What happens if you don't check feof in C?

During read/write operations is it absolutely necessary to check feof()?

The reason I ask is because I have a program that performs read/write ONCE, here is the code below:

while (1) {

      data = read_file_data(file);
      write_file_data(data, filename);

      if (feof(file)) 
          print(read error)
}

This is just pseudocode but is it necessary to check feof() in a case like this where a read will occur once? Currently, I think it is only necessary if you will do ANOTHER read after the one above like this:

while (1) {

      data = read_file_data(file);
      write_file_data(data, filename);

      if (feof(file))       // feof error occurred oops
          print(read error)

      data = read_file_data(file); // reading after error    
}

Lastly, what can the consequences be of reading even after an EOF reached error (reading past EOF) occurs?

Solution

During read/write operations is it absolutely necessary to check feof()?

No. During normal operations, the best way of detecting EOF is to check the return value of the particular input call you're using. Input calls can always fail, so their return values should always be checked. This is especially true of scanf and fgets, which many beginning programmers (and unfortunately many beginning programming tutorials) conspicuously neglect to check.

Explicit calls to feof are rarely necessary. You might need to call feof if:

An input call has just returned 0 or EOF, but you'd like to know whether that was due to an actual end-of-file, or a more serious error.
You've used an input call such as getw() that has no way to indicate EOF or error.
(rare, and arguably poor form) You've made a long series of input calls in a row, and you didn't expect EOF from any of them, so you didn't feel like checking all of them explicitly, but decided to check with one catch-all call to feof() at the end.

The other thing to know is that feof() only tells you that you did hit end-of-file — that is, past tense. It does not predict the future; it does not tell you that next input call you try to make will hit EOF. It tells you that the previous call you made did hit EOF. See also Why is “while( !feof(file) )” always wrong?

what can the consequences be of reading even after an EOF reached error (reading past EOF) occurs?

That's a good question. The answer is, "it depends," and since unpredictability can be a real problem, your best bet is usually not to try to read past EOF.

When I was first learning C, if you got an EOF, but tried reading some more, and if the EOF had somehow "gone away", your next read might succeed. And it could be quite common for the EOF to "go away" like that, if you were reading from the keyboard, and the user indicated EOF by typing control-D. But if they typed more input after typing control-D, you could go ahead and read it.

But that was in the old days. These days EOF is "sticky", and once the end-of-file flag has been set for a stream, I think any future attempts to read are supposed to immediately return EOF. These days, if the user hits control-D and you want to keep reading, you have to call clearerr().

I'm pretty sure everything I just said abut feof() and the per-stream EOF flag is also true of ferror() and the per-stream error flag.

In a comment, @0___________ said that "if you ignore I/O errors and continue you invoke Undefined Behaviour". I don't think this is true, but I don't have my copy of the Standard handy to check.