trailing a growing file

Thomas Lange thomas@REDACTED
Thu Jan 1 15:18:24 CET 2004

Debian Linux has "fam - file alteration monitor"

 From manual:
"fam is informed of filesystem changes as  they  happen  by  the  kernel
  through the DNotify fcntl available in current 2.4.x kernels."


Shawn Pearce wrote:
> tail -f just does the same thing you are doing by reading the log,
> hitting EOF, and waiting a short while.
> Classic UNIX/POSIX doesn't have a way to get new data from a file, or
> to find out when a file/directory is modified - applications just have
> to poll.
> FreeBSD and IRIX (that I know of) have methods where an application
> can open a directory or file, and tell the OS it is interested in
> being notified when a change is made.  Then you read from it and
> carry on.  Under IRIX this is called FAM (File Activity Monitor).  I
> don't know what FreeBSD calls it; and to my knowledge Linux does not
> have support for this.
> If the log writer is another Erlang node I would suggest just sending
> the log data to the target node, as well as write it to disk.  If the
> log writer is an external process, well, you are stuck.  If it is a
> syslogd maintained file, perhaps you could put an Erlang node into the
> list of targets syslogd will write the logs to.
> If the log writer is anything else, you'll need to either patch the
> application (example would be with Apache build or reuse a mod_* that
> can send the logs over the network to an Erlang node), or just poll
> the log file...
> If it was me, I'd be polling from within Erlang using a raw file (so
> no extra process is started) rather than using a port to tail -f.
> Raphael Bellec <raphael.bellec@REDACTED> wrote:
>>I am actually working on a multi-log multi-host monitoring (and
>>reacting) system, made in erlang and I need some advice:
>>I would like to read logs file as soon as data is written to them. I
>>actually have found 2 ways for this purpose:
>>- With Io module, pooling (each time I get an eof, I wait a small time
>>then try again).
>>- With ports, spawning "tail -f -n 1 /path/to/file", which is the method
>>I currently use.
>>Is there a other way to "receive" news lines as they are written in the
>>file without pooling or using an external application? I read few
>>messages saying that there was no blocking IO in erlang, but I think
>>this was mainly from a VM perspective. (I will have few hundreds logs to
>>I currently use the port solution and I sometimes have a "broken pipe"
>>error, I think this is because of the "in" parameter in open_port, but I
>>hat it sometime without this parameter, anybody have a more detailed
>>explanation of what could happen ?
>>Ps: the subject is directly taken from a point in the erlang PLEAC which
>>is still empty.

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