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 ： This issue focuses on ： Explanation of knowledge related to document operation

    The first part mainly talks about the sequential reading and writing of documents , Little friends who don't understand this point ——》  file 1

I hope everyone can study and work happily every day .

 

Let's move on to today's study ：

So how to read and write files randomly ?

So how to calculate the offset ？

How to make the file pointer point to the starting position ？

The difference between binary files and text files

Judgment of the end of file reading

Whether there is the concept of buffer ：

scanf and fscanf and sscanf

printf and fprintf function sprintf

fprintf and fscanf example

sscanf and sprint example ：

So how to read and write files randomly ?

Make the same fseek Function for random reading and writing , Let's look at the function prototype for analysis ：

  First , The first function parameter is the file stream , Nothing to say.

The second parameter is the offset , The offset has a relative position , Offset from file pointer position .

The third parameter is the file pointer position , There are three states

SEEK_CUR

The current location of the file pointer

SEEK_END

End of file pointer

SEEK_SET

File pointer start position

in summary ： We have to read and write randomly , It's at the position of the file pointer , Adding the offset can realize the random reading and writing of the file .

example ：

int main()
{
	FILE *p = fopen(" Populus euphratica .dat", "wb");
	if (p == NULL)
	{
		perror("main:");
		exit(-1);
	}

	fputs("Happy life", p);

	fseek(p, 6, SEEK_SET);

	fputs("every day", p);

	fclose(p);
	return 0;
}

  First write “Happy life”, Then we start at the beginning of the file pointer , The offset for the 6, To write , It will cover “Happy ” After that , It becomes "Happy every day".

Random reading of files ：

int main()
{
	FILE *p = fopen(" Populus euphratica .dat", "rb");
	if (p == NULL)
	{
		perror("main:");
		exit(-1);
	}

	char a[10] = { 0 };

	fseek(p, 4, SEEK_SET);

	fgets(a, 10, p);

	fclose(p);
	return 0;
}

  First, the file pointer is the starting position of the file , The offset for the 4, So, from ’y‘ Characters begin to read , Read back 9 Characters , because fgets Finally, I will read a '\0'.

So how to calculate the offset ？

We use it ftell Function to confirm the offset of the current position from the starting position .

The function prototype is

  Just pass in the file pointer of the current location .

Look at an example ：

int main()
{
	FILE *p = fopen(" Populus euphratica .dat", "rb");
	if (p == NULL)
	{
		perror("main:");
		exit(-1);
	}

	fseek(p, -3, SEEK_END);
	printf("%c",fgetc(p));

	int a = ftell(p);
	printf("\n%d\n", a);

	fclose(p);
	return 0;
}

Under the analysis of ： First, we find the end of the file pointer , Then the offset is -3 , Is to offset forward from the end of the file 3 Characters , Then read is d（'a'  'y'  ’\0‘）, At this time, the file pointer is located at 'a' It's about , The distance from the start of the file is 13 The offset .

How to make the file pointer point to the starting position ？

We can use rewind function To achieve

int main()
{
	FILE *p = fopen(" Populus euphratica .dat", "rb");
	if (p == NULL)
	{
		perror("main:");
		exit(-1);
	}

	fseek(p, -3, SEEK_END);
	printf("%c",fgetc(p));

	int a = ftell(p);
	printf("\n%d\n", a);

	rewind(p);

	a = ftell(p);
	printf("%d\n", a);

	fclose(p);
	return 0;
}

  Use rewind Then the offset of the file is 0 La .

The difference between binary files and text files

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 , Binary files . .
With ASCII A file stored in the form of characters is a text file .

All characters are written in ASCII stored , Numerical data can be used either ASCII stored , It can also be stored in binary form .

Let's take an example to understand ：

int main()
{
	int a = 10000;
	FILE* pf = fopen(" Under the tree .txt", "wb");
	fwrite(&a, 4, 1, pf);
	fclose(pf);
	pf = NULL;
	return 0;
}

We put 10000 Write to the file in binary , Let's have a look , First use Text form Call and have a look .

  We can't understand this , So we're going to Binary call See if you can understand .

  We don't seem to understand the two-level system , So let's see 10000 stay How to store... In memory . .

  We found that what is stored in memory seems to be the same as what is stored in binary . And it takes up 4 Bytes . We know that the display in text form is 10000 yes 5 Characters , That is to say 5 Bytes , In memory and binary, it is 4 Bytes .

Judgment of the end of file reading

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 .

It is Apply when the file reading is finished , 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 .

About fread Put... Under the description ：

  The return value is the number of reads , Parameters 1 It's a storage space , Parameters 2 Is to read each size , Parameters 3 Is the number to read , Parameters 4 Is a file stream . If The number of actual reads is less than the number to be read , Then it means that the file reading is over .

Whether there is the concept of buffer ：
 

ANSIC The standard is “ Buffer file system ” Processing of data files , The so-called buffered file system means that the system automatically creates files for programs in memory Open up a block for each file in use “ File buffer ” . Data output from memory to disk is first sent to a buffer in memory , loading When the buffer is full, 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 According to the C The compiler system decides .

Let's take a look at an example to prove ：

//VS2013 WIN10 Environmental testing 
int main()
{
	FILE*pf = fopen(" Under Populus euphratica .txt", "w");
	fputs("qwertyuiop", pf);// Put the code in the output buffer first 
	printf(" sleep 20 second - The data has been written , Open under the Populus euphratica .txt file , Found no content in the file \n");
	Sleep(20000);
	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 the poplar tree again .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;
}

 

 

scanf and fscanf and sscanf

Let's compare scanf and fsacnf and sscanf function

 

 

  These are the similarities between the three functions ,scanf It's called formatted input ,fscanf Is a formatted input function for all input streams ,sscanf Is to convert a string into formatted data .

printf and fprintf function sprintf

Look at the function prototype

 

 

printf Is the format output function ,fprintf Is a formatted output function for all output streams ,sprintf Is to convert a formatted data into a string . 

fprintf and fscanf example

typedef struct stu
{
	int age;
	char name[20];
	double wight;
}stu;
int main()
{
	stu s1 = { 18," Under Populus euphratica ",66.6 };
	FILE *p = fopen(" Under Populus euphratica .abc", "w");
	if (NULL == p)
	{
		perror("fopen:");
		exit(-1);
	}

	fprintf(p, "%d %s %lf", s1.age, s1.name, s1.wight);

	fclose(p);
	
	return 0;
}

  We wrote , Now read out .

int main()
{
	stu s2 = { 0 };
	FILE *p = fopen(" Under Populus euphratica .abc", "r");
	if (NULL == p)
	{
		perror("fopen:");
		exit(-1);
	}

	fscanf(p, "%d %s %lf", &s2.age, s2.name, &s2.wight);

	printf("%d %s %lf", s2.age, s2.name, s2.wight);

	fclose(p);

	return 0;
}

sscanf and sprint example ：

typedef struct stu
{
	int age;
	char name[20];
	double wight;
}stu;

int main()
{
	char arr[256] = { 0 };
	stu s3 = { 18, " Under Populus euphratica ", 66.6 };
	stu tmp = { 0 }; 

	sprintf(arr, "%d %s %lf", s3.age, s3.name, s3.wight);
	printf(" Print arr Content  :%s\n", arr);

	sscanf(arr, "%d %s %lf", &tmp.age, tmp.name, &tmp.wight);
	printf(" Print tmp Content  :%d %s %lf\n", tmp.age, tmp.name, tmp.wight);

	return 0;
}

  Here, we first convert and formatted data into string output , Then convert the string into formatted data for output .