Re: EOF... how does it work?

"James Kanze" <>
Tue, 13 Mar 2007 08:48:28 CST
On Mar 12, 8:36 pm, wrote:

Hi I'm wondering if anyone could explain to me, how the ios::eof()
function knows when the end of file has been reached?

In a certain sense, it doesn't. All it knows is whether the
eofbit has been set or not (and there's nothing to prevent a
user defined << or >> to set it arbitrarily).

The convention, of course (followed by every standard << and >>,
and by correctly written user operators as well) is to set it
whenever you attempt to read, and there are no more characters
available. In istream and ostream, this is whenever overflow or
underflow return EOF. Which in turn depends entirely on the
streambuf itself.

Is there a eof
symbol in each file like the end of string symbol "\0"? How does it
work? And does it work differently for binary files as opposed to
ascii files? I'm particularly interested to know how finding the eof
for binary files works.

How it works for files is implemented in filebuf, but almost
always, it is the result of a system read returning 0, or a
system write returning less than the number bytes written. As
for the system... it depends on the system; most modern systems
maintain the length of the file somewhere, and use this (for
disk files, of course... if you're reading from the keyboard, or
from a socket or pipe, different rules apply).

Note that in istream, the bit is normally only set when an
attempted read fails. But this refers to internal attempted
reads. The result is that eofbit may be unset when there is no
data left to be read, because no attempt has been made to read
it, or set, even though the last read succeeded, because the
last read required look-ahead to tell it when to stop, and that
look-ahead found the end of the file. The result is that you
practically never use ios::eof(), and never before failure. (If
a read fails, and ios::eof() and ios::bad() are both false, you
know that it is because of a format error in the input.
Regretfully, is ios::eof() is true, you can't always be 100%
certain that there was no format error.) The standard read
idiom is thus something like:

    while ( input >> data ) {
        // process data...
    if ( input.bad() ) {
        // serious error (except that a lot of implementations
        // never generate this case).
    } else if ( ! input.eof() ) {
        // format error in the input file; e.g. we tried to
        // read an int, and only found "abc".
    } else {
        // probably OK, but this can mask a few more exotic
        // format errors as well.

James Kanze (GABI Software)
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