clearerr() is an I/O function that is used to reset the "error flag" associated with the stream. It also resets the end of file(EOF) indicator. When a stream function fails either because of an error or if the end of the file(EOF) has been reached, one of these internal indicators may be set.

    void clearerr(FILE *stream);

   #include <stdio.h>
   int main ()
   {
      FILE *fil;
      fil= fopen("clearerreg1.txt","r");
      fputc ('x',fil);
      if (ferror (fil))
        {
           printf ("ERROR!! No Write Permission\n");
           clearerr (fil);
        }
      fgetc (fil);
      if (!ferror (fil))
        {
           printf ("ERROR!! Cleared\n"); 
        }
      fclose (fil);
      return 0;
   }

    ERROR!! No Write Permission
    No ERROR!! File has read permission

In the above example a stream "fil" is used to open the file "clearerreg1.txt" with "readonly" permission. The "fputc" function tries to write a character "x" which triggers an error that is cleared by the "clearerr()" .