret);

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

1.3、rewind function

rewind The file pointer offset() function sets the file pointer offset to 0, The file pointer points back to the beginning of the file

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main()
{
    
	FILE* pf = fopen("text.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
	}
	// Reading documents 
	int ch = fgetc(pf);
	printf("%c\n", ch);
	ch = fgetc(pf);
	printf("%c\n", ch);

	int ret = ftell(pf);
	printf("%d\n", ret);

	rewind(pf);
	ret = ftell(pf);
	printf("%d\n", ret);

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

2、 Judge whether the file reading is over

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

for example ：
fgetc Judge whether it is EOF;
fgets Determine whether the return value is NULL;

2、 Binary file reading end judgment , 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

3、 File buffer

When documents are stored , The data will be stored in the buffer first , After the buffer is full , Write to file at one time

The same is true when reading data from a file , The data will be stored in the buffer first , After the buffer is full , Read it at one time

As long as you close the file , Will flush the buffer