[C language] file operation

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1. Why use files ？

for instance , For example, we are writing our address book , Student management system , Parking lot management system , When the program runs , We can increase 、 The function of deleting data , At this time, the data is stored in memory , When the program exits , The data in the program naturally does not exist , The developed memory is also returned to the operating system , The next time you run the address book program , The data has to be re entered , It's complicated .
And the role of documents is ： When using files, we can write data into a file , Directly stored on the hard disk of the computer , Read the data in the file directly when you want to use it next time , You can get the data written before , Data persistence is achieved .

2. What is a document ？

A file on disk is a file .
Generally in programming , We divide documents into two kinds ： Program files and data files .

2.1 Program files

Include source files （ The suffix is .c）, Target file （windows Environment suffix is .obj）, Executable program （windows Environmental Science
The suffix is .exe）.

2.2 Data files

The content of the file is not necessarily a program , It's the data that the program reads and writes when it runs , For example, the file from which the program needs to read data , Or output content file .

2.3 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 example ： c:\code\test.txt
c:\code\ Is the location of the file , Also known as file path
test Is the file name trunk
.txt It's a file suffix

2.4 File operation steps

1. Open file
2. read / Writing documents
3. Close file

3. Opening and closing of files

3.1 The file pointer

Buffer file system , The key concept is “ File type pointer ”, abbreviation “ The file pointer ”.

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

stay VS2013 in , There are the following Document type declaration

struct _iobuf {
    
    char *_ptr;
    int  _cnt;
    char *_base;
    int  _flag;
    int  _file;
    int  _charbuf;
    int  _bufsiz;
    char *_tmpfname;
   };
typedef struct _iobuf FILE;

Every time you open a file , The system will automatically create a FILE Structure variables of structure type , And fill in the information . We usually pass FILE The pointer maintains this FILE Structural variables . File pointer variable ：FILE* pf.
pf Point to the file information area of a file （ Structural variable ）, The file can be accessed through the information in the file information area . in other words , Through the file pointer variable, you can find the file associated with it .

3.2 Open mode

3.3 Sequential reading and writing of files

3.4 Read and write characters

fputc
int fputc( int c, FILE *stream );

#include<stdio.h>
#include<string.h>
#include<errno.h>
int main()
{
    
	FILE* pf = fopen("test.txt", "w");// Open the file as written 
	if (pf == NULL)
	{
    
		printf("%s\n",strerror(errno));//No such file or directory
		return 1;
	}
	// Writing documents 
	char i = 'a';
	for (i = 'a'; i <= 'z'; i++)
	{
    
		fputc(i, pf);// Character output function 
	}
	fclose(pf);
	pf = NULL;

	return 0;
}

result ：

fgetc
int fgetc( FILE *stream );

#include<stdio.h>
#include<string.h>
#include<errno.h>
int main()
{
    
	FILE* pf = fopen("test.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 1;
	}
	int ch;
	while ((ch = fgetc(pf)) != EOF)
	{
    
		printf("%c", ch);
	}
	fclose(pf);
	pf = NULL;
	return 0;
}

3.5 Read and write a line of data

Note that in the next write , Previously written data will be destroyed , Don't want the previous data to be destroyed , You can choose "a" Append function
fputs- Write string

#include<stdio.h>
#include<string.h>
#include<errno.h>
int main()
{
    
	FILE* pf = fopen("test.txt", "w");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
	}
	fputs("hello world", pf);
    // read ： You need to create a new array 
	char arr[100] = {
     0 };
	fgets(arr, 6, pf);
	printf("%s\n", arr);//hello
	
	fclose(pf);
	pf = NULL;
	
	return 0;
}

3.6 Read and write text data

fprintf- Text writes data
fscanf- Text read data

#include<stdio.h>
#include<string.h>
#include<errno.h>
struct a
{
    
	char name[20];
	int age;
	float score;
};
int main()
{
    
	struct a s = {
     "zhangsan","10",50.5f };
	FILE* pf = fopen("test.txt", "w");
	if (pf == NULL)
	{
    
		perror(pf);
		return 1;
	}
	// Write text data 
	fprintf(pf, "%s %d %f", s.name, s.age, s.score);
	fclose(pf);
	pf = NULL;
	return 0;
}

3.7 Binary read-write

size_t fwrite( const void *buffer, size_t size, size_t count, FILE *stream );

// Binary read-write 
#include<stdio.h>
#include<string.h>
#include<errno.h>
struct a
{
    
	char name[20];
	int age;
    float score;
};
int main()
{
    
	//struct a s = { "zhangsan","10",50.5f };
	struct a s = {
     0 };
	FILE* pf = fopen("test.txt", "rb");
	//FILE* pf = fopen("test.txt", "wb");

	if (pf == NULL)
	{
    
		perror(pf);
		return 1;
	}
	// Binary write 
	//size_t fwrite( const void *buffer, size_t size, size_t count, FILE *stream );
	//fwrite(&s, sizeof(struct a), 1, pf);
	fread(&s, sizeof(struct a), 1, pf);
    printf("%s %d %f", s.name, s.age, s.score);


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

4. File stream

Any one of them C Language program , As long as it runs, it will open by default 3 A flow ：
FILE* stdin- Standard input stream （ keyboard ）
FILE* stdout- Standard output stream （ The screen ）
FILE* strerr- Standard error flow ( The screen )
such as ：fprintf(stdout,“%d”,12);

5. other

scanf It is a formatted input statement for standard input
printf It is a formatted output statement for standard output
fscanf It is a formatted input statement for all input streams
fprintf Is a formatted output statement for all output streams
sprintf Convert a formatted data into a string
sscanf Convert a format into formatted data

struct S
	{
    
		char arr[10];
		int age;
		float score;
	};
	
	int main()
	{
    
		struct S s = {
     "zhangsan", 20, 55.5f };
		struct S tmp = {
     0 };
		char buf[100] = {
     0 };
		// hold s The formatted data in is converted into a string and placed in buf in 
		sprintf(buf, "%s %d %f", s.arr, s.age, s.score);
	
		//"zhangsan 20 55.500000";
		printf(" character string ：%s\n", buf);
	
		// From a string buf Get a formatted data to tmp in 
		sscanf(buf, "%s %d %f", tmp.arr, &(tmp.age), &(tmp.score));
		printf(" format ：%s %d %f\n", tmp.arr, tmp.age, tmp.score);
		return 0;
	}

6. Text files and binaries

According to the organization of data , Data files are called Text file or binary file .
Data is stored in memory in binary form , If the output without conversion is to external memory , Binary files .
If it's required to use ASCII In the form of code , You need to convert before storing . With ASCII A file stored in the form of characters is a text file .

#include<stdio.h>
int main()
{
    
	int a = 10000;
	FILE* pf = fopen("test.txt", "wb");
	fwrite(&a, 4, 1, pf);// The binary form is written to the file 
	fclose(pf);
	pf = NULL;

	return 0;
}

Binary values

ASCII Code form

7. Determination of the end of file reading

During file reading , Out-of-service feof The return value of the function is directly used to determine whether the end of the file .
Instead, it applies 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 ：
   fgetc Judge whether it is EOF .
   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 .

8. File buffer

ANSIC The standard is “ Buffer file system ” Processing of data files , The so-called buffer file system 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 memory
Impact area （ 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 .