question about feof

[alzar]

if we have this code for files

while(!feof(from);
  ch=fgetc(from);
  if(ferror(from){
      printf("error reading source file\n");
      exit(1);
      }
   if(!feof(from)) fputc(ch,to)/*IN THIS line why we check again with feof? */
    if(ferror(to){
      printf("error writing destination file\n");
      exit(1);
      }
 }

my question is on the code:why we check again with if and feof?( we are in the while loop so we have already !feof(from) )

[zacs7]

Don't use feof() to control loops, see the FAQ. Other than the logic being broken, you've got a few syntax errors

What you want is probably,

int c;

while((c = fgetc(from)) != EOF)
{
    fputc(c, to);
}

[alzar]

yes but i must use feof (it is for an execise and says that i must use feof)

[vart]

yes but i must use feof (it is for an execise and says that i must use feof)

Use it to check if the read failed due to error or end of file, but not to control loop

[bithub]

The reason the second feof() is needed in your code is due to the end of file marker not being set until after the fgetc() call. If you didn't have the second feof() you could process your ch variable even when fgetc() didn't return anything due to the end of the file being reached.

If you must call feof(), you can rearange your code to be something like:

while(1)
{
   read from file
   if(feof())
      break;
   process what was read from the file
}

Keep in mind that what zacs posted is a cleaner solution though.

[jawahar]

yeah....instead of using feof it is better to use

while( (ch=fgetc(from))!=EOF)

this even avoids the over head of calling the function everytime...
cheers

[alzar]

If you didn't have the second feof() you could process your ch variable even when fgetc() didn't return anything due to the end of the file being reached.

How can this happen?
we are in a while loop while(!feof(my_file)) and this means that the End of file not reached
???

[vart]

the flag of end of file is set during read that failed, not before

[matsp]

Yes, if you consider an input stream of "abc<eof>", with the following code:

int ch;
while(!feof(file)) {
   ch = fgetc(file);
   printf("%d", ch);
}

It will produce:
97
98
99
-1
The loop will not end until AFTER you have read beyond the end of the file, and if you then don't check feof() AFTER the read, you will "process" the end of file input - not so bad here, but consider a fgets() where the string is not overwritten because the input was at end of file - now you may have a full string of text that was the previous last line of data, processed again!

--
Mats

[alzar]

Thanks i understand it!!