File operation (detailed!)

file

1, What is a document

Program files ： Include source files （ The suffix is .c）, Target file （windows Environment suffix is .obj）, Executable program （windows Environment suffix is .exe）
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
In the past, input refers to input from the keyboard , Output refers to display on the screen ,
But sometimes we output information to disk , When necessary, read the data from the disk to the memory for use , What we are dealing with here is the files on the disk

2, The file pointer FILE

seeing the name of a thing one thinks of its function ： Through the file pointer variable, you can find the file associated with it .
stay vs2013 Yes FILE Have the following statement

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

Different compilers for FILE The definition of is different , But it's the same , Every time we open a file , The system will automatically create a FILE Structural variables , And supplement the information , Generally, we use file pointers to perform relevant operations on files .

3, Opening and closing of files

How files are used meaning If the specified file does not exist
“r”（ read-only ） To enter data , Open an existing text file error
“w”（ Just write ） To output data , Open a text file Create a new file
“a”（ Additional ） Add data to the end of the text file Create a new file
“rb”（ read-only ） To enter data , Open a binary file error
“wb”（ Just write ） To output data , Open a binary file Create a new file
“ab”（ Additional ） Add data to the end of a binary file error
“r+”（ Reading and writing ） For reading and writing , Open a text file error
“w+”（ Reading and writing ） For reading and writing , Suggest a new file Create a new file
“a+”（ Reading and writing ） Open a file , Read and write at the end of the file Create a new file
“rb+”（ Reading and writing ） Open a binary file for reading and writing error
“wb+”（ Reading and writing ） For reading and writing , Create a new binary file Create a new file
“ab+”（ Reading and writing ） Open a binary file , Read and write at the end of the file Create a new file

// Open file 
FILE * fopen ( const char * filename, const char * mode );
 The first parameter ： Open file name 
 The second parameter ： Open mode , Must be a double quotation mark , Single quotation marks don't work 
// Close file 
int fclose ( FILE * stream );

Each time you open a file, you only need to write the corresponding opening method .
eg：

// Open the file as a write 
	FILE* pf = fopen("text.txt", "w");// To output data , Open a text file  
	                  // If the asking price doesn't exist, create a new document 
	                   // If the file exists, eliminate the original contents in the file 
	if (pf == NULL)
	{
    
		printf(" fail to open file \n");
		printf("%s\n", strerror(errno));// Print error messages related to file opening failure 
		return 0;

	}
	fclose(pf);
	pf = NULL;// Be sure to leave the file empty after closing , Otherwise, wild pointers may appear 

When you open the same folder, a

Open file

	FILE* pf = fopen("text.txt", "r");// Only existing files can be opened , If it does not exist , Then return to NULL
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	fclose(pf);

Other opening methods are similar

4, Sequential reading and writing of files

The reading of file data can be divided into Sequential reading and writing , Random, speaking, reading and writing , Sequential read-write, as the name suggests, is to write and read files in the order we input and output , Random reading and writing is just random reading and writing in a sense , What is it? Let's see below .

function Function name Apply to
Character input function fgetc All input streams
Character output function fputc All output streams
Text line input function fgets All input streams
Text line output function fputs All output streams
Format input function fscanf All input streams
Format output function fprintf All output streams
Binary input fread file
Binary output fwrite file

fgetc,fputc

fputc:int fputc( int c, FILE stream ); Output a character to the stream
c: Indicates that the characters entering the stream need to be read
stream: flow
fputc Applicable to all streams ？ What does it mean ？
Take the output as an example ： At present, what I know about the output is two streams , Does it flow to the screen or to the file .
Flow can be compared to water , Then the screen and file are in two directions , The direction refers to which direction the water flows, so the data is input or output to .
When running a program from a language , Three streams are opened by default ：
Standard input stream ：stdin
Standard output stream ：stdout These three streams The types of FILE
Standard error flow ：stderr
for example ：
printf： Standard output stream ：stdout（ Flow to screen ）
scanf： Standard output stream ：stdout（ keyboard ）


fgetc:int fgetc( FILE *stream ); Read characters from the stream into memory , And return the read characters

FILE* pf = fopen("data.txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	int ch = 0;
	//while ((ch = fgetc(stdin)) != EOF)// When the error or the end of the file is returned EOF
	while ((ch = fgetc(pf)) != EOF)// When the error or the end of the file is returned EOF
	{
    
		printf("%c ", ch);
	}

fputs,fgets

fputs：int fputs( const char *string, FILE *stream );
string: The string to be read
streat： The stream to which the string should flow
Return value ： If executed successfully , These functions will return a non negative value . When something goes wrong ,fputs return EOF
fgets:char *fgets( char *string, int n, FILE *stream );
string: The data extracted from the stream is roughly stored in the string
n:fgets Read characters from the current position of the stream , Including the first line break , Read to the end of the stream , Or until the number of characters read is equal to n - 1, Stored at the end of the character will automatically add a ’\0’
stream: Which stream to extract data from

fscanf ,fprint

fread/fwrite

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

Return value ： Returns the number of completed entries actually read
buffer: The entries read from the stream are stored in buffer in
size: The overall size of each read （ In bytes ）
count: The maximum number of entries read
stream： Read the source of the entry , It's from the standard input stream （ keyboard ） Or file input

size_t fwrite( const void *buffer, size_t size, size_t count, FILE *stream );
Return value ： Returns the number of complete entries successfully written to the stream
size： The size of each entry written
count： Maximum number of entries written
stream： Same as fread

feof/ferror

int main()
{
    
	FILE* pf = fopen("data,txt", "r");
	if (pf == NULL)
	{
    
		printf("%s\n", strerror(errno));
		return 0;
	}
	int ch = 0;
	//fgetc  When reading fails or the end of the file is encountered , Will return to EOF
	while ((ch = fgetc(pf)) != EOF);// Label read file operation cycle 

	// Judge what caused it 
	if (ferror(pf))// If there is no error on the stream ,ferror return 0. otherwise , It returns a non-zero value .
		puts("I/O error when reading");
	else if (feof(pf))//eof Function returns a non-zero value after the first read operation attempts to read to the end of the file .
		             // If the current location is not at the end of the file , Then return to 0.
		puts("End of file reached successfully");
	fclose(pf);
	return 0;
}