C/stdio.h/ungetc

    

//Declaration:  int ungetc(int ch, FILE *stream); 
//Return:       returns ch on success or EOF on failure. 
#include <stdio.h>
int main ()
{
  FILE * fp;
  int c;
  char buffer [256];
  fp = fopen ("test.txt","rt");
 
  if (fp==NULL)
    perror ("Error opening file");
  else {
    while (!feof (fp))
    {
      c=getc (fp);
      if (c == "#")
        ungetc ("@",fp);
      else
        ungetc (c,fp);
      fgets (buffer,255,fp);
      fputs (buffer,stdout);
    }
  }
  return 0; 
}