kqueue alternative?

[Joshua Oreman]

On Sun, Jun 15, 2003 at 09:50:24PM -0400 or thereabouts, Matthew Hagerty seemed to write:
> > Joshua Oreman wrote:
> >> On Sun, 15 Jun 2003, Matthew Hagerty wrote:
> >>
> >>>I'm writing a little application that needs to watch a file that another
> >>>process is writing to, think 'tail -F'.  kqueue and kevent are going to
> >>>do it for me on *BSD, but I'm also trying to support *cough* linux and
> >>>other UN*X types OSes.
> >>>
> >>>>From what I can find on google, the linux community seems very opposed
> >>>to kqueue and has not yet implemented it (they say: blah blah blah,
> >>>aio_*, blah blah balh.)  What alternatives do I have with OSes that
> >>>don't support kqueue?  I'd really hate to poll with stat(), but do I
> >>>have any other choices?
> >>
> >>
> >> I would say, use select(2).
> >> Is there a reason this wouldn't work?
> >>
> >> -- Josh
> >
> >
> > Either select(2) or poll(2) should work.
> >
> >
> >
> > --
> > Stephen Montgomery-Smith
> > stephen at math.missouri.edu
> > http://www.math.missouri.edu/~stephen
> >
> 
> Last time I did a test, it seemed that I could not read to the end of a
> file that another process was writing to, without calling stat() to get
> the real size?
> 
> So, I open the file for reading, and read/process the data until EOF, at
> which point I... ?  call select()?  What should I be watching for if
> another process is doing the writing?  Will my select() return if I'm
> watching for a ready to read condition while another process is writing?
> 
> While writing this response I'm thinking of a function I didn't know about
> that I saw in forward.c (part of the tail command's code), clearerr().  If
> I hit EOF, and another process writes to the file, will calling clearerr()
> on my file pointer allow me to continue reading the new data that was just
> written by the other process?  Am I making sense?

Are you using (FILE *) file pointers or (int) file descriptors?
I recommend you use file pointers for this program; easier line input.
However, select() needs a file descriptor, which you can obtain with
fileno(my_file_pointer).

Here's some pseudocode (assuming you want to print the data):
FILE *file;
char buffer[101];
fd_set readfds;

file = fopen ("/path/to/my/file");
while (fgets (buffer, 100, file)) fputs (buffer, stdout);
clearerr (file);
FD_ZERO (&readfds);
FD_SET (fileno (file), &readfds);
while (1) {
	select (fileno (file), &readfds, (fd_set *)0, (fd_set *)0, (struct timeval *)0);
	while (fgets (buffer, 100, file)) fputs (buffer, stdout);
	clearerr (file);
}

Note: it's not tested, it is very realistic pseudocode but it is
pseudocode after all :-)

-- Josh

> 
> Thanks for the insight!
> 
> Matthew
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