On Thu, 17 May 2001, kosta wrote:
> Question: What is "flush"? and how can a filehandle be flushed? I saw
> such a phrase while looking for info about file locking.
Found in /usr/lib/perl5/5.6.1/pods/perlfaq5.pod
How do I flush/unbuffer an output filehandle? Why must I
do this?
The C standard I/O library (stdio) normally buffers
characters sent to devices. This is done for efficiency
reasons so that there isn't a system call for each byte.
Any time you use print() or write() in Perl, you go though
this buffering. syswrite() circumvents stdio and
buffering.
In most stdio implementations, the type of output
buffering and the size of the buffer varies according to
the type of device. Disk files are block buffered, often
with a buffer size of more than 2k. Pipes and sockets are
often buffered with a buffer size between 1/2 and 2k.
Serial devices (e.g. modems, terminals) are normally line-
buffered, and stdio sends the entire line when it gets the
newline.
Perl does not support truly unbuffered output (except
insofar as you can "syswrite(OUT, $char, 1)"). What it
does instead support is "command buffering", in which a
physical write is performed after every output command.
This isn't as hard on your system as unbuffering, but does
get the output where you want it when you want it.
If you expect characters to get to your device when you
print them there, you'll want to autoflush its handle.
Use select() and the "$|" variable to control autoflushing
(see perlvar/$ and the select entry in the perlfunc
manpage):
$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);
Or using the traditional idiom:
select((select(OUTPUT_HANDLE), $| = 1)[0]);
Or if don't mind slowly loading several thousand lines of
module code just because you're afraid of the "$|"
variable:
use FileHandle;
open(DEV, "+</dev/tty"); # ceci n'est pas une pipe
DEV->autoflush(1);
or the newer IO::* modules:
use IO::Handle;
open(DEV, ">/dev/printer"); # but is this?
DEV->autoflush(1);
or even this:
use IO::Socket; # this one is kinda a pipe?
$sock = IO::Socket::INET->new(PeerAddr => 'www.perl.com',
PeerPort => 'http(80)',
Proto => 'tcp');
die "$!" unless $sock;
$sock->autoflush();
print $sock "GET / HTTP/1.0" . "\015\012" x 2;
$document = join('', <$sock>);
print "DOC IS: $document\n";
Note the bizarrely hardcoded carriage return and newline
in their octal equivalents. This is the ONLY way
(currently) to assure a proper flush on all platforms,
including Macintosh. That's the way things work in
network programming: you really should specify the exact
bit pattern on the network line terminator. In practice,
""\n\n"" often works, but this is not portable.
See the perlfaq9 manpage for other examples of fetching
URLs over the web.
-- Brett
