C language file operation analysis detailed explanation and example code

C Language File Operation Analysis

                                                                                                      In addition to open operations and read and write operations, there are several more common operations in file operations. The functions involved in these operations are introduced below.

1. Functions to move the position pointer

rewind function and fseek function, the prototypes of these two functions are:

void rewind(FILE *fp); Move the position pointer to the beginning of the file

int fseek(FILE *fp ,long int offset,int origin); Move the position pointer to the offset number of bytes from origin. For the parameters in the fseek function, origin is the starting point, and offset is the offset bytes from origin. There are three values ​​for origin: SEEK_SET(0 )—>The beginning of the file, SEEK_CUR(1)—>The current position, SEEK_END(2)—>The end of the file.

Note: 1) If the file is opened in append mode, these two functions will not work when writing. No matter where the position pointer is moved, the added data will always be appended to the end of the file.

2. Other commonly used functions

1. ftell function

long int ftell(FILE *fp);

Calculate the number of bytes from the current position pointer to the beginning of the file. If an error occurs, -1L is returned.

Use the ftell function to calculate the file size.

2.feof function

int feof(FILE *fp);

Detects whether the current position pointer reaches the end of the file. If it reaches the end of the file, it returns a non-zero value, otherwise it returns 0.

3.ferror function

int ferror(FILE *fp);

Detects whether there is an error during the file operation. If an error occurs, it returns a non-zero value, otherwise it returns 0

4.remove function

int remove( const char *filename);

Delete the file. If the deletion is successful, return 0, otherwise return a non-zero value

5.rename function

int rename(const char *oldname, const char *newname);

Replace the file Rename, return 0 if the rename is successful, otherwise return a non-zero value.

6.freopen function

FILE* freopen(const char *filename,const char *mode,FILE *stream);

implements redirected input and output. This function is often used when testing data.

7.fclose function

int fclose(FILE *stream);

Closes a stream. If successful, it returns 0, otherwise it returns -1. Note that the stream must be closed after each operation on the file, otherwise it may cause data lost.

Test program:

#include<stdio.h>
#include<stdlib.h>
  
int main(void)
{
  freopen("input.txt","r",stdin);
  freopen("output.txt","w+",stdout);
  int i;
  int a[10];
  for(i=0;i<10;i++)
  {
    scanf("%d",&a[i]);
  }
  for(i=0;i<10;i++)
  {
    printf("%d\n",a[i]);
  }
  return 0;
}

Assume input.txt already exists in the project directory, and the data in the file is 1 2 -1 3 4 5 7 8 9 10. After running, there is no need to input data from the console. The program reads data directly from input.txt, and then outputs the results to output.txt, without directly outputting the results to the console.

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