X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq5.pod;h=83e34942c06c732ebf69bc762c831d3eecdf5706;hb=351f32542342b92f7303dec0a812c5301714120f;hp=521cb6f263ad92bcd3d8a5c9e7c256dad617bafe;hpb=30852c57b1a5333a45e16bf8bdc51e232a28eed0;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq5.pod b/pod/perlfaq5.pod index 521cb6f..83e3494 100644 --- a/pod/perlfaq5.pod +++ b/pod/perlfaq5.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq5 - Files and Formats ($Revision: 1.14 $, $Date: 2002/04/07 18:33:45 $) +perlfaq5 - Files and Formats ($Revision: 1.18 $, $Date: 2002/05/30 07:04:25 $) =head1 DESCRIPTION @@ -9,72 +9,56 @@ formats, and footers. =head2 How do I flush/unbuffer an output filehandle? Why must I do this? -The C standard I/O library (stdio) normally buffers characters sent to -devices. This is done for efficiency reasons so that there isn't a -system call for each byte. Any time you use print() or write() in -Perl, you go though this buffering. syswrite() circumvents stdio and -buffering. - -In most stdio implementations, the type of output buffering and the size of -the buffer varies according to the type of device. Disk files are block -buffered, often with a buffer size of more than 2k. Pipes and sockets -are often buffered with a buffer size between 1/2 and 2k. Serial devices -(e.g. modems, terminals) are normally line-buffered, and stdio sends -the entire line when it gets the newline. - -Perl does not support truly unbuffered output (except insofar as you can -C). What it does instead support is "command -buffering", in which a physical write is performed after every output -command. This isn't as hard on your system as unbuffering, but does -get the output where you want it when you want it. - -If you expect characters to get to your device when you print them there, -you'll want to autoflush its handle. -Use select() and the C<$|> variable to control autoflushing -(see L and L): +Perl does not support truly unbuffered output (except +insofar as you can C), although it +does support is "command buffering", in which a physical +write is performed after every output command. + +The C standard I/O library (stdio) normally buffers +characters sent to devices so that there isn't a system call +for each byte. In most stdio implementations, the type of +output buffering and the size of the buffer varies according +to the type of device. Perl's print() and write() functions +normally buffer output, while syswrite() bypasses buffering +all together. + +If you want your output to be sent immediately when you +execute print() or write() (for instance, for some network +protocols), you must set the handle's autoflush flag. This +flag is the Perl variable $| and when it is set to a true +value, Perl will flush the handle's buffer after each +print() or write(). Setting $| affects buffering only for +the currently selected default file handle. You choose this +handle with the one argument select() call (see +L and L). + +Use select() to choose the desired handle, then set its +per-filehandle variables. $old_fh = select(OUTPUT_HANDLE); $| = 1; select($old_fh); -Or using the traditional idiom: +Some idioms can handle this in a single statement: select((select(OUTPUT_HANDLE), $| = 1)[0]); -Or if don't mind slowly loading several thousand lines of module code -just because you're afraid of the C<$|> variable: + $| = 1, select $_ for select OUTPUT_HANDLE; - use FileHandle; - open(DEV, "+autoflush(1); - -or the newer IO::* modules: +Some modules offer object-oriented access to handles and their +variables, although they may be overkill if this is the only +thing you do with them. You can use IO::Handle: use IO::Handle; open(DEV, ">/dev/printer"); # but is this? DEV->autoflush(1); -or even this: +or IO::Socket: 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; + my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ; $sock->autoflush(); - print $sock "GET / HTTP/1.0" . "\015\012" x 2; - $document = join('', <$sock>); - print "DOC IS: $document\n"; - -Note the bizarrely hard coded 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, C<"\n\n"> often works, -but this is not portable. - -See L for other examples of fetching URLs over the web. =head2 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file? @@ -172,74 +156,31 @@ well. It also only works on global variables, not lexicals. =head2 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles? -The fastest, simplest, and most direct way is to localize the typeglob -of the filehandle in question: - - local *TmpHandle; - -Typeglobs are fast (especially compared with the alternatives) and -reasonably easy to use, but they also have one subtle drawback. If you -had, for example, a function named TmpHandle(), or a variable named -%TmpHandle, you just hid it from yourself. - - sub findme { - local *HostFile; - open(HostFile, ") { - print if /\b127\.(0\.0\.)?1\b/; - } - # *HostFile automatically closes/disappears here - } - -Here's how to use typeglobs in a loop to open and store a bunch of -filehandles. We'll use as values of the hash an ordered -pair to make it easy to sort the hash in insertion order. - - @names = qw(motd termcap passwd hosts); - my $i = 0; - foreach $filename (@names) { - local *FH; - open(FH, "/etc/$filename") || die "$filename: $!"; - $file{$filename} = [ $i++, *FH ]; - } +As of perl5.6, open() autovivifies file and directory handles +as references if you pass it an uninitialized scalar variable. +You can then pass these references just like any other scalar, +and use them in the place of named handles. - # Using the filehandles in the array - foreach $name (sort { $file{$a}[0] <=> $file{$b}[0] } keys %file) { - my $fh = $file{$name}[1]; - my $line = <$fh>; - print "$name $. $line"; - } + open my $fh, $file_name; -For passing filehandles to functions, the easiest way is to -preface them with a star, as in func(*STDIN). -See L for details. + open local $fh, $file_name; -If you want to create many anonymous handles, you should check out the -Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent -code with Symbol::gensym, which is reasonably light-weight: + print $fh "Hello World!\n"; - foreach $filename (@names) { - use Symbol; - my $fh = gensym(); - open($fh, "/etc/$filename") || die "open /etc/$filename: $!"; - $file{$filename} = [ $i++, $fh ]; - } + process_file( $fh ); -Here's using the semi-object-oriented FileHandle module, which certainly -isn't light-weight: +Before perl5.6, you had to deal with various typeglob idioms +which you may see in older code. - use FileHandle; + open FILE, "> $filename"; + process_typeglob( *FILE ); + process_reference( \*FILE ); - foreach $filename (@names) { - my $fh = FileHandle->new("/etc/$filename") or die "$filename: $!"; - $file{$filename} = [ $i++, $fh ]; - } + sub process_typeglob { local *FH = shift; print FH "Typeglob!" } + sub process_reference { local $fh = shift; print $fh "Reference!" } -Please understand that whether the filehandle happens to be a (probably -localized) typeglob or an anonymous handle from one of the modules -in no way affects the bizarre rules for managing indirect handles. -See the next question. +If you want to create many anonymous handles, you should +check out the Symbol or IO::Handle modules. =head2 How can I use a filehandle indirectly? @@ -253,13 +194,10 @@ to get indirect filehandles: $fh = \*SOME_FH; # ref to typeglob (bless-able) $fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob -Or, you can use the C method from the FileHandle or IO modules to +Or, you can use the C method from one of the IO::* modules to create an anonymous filehandle, store that in a scalar variable, and use it as though it were a normal filehandle. - use FileHandle; - $fh = FileHandle->new(); - use IO::Handle; # 5.004 or higher $fh = IO::Handle->new(); @@ -267,7 +205,7 @@ Then use any of those as you would a normal filehandle. Anywhere that Perl is expecting a filehandle, an indirect filehandle may be used instead. An indirect filehandle is just a scalar variable that contains a filehandle. Functions like C, C, C, or -the C<< >> diamond operator will accept either a read filehandle +the C<< >> diamond operator will accept either a named filehandle or a scalar variable containing one: ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR); @@ -327,9 +265,9 @@ This approach of treating C and C like object methods calls doesn't work for the diamond operator. That's because it's a real operator, not just a function with a comma-less argument. Assuming you've been storing typeglobs in your structure as we did above, you -can use the built-in function named C to reads a record just +can use the built-in function named C to read a record just as C<< <> >> does. Given the initialization shown above for @fd, this -would work, but only because readline() require a typeglob. It doesn't +would work, but only because readline() requires a typeglob. It doesn't work with objects or strings, which might be a bug we haven't fixed yet. $got = readline($fd[0]);