using pipe with tail -f

mfofo · 07-10-2008, 02:48 PM

I am trying to read the input from stdin, which works fine. However, it does not work when I try to read a pipe from tail -f.
The code:

tail logfile | ./a.out //this one works
but the code:

tail -f logfile | ./a.out //This one does not, it seems like nothing is bein read from the pipe. in my code I am using:

fread(&buffer,1,BUFFER_SIZE,stdin)

Mr. C. · 07-10-2008, 03:23 PM

The -f option to tail says don't close STDOUT, and just keep reading the file and output forever. Since STDOUT isn't closed, your program's read doesn't complete.

If enough data gets pushed through the pipe, your program will eventually get some data on the read, but not until then.

If you want your program to read a growing file, do it internally, not with an external tool.

helptonewbie · 07-10-2008, 04:17 PM

I'm not totally sure on what your trying to acheive but looking at the fact your trying to do a tail -f and then constantly read data there...i may sugest looking at mkfifo

http://linux.die.net/man/3/mkfifo

Take a look it may be what your looking for, this isn't a file though and data written to it won't be stored, its simply a method to pass data from one program to another

Hope it helps

chrism01 · 07-11-2008, 01:02 AM

If you are willing to use Perl, this does exactly what you need: http://search.cpan.org/~mgrabnar/Fil...0.99.3/Tail.pm

mfofo · 07-11-2008, 08:05 AM

I figured this out, the problem is in the fread() buffering a lot of the data, so I used read() instead and everything is working fine.

Mr. C. · 07-11-2008, 12:53 PM

Right, but the point is, if you are going to use read(), just read() continually, with a timeout on the read() until the program terminates with a SIGQUIT. Your program is duplicating the behavior of tail -f, so just do it yourself.