File operation in C language

Preface ： We know that a program is running , Data is stored in memory , When you exit the program , The data is destroyed , The next time you run the program, you have to re-enter the data , If we want to reach the program exit , The data still exists , The data is not destroyed until we choose to delete it , How to do it ？ This involves how to write data into the hard disk of the computer , In order to achieve the desired effect , It also achieves data persistence .

FILE*

First we need to understand this FILE The pointer , It is a file pointer that is used to create a FILE* To access a file , Generally, we use this file pointer to find the file associated with it .
How to open and close a file ,ANSIC To prescribe the use of fopen Function to open a file , use fclose Function to close the file .
## fopen The function prototype

## fclose The function prototype

When we know the function used to open the file, we also need to know how to open the file , The following figure will introduce one by one :

#include<stdio.h>
int main()
{
    
	FILE* pf;                   // Create a file pointer type variable 
	pf = fopen("li.txt", "w");	// Open file , If it does not exist, create a new 
	if (pf == NULL)             // Judge fopen Whether the return value of is a null pointer , Failure will return NULL The pointer 
	{
    
		perror("fopen:");
		return 1;
	}
	fclose(pf);                // Close file 
	pf = NULL;                 // The pointer is set to null 
	return 0;
}

Here, after the program runs , At this point, a new file will be created under our path .

Sequential reading and writing of files

Sequential read / write functions and functions of files ：

#include<stdio.h>
int main()
{
    
	FILE* pf = fopen("li.txt", "w");     // Open the file as a write 
	if (pf == NULL)                      
	{
    
		perror("fopen:");
		return 1;
	}
	char ch = 'a';
	for (ch = 'a'; ch <= 'z'; ch++)   
	{
    
		fputc(ch, pf);                // Enter information characters into this file 'a'~'z'
	}
	fclose(pf);                       // Close file 
	pf = NULL;
	return 0;
}

Random reading and writing of documents

1.fseek The function prototype

This function locates the file pointer according to the position of the file pointer and the offset from the starting position .

#include<stdio.h>
int main()
{
    
	FILE* pf = fopen("li.txt", "r");   // Because we put characters in this file 'a'~'z'
	if (pf == NULL)
	{
    
		perror("fopen:");
		return 1;
	}
	int ch = fgetc(pf);               // So this is a
	printf("%c\n", ch);

	fseek(pf, 3, SEEK_SET);           // Offset backward from the starting position 3, Point to d

	ch = fgetc(pf);
	printf("%c\n", ch);             

    ch = fgetc(pf);                  // Last time the pointer pointed to d, Walk backwards, this time pointing to e
	printf("%c\n", ch);

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

verification ：

2.ftell The function prototype

This function is used to calculate the offset of the file pointer from the starting position at this time , And back to it .

#include<stdio.h>
int main()
{
    
	FILE* pf = fopen("li.txt", "r");   // Because we put characters in this file 'a'~'z'
	if (pf == NULL)
	{
    
		perror("fopen:");
		return 1;
	}
	int ch = fgetc(pf);               // So this is a
	printf("%c\n", ch);

	fseek(pf, 3, SEEK_SET);           // Offset backward from the starting position 3, Point to d

	ch = fgetc(pf);
	printf("%c\n", ch);             

    ch = fgetc(pf);                  // Last time the pointer pointed to d, Walk backwards, this time pointing to e
	printf("%c\n", ch);

	long flag = ftell(pf);           // Calculate the offset 
	printf(" The offset for the starting position is :%ld\n", flag);// The offset for the 5

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

verification ：

3.rewind The function prototype

This function returns the position of the file pointer to the starting position

#include<stdio.h>
int main()
{
    
	FILE* pf = fopen("li.txt", "r");   // Because we put characters in this file 'a'~'z'
	if (pf == NULL)
	{
    
		perror("fopen:");
		return 1;
	}
	fseek(pf, 3, SEEK_SET);           // Offset backward from the starting position 3
	long flag = ftell(pf);           // Calculate the offset 
	printf(" The offset for the starting position is :%ld\n", flag);

	rewind(pf);                      // Use rewind Function returns the file pointer to the starting position 
	
	flag = ftell(pf);                // Calculate the offset again 
	printf(" The offset for the starting position is :%ld\n", flag);// Here is 0

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

verification ：