When printing to a filehandle, output doesn't appear immediately. This is a problem in CGI scripts running on some programmer-hostile web servers where, if the web server sees warnings from Perl before it sees the (buffered) output of your script, it sends the browser an uninformative
Error. These buffering problems arise with concurrent access to files by multiple programs and when talking with devices or sockets.
Disable buffering by setting the per-filehandle variable
$| to a true value, customarily
$old_fh = select(OUTPUT_HANDLE); $| = 1; select($old_fh);
Or, if you don't mind the expense, disable it by calling the
autoflush method from the IO modules:
use IO::Handle; OUTPUT_HANDLE->autoflush(1);
In most stdio implementations, buffering varies with the type of output 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, including terminals, modems, mice, and joysticks, are normally line-buffered; stdio sends the entire line out only when it gets the newline.
Control output buffering through the
$| special variable. Enable command buffering by setting it to a true value. It has no effect upon input; see Recipes Recipe 15.6 and Recipe 15.8 for unbuffered input. Set this variable to a false value to use default stdio buffering. Example 7.6 illustrates the difference.
If you call this program with no arguments, STDOUT is not command buffered. Your terminal (console, window, telnet session, whatever) doesn't receive output until the entire line is completed, so you see nothing for two seconds and then get the full line
do". If you call the program with at least one argument, STDOUT is command buffered. That means you first see
it...", and then after two seconds you finally see
The dubious quest for increasingly compact code has led programmers to use the return value of
select, the filehandle that was currently selected, as part of the second
select((select(OUTPUT_HANDLE), $| = 1));
There's another way. The FileHandle and IO modules provide a class method called
autoflush. Call it with true or false values (the default value is true) to control autoflushing on a particular output handle:
use FileHandle; STDERR->autoflush; # already unbuffered in stdio $filehandle->autoflush(0);
If you're willing to accept the oddities of indirect object notation covered in Chapter 13, Classes, Objects, and Ties, you can even write something reasonably close to English:
use IO::Handle; # assume REMOTE_CONN is an interactive socket handle, # but DISK_FILE is a handle to a regular file. autoflush REMOTE_CONN 1; # unbuffer for clarity autoflush DISK_FILE 0; # buffer this for speed
This avoids the bizarre
select business, and makes your code much more readable. Unfortunately, your program takes longer to compile because you're now including the IO::Handle module, so thousands and thousands of lines must first be read and compiled. Learn to manipulate
$| directly, and you'll be happy.
To ensure that your output gets where you want it, when you want it, buffer flushing is important. It's particularly important with sockets, pipes, and devices, because you may be trying to do interactive I/O with these - more so, in fact, because you can't assume line-buffering. Consider the program in Example 7.7.
#!/usr/bin/perl # getpcomidx - fetch www.perl.com's index.html document use IO::Socket; $sock = new IO::Socket::INET (PeerAddr => 'www.perl.com', PeerPort => 'http(80)'); die "Couldn't create socket: $@" unless $sock; # the library doesn't support $! setting; it uses $@ $sock->autoflush(1); # Mac *must* have \015\012\015\012 instead of \n\n here. # It's a good idea for others, too, as that's the spec, # but implementations are encouraged to accept "\cJ\cJ" too, # and as far as we've seen, they do. $sock->print("GET /index.html http/1.1\n\n"); $document = join('', $sock->getlines()); print "DOC IS: $document\n";