File -- first acquaintance

---

Preface

When to use files ？：
When we need to persist data , Need to use files . The general methods of persisting data are ： Store the data on the disk file , Store the data in the database .
Using files, we can directly store the data on the hard disk of the computer , Data persistence .

One 、 Classification of documents

Classification of documents in programming ：

The classification of documents in the form of data organization ：

Two 、 Some functions about files

Open file ：fopen
Close file ：fclose
Different opening methods of files correspond to different ：

1. Opening and closing of files

The code is as follows （ Example ）：

int main() {
    
	// Open file 

	FILE* pf;
	// Create a FILE*  File pointer to 
	pf = fopen("class1.txt", "w");
	// Use fopen Function to open a file 
	//"class1.txt" For file name  
	// Because we didn't create the file just now   So use "w" The way to create a file directly in the open 

	// Close file 
	fclose(pf);
	//fclose The function passes parameters to the file pointer to close pf
	pf = NULL;
	// Remember to set the closed file pointer to null 
	return 0;
}

After the code runs , You can find the file we created in the corresponding path class1.txt

2. Data input

1. Single character input

The code is as follows （ Example ）：

// Writing documents 
// Writing of a single character 
	fputc('w', pf);
// The characters in single quotation marks indicate what we want to write   The back pf To give pf Write in 

When the program runs , We can find that the file size just now becomes 1KB, After opening the file , You can see what we write .

2. Input of a string of characters

// Writing documents 
	fputs("wang", pf);
//fputs You can enter one line at a time 
// Note that the file should be opened in 'w'
// The line to be entered should use double quotation marks 

After program running , We can see the written content in the document wang

3. Data output

1. Single character output

	// Reading documents 
	int ret = fgetc(pf);
	printf("%c\n", ret);
	// Note that here is reading file   Therefore, the above file opening method should be changed to "w"

When the code runs , You can see the characters we read “w" Printed on the screen .

2. Output of a string of characters

char ret[10] = "";
fgets(ret, 3, pf);
printf("%s\n", ret);
// Note that here is reading file   The file should be opened in r
//fgets The function takes three arguments  
// The first parameter indicates where to put the content after reading 
// The second parameter indicates reading n-1 Characters 
// The third parameter indicates where to read 

After the program runs, we can see what it reads n-1 Characters are printed on the screen .

4. Binary

1. Reading of binary files

	fread(&pf, sizeof(char), 1, pf);
	printf("%c\n", pf);
	//fread Function requires four parameters 
	// first &fp Represents the address where the read-out data is stored 
	// the second sizeof(char) Represents the number of unit bytes of content to be read 
	// The third number 1 Delegate to read sizeof The number of bytes 
	// Address of the fourth read data 

Here we read 1 individual size by char The data of , So printing out is stored in class1.txt The first character in .

3、 ... and 、 Judgment of the end of the document

1.EOF function
EOF Function should be used to judge when the file reading is over , Whether it ends with the end of the file or with a read error .
2. End of reading text file
Whether the reading of text file is over , application fgetc Function to determine whether the return value is EOF Or use fgets Function to determine whether the return value is NULL;
3. The reading of binary file ends
Whether the reading of binary file is over , You should use fread Function to determine whether the return value is less than the number that should be read .