Problem with EOF when determining stream end

Question

When I try to determine end of file with function feof(FILE *), I find it does not work as I expected: an extra read is required even if the stream does end. E.g., feof(FILE*) will not return true if invoked on a file with 10 bytes data just after reading 10 bytes out. I need an extra read operation which of course returns 0. Then feof(FILE *) will say "OK, now you reach the end."

Why is one more read required and how can I determine end of file or how can I know how many bytes are left in a file stream if I don't want the feof-style?

Answer 1

Do not use feof() or any variants—it is as simple as that. You want it to somehow predict the next read will fail, but that's not what it does - it tells you what the result of the previous read was. The correct way to read a file is (in pseudocode):

while(read(file, buffer)) {
   // Do something with buffer
}

In other words, you need to test the result of the read operation. This is true for both C streams and C++ iostreams.