X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq5.pod;h=31db204129115cf2b4b8a5da0ec9d100f0424d17;hb=af9e49b40a4cc2d6c0d5ebad7e84fb62143b24e1;hp=ca2fb7e87c3bd35da463073446ea10d06cf5aaa8;hpb=49d635f9372392ae44fe4c5b62b06e41912ae0c9;p=p5sagit%2Fp5-mst-13.2.git

diff --git a/pod/perlfaq5.pod b/pod/perlfaq5.pod
index ca2fb7e..31db204 100644
--- a/pod/perlfaq5.pod
+++ b/pod/perlfaq5.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
-perlfaq5 - Files and Formats ($Revision: 1.26 $, $Date: 2002/09/21 21:04:17 $)
+perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 2005/10/13 19:49:13 $)
 
 =head1 DESCRIPTION
 
@@ -8,6 +8,7 @@ This section deals with I/O and the "f" issues: filehandles, flushing,
 formats, and footers.
 
 =head2 How do I flush/unbuffer an output filehandle?  Why must I do this?
+X<flush> X<buffer> X<unbuffer> X<autoflush>
 
 Perl does not support truly unbuffered output (except
 insofar as you can C<syswrite(OUT, $char, 1)>), although it
@@ -30,7 +31,7 @@ value, Perl will flush the handle's buffer after each
 print() or write(). Setting $| affects buffering only for
 the currently selected default file handle. You choose this
 handle with the one argument select() call (see
-L<perlvar/$|> and L<perlfunc/select>).
+L<perlvar/$E<verbar>> and L<perlfunc/select>).
 
 Use select() to choose the desired handle, then set its
 per-filehandle variables.
@@ -61,11 +62,13 @@ or IO::Socket:
     $sock->autoflush();
 
 =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?
+X<file, editing>
 
 Use the Tie::File module, which is included in the standard
 distribution since Perl 5.8.0.
 
 =head2 How do I count the number of lines in a file?
+X<file, counting lines> X<lines> X<line>
 
 One fairly efficient way is to count newlines in the file. The
 following program uses a feature of tr///, as documented in L<perlop>.
@@ -82,6 +85,7 @@ proper text file, so this may report one fewer line than you expect.
 This assumes no funny games with newline translations.
 
 =head2 How can I use Perl's C<-i> option from within a program?
+X<-i> X<in-place>
 
 C<-i> sets the value of Perl's C<$^I> variable, which in turn affects
 the behavior of C<< <> >>; see L<perlrun> for more details.  By
@@ -106,11 +110,35 @@ This block modifies all the C<.c> files in the current directory,
 leaving a backup of the original data from each file in a new
 C<.c.orig> file.
 
+=head2 How can I copy a file?
+X<copy> X<file, copy>
+
+(contributed by brian d foy)
+
+Use the File::Copy module. It comes with Perl and can do a
+true copy across file systems, and it does its magic in
+a portable fashion.
+
+	use File::Copy;
+
+	copy( $original, $new_copy ) or die "Copy failed: $!";
+
+If you can't use File::Copy, you'll have to do the work yourself:
+open the original file, open the destination file, then print
+to the destination file as you read the original.
+
 =head2 How do I make a temporary file name?
+X<file, temporary>
+
+If you don't need to know the name of the file, you can use C<open()>
+with C<undef> in place of the file name.  The C<open()> function
+creates an anonymous temporary file.
 
-Use the File::Temp module, see L<File::Temp> for more information.
+	open my $tmp, '+>', undef or die $!;
 
-  use File::Temp qw/ tempfile tempdir /; 
+Otherwise, you can use the File::Temp module.
+
+  use File::Temp qw/ tempfile tempdir /;
 
   $dir = tempdir( CLEANUP => 1 );
   ($fh, $filename) = tempfile( DIR => $dir );
@@ -141,6 +169,7 @@ temporary files in one process, use a counter:
 	    my $count = 0;
 	    until (defined(fileno(FH)) || $count++ > 100) {
 		$base_name =~ s/-(\d+)$/"-" . (1 + $1)/e;
+		# O_EXCL is required for security reasons.
 		sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
 	    }
 	    if (defined(fileno(FH))
@@ -152,9 +181,12 @@ temporary files in one process, use a counter:
     }
 
 =head2 How can I manipulate fixed-record-length files?
+X<fixed-length> X<file, fixed-length records>
 
-The most efficient way is using pack() and unpack().  This is faster than
-using substr() when taking many, many strings.  It is slower for just a few.
+The most efficient way is using L<pack()|perlfunc/"pack"> and
+L<unpack()|perlfunc/"unpack">.  This is faster than using
+L<substr()|perlfunc/"substr"> when taking many, many strings.  It is
+slower for just a few.
 
 Here is a sample chunk of code to break up and put back together again
 some fixed-format input lines, in this case from the output of a normal,
@@ -162,24 +194,26 @@ Berkeley-style ps:
 
     # sample input line:
     #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
-    $PS_T = 'A6 A4 A7 A5 A*';
-    open(PS, "ps|");
-    print scalar <PS>; 
-    while (<PS>) {
-	($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
-	for $var (qw!pid tt stat time command!) {
-	    print "$var: <$$var>\n";
+    my $PS_T = 'A6 A4 A7 A5 A*';
+    open my $ps, '-|', 'ps';
+    print scalar <$ps>;
+    my @fields = qw( pid tt stat time command );
+    while (<$ps>) {
+        my %process;
+        @process{@fields} = unpack($PS_T, $_);
+	for my $field ( @fields ) {
+	    print "$field: <$process{$field}>\n";
 	}
-	print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),
-		"\n";
+	print 'line=', pack($PS_T, @process{@fields} ), "\n";
     }
 
-We've used C<$$var> in a way that forbidden by C<use strict 'refs'>.
-That is, we've promoted a string to a scalar variable reference using
-symbolic references.  This is okay in small programs, but doesn't scale
-well.   It also only works on global variables, not lexicals.
+We've used a hash slice in order to easily handle the fields of each row.
+Storing the keys in an array means it's easy to operate on them as a
+group or loop over them with for. It also avoids polluting the program
+with global variables and using symbolic references.
 
 =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?
+X<filehandle, local> X<filehandle, passing> X<filehandle, reference>
 
 As of perl5.6, open() autovivifies file and directory handles
 as references if you pass it an uninitialized scalar variable.
@@ -208,6 +242,7 @@ 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?
+X<filehandle, indirect>
 
 An indirect filehandle is using something other than a symbol
 in a place that a filehandle is expected.  Here are ways
@@ -282,9 +317,9 @@ an expression where you would place the filehandle:
 That block is a proper block like any other, so you can put more
 complicated code there.  This sends the message out to one of two places:
 
-    $ok = -x "/bin/cat";                
+    $ok = -x "/bin/cat";
     print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
-    print { $fd[ 1+ ($ok || 0) ]  } "cat stat $ok\n";           
+    print { $fd[ 1+ ($ok || 0) ]  } "cat stat $ok\n";
 
 This approach of treating C<print> and C<printf> like object methods
 calls doesn't work for the diamond operator.  That's because it's a
@@ -303,15 +338,25 @@ It's the syntax of the fundamental operators.  Playing the object
 game doesn't help you at all here.
 
 =head2 How can I set up a footer format to be used with write()?
+X<footer>
 
 There's no builtin way to do this, but L<perlform> has a couple of
 techniques to make it possible for the intrepid hacker.
 
 =head2 How can I write() into a string?
+X<write, into a string>
 
 See L<perlform/"Accessing Formatting Internals"> for an swrite() function.
 
 =head2 How can I output my numbers with commas added?
+X<number, commify>
+
+(contributed by brian d foy and Benjamin Goldberg)
+
+You can use L<Number::Format> to separate places in a number.
+It handles locale information for those of you who want to insert
+full stops instead (or anything else that they want to use,
+really).
 
 This subroutine will add commas to your number:
 
@@ -329,7 +374,7 @@ It is easier to see with comments:
 
    s/(
        ^[-+]?            # beginning of number.
-       \d{1,3}?          # first digits before first comma
+       \d+?              # first digits before first comma
        (?=               # followed by, (but not included in the match) :
           (?>(?:\d{3})+) # some positive multiple of three digits.
           (?!\d)         # an *exact* multiple, not x * 3 + 1 or whatever.
@@ -340,6 +385,7 @@ It is easier to see with comments:
    )/$1,/xg;
 
 =head2 How can I translate tildes (~) in a filename?
+X<tilde> X<tilde expansion>
 
 Use the <> (glob()) operator, documented in L<perlfunc>.  Older
 versions of Perl require that you have a shell installed that groks
@@ -362,6 +408,7 @@ Within Perl, you may use this directly:
 	}ex;
 
 =head2 How come when I open a file read-write it wipes it out?
+X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating>
 
 Because you're using something like this, which truncates the file and
 I<then> gives you read-write access:
@@ -369,7 +416,7 @@ I<then> gives you read-write access:
     open(FH, "+> /path/name");		# WRONG (almost always)
 
 Whoops.  You should instead use this, which will fail if the file
-doesn't exist.  
+doesn't exist.
 
     open(FH, "+< /path/name");  	# open for update
 
@@ -424,8 +471,8 @@ To open file for update, file must not exist:
 
 To open a file without blocking, creating if necessary:
 
-    sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT)
-	    or die "can't open /tmp/somefile: $!":
+    sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
+	    or die "can't open /foo/somefile: $!":
 
 Be warned that neither creation nor deletion of files is guaranteed to
 be an atomic operation over NFS.  That is, two processes might both
@@ -434,7 +481,8 @@ isn't as exclusive as you might wish.
 
 See also the new L<perlopentut> if you have it (new for 5.6).
 
-=head2 Why do I sometimes get an "Argument list too long" when I use <*>?
+=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
+X<argument list too long>
 
 The C<< <> >> operator performs a globbing operation (see above).
 In Perl versions earlier than v5.6.0, the internal glob() operator forks
@@ -448,6 +496,7 @@ yourself with readdir() and patterns, or use a module like File::KGlob,
 one that doesn't use the shell to do globbing.
 
 =head2 Is there a leak/bug in glob()?
+X<glob>
 
 Due to the current implementation on some operating systems, when you
 use the glob() function or its angle-bracket alias in a scalar
@@ -455,27 +504,24 @@ context, you may cause a memory leak and/or unpredictable behavior.  It's
 best therefore to use glob() only in list context.
 
 =head2 How can I open a file with a leading ">" or trailing blanks?
+X<filename, special characters>
 
-Normally perl ignores trailing blanks in filenames, and interprets
-certain leading characters (or a trailing "|") to mean something
-special. 
+(contributed by Brian McCauley)
 
-The three argument form of open() lets you specify the mode
-separately from the filename.  The open() function treats
-special mode characters and whitespace in the filename as 
-literals
+The special two argument form of Perl's open() function ignores
+trailing blanks in filenames and infers the mode from certain leading
+characters (or a trailing "|"). In older versions of Perl this was the
+only version of open() and so it is prevalent in old code and books.
 
+Unless you have a particular reason to use the two argument form you
+should use the three argument form of open() which does not treat any
+charcters in the filename as special.
+     
 	open FILE, "<", "  file  ";  # filename is "   file   "
 	open FILE, ">", ">file";     # filename is ">file"
 
-It may be a lot clearer to use sysopen(), though:
-
-    use Fcntl;
-    $badpath = "<<<something really wicked   ";
-    sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
-	or die "can't open $badpath: $!";
-
 =head2 How can I reliably rename a file?
+X<rename> X<mv> X<move> X<file, rename> X<ren>
 
 If your operating system supports a proper mv(1) utility or its
 functional equivalent, this works:
@@ -491,6 +537,7 @@ permissions, timestamps, inode info, etc.
 Newer versions of File::Copy export a move() function.
 
 =head2 How can I lock a file?
+X<lock> X<file, lock> X<flock>
 
 Perl's builtin flock() function (see L<perlfunc> for details) will call
 flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
@@ -532,12 +579,13 @@ for your own system's idiosyncrasies (sometimes called "features").
 Slavish adherence to portability concerns shouldn't get in the way of
 your getting your job done.)
 
-For more information on file locking, see also 
+For more information on file locking, see also
 L<perlopentut/"File Locking"> if you have it (new for 5.6).
 
 =back
 
-=head2 Why can't I just open(FH, ">file.lock")?
+=head2 Why can't I just open(FH, "E<gt>file.lock")?
+X<lock, lockfile race condition>
 
 A common bit of code B<NOT TO USE> is this:
 
@@ -549,7 +597,7 @@ which must be done in one.  That's why computer hardware provides an
 atomic test-and-set instruction.   In theory, this "ought" to work:
 
     sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
-		or die "can't open  file.lock: $!":
+		or die "can't open  file.lock: $!";
 
 except that lamentably, file creation (and deletion) is not atomic
 over NFS, so this won't work (at least, not every time) over the net.
@@ -557,6 +605,7 @@ Various schemes involving link() have been suggested, but
 these tend to involve busy-wait, which is also subdesirable.
 
 =head2 I still don't get locking.  I just want to increment the number in the file.  How can I do this?
+X<counter> X<file, counter>
 
 Didn't anyone ever tell you web-page hit counters were useless?
 They don't count number of hits, they're a waste of time, and they serve
@@ -581,6 +630,7 @@ Here's a much better web-page hit counter:
 If the count doesn't impress your friends, then the code might.  :-)
 
 =head2 All I want to do is append a small amount of text to the end of a file.  Do I still have to use locking?
+X<append> X<file, append>
 
 If you are on a system that correctly implements flock() and you use the
 example appending code from "perldoc -f flock" everything will be OK
@@ -609,6 +659,7 @@ level write()s even if the buffer was empty to start.  There may be some
 systems where this probability is reduced to zero.
 
 =head2 How do I randomly update a binary file?
+X<file, binary patch>
 
 If you're just trying to patch a binary, in many cases something as
 simple as this works:
@@ -632,6 +683,7 @@ Locking and error checking are left as an exercise for the reader.
 Don't forget them or you'll be quite sorry.
 
 =head2 How do I get a file's timestamp in perl?
+X<timestamp> X<file, timestamp>
 
 If you want to retrieve the time at which the file was last
 read, written, or had its meta-data (owner, etc) changed,
@@ -665,6 +717,7 @@ in theory, independent of the current locale.  See L<perllocale>
 for details.
 
 =head2 How do I set a file's timestamp in perl?
+X<timestamp> X<file, timestamp>
 
 You use the utime() function documented in L<perlfunc/utime>.
 By way of example, here's a little program that copies the
@@ -680,11 +733,17 @@ of them.
 
 Error checking is, as usual, left as an exercise for the reader.
 
-Note that utime() currently doesn't work correctly with Win95/NT
-ports.  A bug has been reported.  Check it carefully before using
-utime() on those platforms.
+The perldoc for utime also has an example that has the same
+effect as touch(1) on files that I<already exist>.
+
+Certain file systems have a limited ability to store the times
+on a file at the expected level of precision.  For example, the
+FAT and HPFS filesystem are unable to create dates on files with
+a finer granularity than two seconds.  This is a limitation of
+the filesystems, not of utime().
 
 =head2 How do I print to more than one file at once?
+X<print, to multiple files>
 
 To connect one filehandle to several output filehandles,
 you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
@@ -695,11 +754,12 @@ to each filehandle.
     for $fh (FH1, FH2, FH3) { print $fh "whatever\n" }
 
 =head2 How can I read in an entire file all at once?
+X<slurp> X<file, slurping>
 
 You can use the File::Slurp module to do it in one step.
 
 	use File::Slurp;
-	
+
 	$all_of_it = read_file($filename); # entire file in scalar
     @all_lines = read_file($filename); # one line perl element
 
@@ -710,7 +770,7 @@ do so one line at a time:
     while (<INPUT>) {
 	chomp;
 	# do something with $_
-    } 
+    }
     close(INPUT)	    	|| die "can't close $file: $!";
 
 This is tremendously more efficient than reading the entire file into
@@ -735,7 +795,7 @@ You can read the entire filehandle contents into a scalar.
 	$var = <INPUT>;
     }
 
-That temporarily undefs your record separator, and will automatically 
+That temporarily undefs your record separator, and will automatically
 close the file at block exit.  If the file is already open, just use this:
 
     $var = do { local $/; <INPUT> };
@@ -748,16 +808,18 @@ The third argument tests the byte size of the data on the INPUT filehandle
 and reads that many bytes into the buffer $var.
 
 =head2 How can I read in a file by paragraphs?
+X<file, reading by paragraphs>
 
 Use the C<$/> variable (see L<perlvar> for details).  You can either
 set it to C<""> to eliminate empty paragraphs (C<"abc\n\n\n\ndef">,
 for instance, gets treated as two paragraphs and not three), or
 C<"\n\n"> to accept empty paragraphs.
 
-Note that a blank line must have no blanks in it.  Thus 
+Note that a blank line must have no blanks in it.  Thus
 S<C<"fred\n \nstuff\n\n">> is one paragraph, but C<"fred\n\nstuff\n\n"> is two.
 
 =head2 How can I read a single character from a file?  From the keyboard?
+X<getc> X<file, reading one character at a time>
 
 You can use the builtin C<getc()> function for most filehandles, but
 it won't (easily) work on a terminal device.  For STDIN, either use
@@ -889,6 +951,7 @@ FIONREAD requires a filehandle connected to a stream, meaning that sockets,
 pipes, and tty devices work, but I<not> files.
 
 =head2 How do I do a C<tail -f> in perl?
+X<tail>
 
 First try
 
@@ -917,11 +980,12 @@ more.  Lather, rinse, repeat.
 There's also a File::Tail module from CPAN.
 
 =head2 How do I dup() a filehandle in Perl?
+X<dup>
 
 If you check L<perlfunc/open>, you'll see that several of the ways
 to call open() should do the trick.  For example:
 
-    open(LOG, ">>/tmp/logfile");
+    open(LOG, ">>/foo/logfile");
     open(STDERR, ">&LOG");
 
 Or even with a literal numeric descriptor:
@@ -931,12 +995,13 @@ Or even with a literal numeric descriptor:
 
 Note that "<&STDIN" makes a copy, but "<&=STDIN" make
 an alias.  That means if you close an aliased handle, all
-aliases become inaccessible.  This is not true with 
+aliases become inaccessible.  This is not true with
 a copied one.
 
 Error checking, as always, has been left as an exercise for the reader.
 
 =head2 How do I close a file descriptor by number?
+X<file, closing file descriptors>
 
 This should rarely be necessary, as the Perl close() function is to be
 used for things that Perl opened itself, even if it was a dup of a
@@ -949,13 +1014,14 @@ to, you may be able to do this:
 
 Or, just use the fdopen(3S) feature of open():
 
-    { 
-	local *F; 
+    {
+	local *F;
 	open F, "<&=$fd" or die "Cannot reopen fd=$fd: $!";
 	close F;
     }
 
 =head2 Why can't I use "C:\temp\foo" in DOS paths?  Why doesn't `C:\temp\foo.exe` work?
+X<filename, DOS issues>
 
 Whoops!  You just put a tab and a formfeed into that filename!
 Remember that within double quoted strings ("like\this"), the
@@ -972,6 +1038,7 @@ awk, Tcl, Java, or Python, just to mention a few.  POSIX paths
 are more portable, too.
 
 =head2 Why doesn't glob("*.*") get all the files?
+X<glob>
 
 Because even on non-Unix ports, Perl's glob function follows standard
 Unix globbing semantics.  You'll need C<glob("*")> to get all (non-hidden)
@@ -994,6 +1061,7 @@ of the directory, not of the file).  If you try to write to the file,
 the permissions of the file govern whether you're allowed to.
 
 =head2 How do I select a random line from a file?
+X<file, selecting a random line>
 
 Here's an algorithm from the Camel Book:
 
@@ -1038,8 +1106,8 @@ If your array contains lines, just print them:
 
 =head1 AUTHOR AND COPYRIGHT
 
-Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
 
 This documentation is free; you can redistribute it and/or modify it
 under the same terms as Perl itself.