|P is for Practical|
How do I flush/unbuffer an output filehandle? Why must I do this?by faq_monk (Initiate)
|on Oct 08, 1999 at 00:23 UTC||Need Help??|
Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
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
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
$old_fh = select(OUTPUT_HANDLE); $| = 1; select($old_fh);
Or using the traditional idiom:
select((select(OUTPUT_HANDLE), $| = 1));
Or if don't mind slowly loading several thousand lines of module code just
because you're afraid of the
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 the way things work in network programming: you really should specify the exact bit pattern on the network line terminator. In practice,
See the perlfaq9 manpage for other examples of fetching URLs over the web.