Question

I’m wondering how you can implement a program similar to tail -f in C/C++, a program that watches for and processes new lines added to a log file?

Answer 1

You can use fseek() to clear the eof condition on the stream. Essentially, read to the end of the file, sleep for a while, fseek() (without changing your position) to clear eof, the read to end of file again. wash, rinse, repeat. man fseek(3) for details.

Here’s what it looks like in perl. perl’s seek() is essentially a wrapper for fseek(3), so the logic is the same:

wembley 0 /home/jj33/swap >#> cat p my $f = shift; open(I, '<$f') || die 'Couldn't open $f: $!\n';  while (1) {   seek(I, 0, 1);   while (defined(my $l = <I>)) {     print 'Got: $l';   }   print 'Hit EOF, sleeping\n';   sleep(10); } wembley 0 /home/jj33/swap >#> cat tfile This is some text in a file wembley 0 /home/jj33/swap >#> perl p tfile Got: This is Got: some Got: text Got: in Got: a file Hit EOF, sleeping 

Then, in another session:

wembley 0 /home/jj33/swap > echo 'another line of text' >> tfile 

And back to the original program output:

Hit EOF, sleeping Got: another line of text Hit EOF, sleeping