C language - file operation

The file pointer :FILE

FILE yes C A structure type declared by a language system , The relevant information of the document will be stored in FILE In variables of type , After opening the file , Maintain the opened file through this structure variable .

After opening a file , The system will automatically create a FILE Variable of type , Through the file pointer variable, you can find the file associated with it .

Opening and closing of files ：

You need to open the file before reading or writing it , You need to close the file after using it .

Use fopen Function to open a file , Use fclose Function to close the file .

fopen The function needs to pass two string type parameters , They are file names , And open mode

Turn on mode

 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 

If it opens successfully ,fopen The function will return a pointer , Need to use file Pointer variables of type receive . Thus, the connection between the file and the pointer is established .

If opening fails , Will return a null pointer NULL, Therefore, you need to judge whether it is successfully opened .

After the file is used , Use fclose function , Pass a pointer to the file , You can close the file , Then you need to set the pointer to null .

Sequential reading and writing of files

 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： From the specified stream stream Read a character , Return to one int This character of type . If reading fails or an error is encountered , Then return to EOF.

fputc: Passing one character ASCII Code value and an output stream , Write this character to the input stream .

fgets: From the specified stream stream Read a line , Maximum read n Characters , And store it in  str  Within the string pointed to . When reading  (n-1)  In characters , Or when a newline character is read , Or at the end of the file , It will stop . return NULL.

fputs: Write to the specified stream str The string in the string pointed to , Terminate with a null character . If the write fails , Then return to EOF.

fscanf： Input data from the specified stream in the specified format .

fprintf： Input the data into the specified stream through the specified format .

fwrite： Write the specified data to the specified stream in binary form , You need to specify the size and number of data elements to be written .

fread： Data will be read from the specified stream , Write to the specified variable in binary form , You need to specify the size and number of elements to write data .

Random reading and writing of documents

fseek function , You can locate the file pointer by the position and offset of the file pointer .

 fseek ( pFile , 9 , SEEK_SET );
        // The file pointer // Offset // Which position to offset 
SEEK_CUR// The current location of the file pointer 
SEEK_END// End of file pointer 
SEEK_SET// File pointer start position 

After locating the file pointer , You can read and write data in the file pointer .

int main ()
{
  FILE * pFile;
  pFile = fopen ( "example.txt" , "wb" );
  fputs ( "This is an apple." , pFile );
  fseek ( pFile , 9 , SEEK_SET );
  fputs ( " sam" , pFile );
  fclose ( pFile );
  return 0; }

ftell function ： Returns the offset of the file pointer from the starting position .

rewind function ： Set the file pointer to the starting position .

Text files and binaries

Data is stored in binary form in memory , Output to file without processing , Then the file is binary .

With ASCII The file stored in the form of code value is a text file .

Determination of the end of file reading

Out-of-service feof To directly determine whether the reading of the file is over .

feof Only when it is read to the end of the file will it return eof, When an error occurs during reading, the file reading will also end .

Whether the reading of text file is finished , Determine whether the return value is EOF （ fgetc ）, perhaps NULL （ fgets ）

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 .

fread Judge whether the return value is less than the actual number to be read .

File buffer

ANSIC The standard is “ Buffer file system ” Processed data files .

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 from disk to

The computer reads data , Then read the data from the disk file and input it into the memory buffer （ Fill the buffer ）, Then from the buffer

Send data to the program data area （ Program variables, etc ）.