Question

I’ve got a program that is tailing a growing file.

I’m trying to avoid grabbing a partial line from the file (e.g. reading before the line is completely written by the other process.) I know it’s happening in my code, so I’m trying to catch it specifically.

Is there a sane way to do this?

Here’s what I’m trying:

if (getline (stream, logbuffer))
{
    if (stream.eof())
    {
        cout << "Partial line found!" << endl;
        return false;
    }
    return true;
}
return false;

However, I can’t easily reproduce the problem so I’m not sure I’m detecting it with this code. std::getline strips off newlines, so I can’t check the buffer for a trailing newline. My log message (above) is NEVER tripping.

Is there some other way of trying to check what I want to detect? Is there a way to know if the last line I read hit EOF without finding a EOL character?

Thanks.

Answer 1

This will never be true:

if (getline (stream, logbuffer))
{
    if (stream.eof())
    {
       /// will never get here

If getline() worked, the stream cannot be in an eof state. The eof() and related state tests only work on the results of a previous read operation such as getline()– they do not predict what the next read will do.

As far as I know, there is no way of doing what you want. However, if the other process writes a line at a time, the problems you say you are experiencing should be very rare (non -existent in my experience), depending to some extent on the OS you are are using. I suspect the problem lies elsewhere, probably in your code. Tailing a file is a very common thing to do, and one does not normally need to resort to special code to do it.

However, should you find you do need to read partial lines, the basic algorithm is as follows:

forever do
   wait for file change
   read all possible input using read or readsome (not getline)
   chop input into lines and possible partial line
   process as required
end