[advanced C language] file operation (I)

1、 Program files

1、 Source file （.c For the suffix ）
2、 Target file （Windows In the environment, the suffix is .obj）
3、 Executable program （Windows In the environment, the suffix is .exe）

2、 Data files

1、 One of the most common types of file systems used on computers . Essentially , It can be any file that stores a piece of data .

2、 You can use program files to read , Or write the data in the data file

3、 The file pointer

1、 When using a data file to read or write , The system will create a file information automatically FILE Structural variable , And fill in the information , Users don't have to care about details

2、 It's usually through a FILE* Pointer variables to maintain FILE This structural variable

3、 Definition pf It's a point FILE Pointer variable of type data , It can be pf Point to the file information area of a file （ It's a 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

4、fopen Function to open a file ,fclose Function to close a file

#include <errno.h>
#include <stdio.h>
int main()
{
    
   FILE* pf = fopen("text.txt","w");
   // Open file 
   if (pf == NULL)
   {
    
      //printf(" fail to open file \n");
      printf("%s\n",strerror(errno));
      return 0;
   }
   // Writing documents 
   // Close file 
   fclose(pf);
   pf = NULL;
   //FILE* pf = fopen("C:\\2021code\\text.txt","w");
   // If you want to generate files in a specific path , You need to add a specific path ,
   // Use two \\ To eliminate the effect of transferred characters 
   return 0;
}

5、 How to open a folder

“r”( read-only )： For input （ Input the data in the file into the program ）, Open an existing text file （ If the specified file does not exist, open the error , Returns a null pointer ）

“w”( Just write )： In order to output （ Write data to a file ）, Open a text file （ If the specified file does not exist, create a new file ）

“a”( Additional )： Add data to the end of the text file （ If the specified file does not exist, create a new file ）

4、 Sequential reading and writing of files

stdout（ Standard output stream ）,stdin（ Standard input stream ）,stderr （ Standard error flow ）

1、 Write a character into the file

#include <stdio.h>
#include <errno.h>
int main()
{
    
	FILE* pf = fopen("data.txt", "w");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	// Writing documents 
	//fputc('a', pf);
	//fputc('b', pf);
	//fputc('c', pf);

	char ch = 0;
	for (ch = 'a'; ch <= 'z'; ch++)
	{
    
		fputc(ch, pf);
	}

	fclose(pf);
	pf = NULL;

	return 0;
}

2、 Read a character in the file

#include <stdio.h>
#include <errno.h>
int main()
{
    
	FILE* pf = fopen("data.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	// Reading documents 

	int ch = 0;
	while ((ch = fgetc(pf)) != EOF)
	{
    
		printf("%c ", ch);
	}

	fclose(pf);
	pf = NULL;

	return 0;
}

3、 Write a line

#include <stdio.h>
#include <errno.h>
int main()
{
    
	FILE* pf = fopen("data.txt", "w");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	//fputs("hello world\n", pf);
	//fputs("hehe\n", pf);
	fputs("hello world\n", stdout);//fputs The function is to write a line 
	//stdout Is the standard output stream , It can be printed directly on the screen 

	fclose(pf);
	pf = NULL;

	return 0;
}

4、 Read a line

#include <stdio.h>
#include <errno.h>
int main()
{
    
	FILE* pf = fopen("data.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	char buf[1000] = {
     0 };
	// Reading documents 
	fgets(buf, 1000, pf);//fgets The function reads a line 
	printf("%s", buf);

	fclose(pf);
	pf = NULL;

	return 0;
}

5、 Copy file code

#include <stdio.h>
#include <errno.h>
int main()
{
    
	// Implement a code , take data.txt Copy a   Generate data2.txt file 
	FILE* pr = fopen("data.txt", "r");
	if (pr == NULL)
	{
    
		printf("open for reading: %s\n", strerror(errno));
		return 0;
	}
	FILE* pw = fopen("data2.txt", "w");
	if (pw == NULL)
	{
    
		printf("open for writting: %s\n", strerror(errno));
		fclose(pr);
		pr = NULL;
		return 0;
	}
	// Copy files 
	int ch = 0;
	while ((ch = fgetc(pr)) != EOF)
	{
    
		fputc(ch, pw);
	}

	fclose(pr);
	pr = NULL;
	fclose(pw);
	pw = NULL;
	return 0;
}

6、 Write formatted data

#include <stdio.h>
#include <errno.h>
struct Stu
{
    
	char name[20];
	int age;
	double d;
};

int main()
{
    
	struct Stu s = {
     " Zhang San ",20,95.5 };
	FILE* pf = fopen("data.txt", "w");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	// Write formatted data 
	fprintf(pf, "%s %d %lf", s.name, s.age, s.d);
	//fprintf(stdout, "%s %d %lf", s.name, s.age, s.d);// Write to screen 

	fclose(pf);
	pf = NULL;

	return 0;
}

7、 Read formatted data

#include <stdio.h>
#include <errno.h>
struct Stu
{
    
	char name[20];
	int age;
	double d;
};

int main()
{
    
	struct Stu s = {
     " Zhang San ",20,95.5 };
	FILE* pf = fopen("data.txt", "w");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	// Read formatted data 
	fscanf(pf,"%s %d %lf", s.name, &(s.age), &(s.d));

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

	fclose(pf);
	pf = NULL;

	return 0;
}

8、 Write in binary

#include <stdio.h>
#include <errno.h>
struct Stu
{
    
	char name[20];
	int age;
	double d;
};

int main()
{
    
	struct Stu s[2] = {
     {
     " Zhang San ",20,95.5 },{
    "lisi",16,66.5} };
	FILE* pf = fopen("data.txt", "wb");//wb It's binary 
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	// Write files in binary mode 
	fwrite(&s,sizeof(struct Stu),2,pf);

	fclose(pf);
	pf = NULL;

	return 0;
}

9、 Read in binary

#include <stdio.h>
#include <errno.h>
struct Stu
{
    
	char name[20];
	int age;
	double d;
};

int main()
{
    
	struct Stu s[2] = {
     0 };
	FILE* pf = fopen("data.txt", "rb");//rb Binary read 
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	// Write files in binary mode 
	fread(&s,sizeof(struct Stu),2,pf);
	printf("%s %d %lf\n", s[0].name, s[0].age, s[0].d);
	printf("%s %d %lf\n", s[1].name, s[1].age, s[1].d);

	fclose(pf);
	pf = NULL;

	return 0;
}

5、 Function comparison

1、scanf（ From standard input stream （stdin） Input function for formatting on ）
2、printf（ To standard output stream （stdout） Output function formatted on ）

3、fcanf（ From the standard input stream , Or specified file stream   Read formatted data on ）
4、fprintf（ Format the data in a way   Output to standard output stream , Or specify a file stream ）

5、sscanf（ From a string , Extract or convert formatted data ）
6、sprintf（ Convert a formatted data into a string ）

1、sscanf and sprintf Usage of

#include <stdio.h>
#include <errno.h>
struct Stu
{
    
	char name[20];
	int age;
	double d;
};

int main()
{
    
	struct Stu s = {
     " Zhang San ",20,95.5 };
	struct Stu tmp = {
     0 };

	char buf[100] = {
     0 };
	sprintf(buf, "%s %d %lf", s.name, s.age, s.d);

	printf("%s\n", buf);
	sscanf(buf,"%s %d %lf", tmp.name, &(tmp.age), &(tmp.d));
	//buf This string , Extracted in the form of structure , Namely sscanf The function of 
	printf("%s %d %lf\n", tmp.name, tmp.age, tmp.d);

	return 0;
}