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Tie::File(3)	       Perl Programmers	Reference Guide		  Tie::File(3)

NAME
       Tie::File - Access the lines of a disk file via a Perl array

SYNOPSIS
	use Tie::File;

	tie @array, 'Tie::File', filename or die ...;

	$array[0] = 'blah';	 # first line of the file is now 'blah'
				   # (line numbering starts at 0)
	print $array[42];	 # display line	43 of the file

	$n_recs	= @array;	 # how many records are	in the file?
	$#array	-= 2;		 # chop	two records off	the end

	for (@array) {
	  s/PERL/Perl/g;	# Replace PERL with Perl everywhere in the file
	}

	# These	are just like regular push, pop, unshift, shift, and splice
	# Except that they modify the file in the way you would	expect

	push @array, new recs...;
	my $r1 = pop @array;
	unshift	@array,	new recs...;
	my $r2 = shift @array;
	@old_recs = splice @array, 3, 7, new recs...;

	untie @array;		 # all finished

DESCRIPTION
       "Tie::File" represents a	regular	text file as a Perl array.  Each
       element in the array corresponds	to a record in the file.  The first
       line of the file	is element 0 of	the array; the second line is element
       1, and so on.

       The file	is not loaded into memory, so this will	work even for gigantic
       files.

       Changes to the array are	reflected in the file immediately.

       Lazy people and beginners may now stop reading the manual.

   "recsep"
       What is a 'record'?  By default,	the meaning is the same	as for the
       "<...>" operator: It's a	string terminated by $/, which is probably
       "\n".  (Minor exception:	on DOS and Win32 systems, a 'record' is	a
       string terminated by "\r\n".)  You may change the definition of
       "record"	by supplying the "recsep" option in the	"tie" call:

	       tie @array, 'Tie::File',	$file, recsep => 'es';

       This says that records are delimited by the string "es".	 If the	file
       contained the following data:

	       Curse these pesky flies!\n

       then the	@array would appear to have four elements:

	       "Curse th"
	       "e p"
	       "ky fli"
	       "!\n"

       An undefined value is not permitted as a	record separator.  Perl's
       special "paragraph mode"	semantics (a la	"$/ = """) are not emulated.

       Records read from the tied array	do not have the	record separator
       string on the end; this is to allow

	       $array[17] .= "extra";

       to work as expected.

       (See "autochomp", below.)  Records stored into the array	will have the
       record separator	string appended	before they are	written	to the file,
       if they don't have one already.	For example, if	the record separator
       string is "\n", then the	following two lines do exactly the same	thing:

	       $array[17] = "Cherry pie";
	       $array[17] = "Cherry pie\n";

       The result is that the contents of line 17 of the file will be replaced
       with "Cherry pie"; a newline character will separate line 17 from line
       18.  This means that this code will do nothing:

	       chomp $array[17];

       Because the "chomp"ed value will	have the separator reattached when it
       is written back to the file.  There is no way to	create a file whose
       trailing	record separator string	is missing.

       Inserting records that contain the record separator string is not
       supported by this module.  It will probably produce a reasonable
       result, but what	this result will be may	change in a future version.
       Use 'splice' to insert records or to replace one	record with several.

   "autochomp"
       Normally, array elements	have the record	separator removed, so that if
       the file	contains the text

	       Gold
	       Frankincense
	       Myrrh

       the tied	array will appear to contain "("Gold", "Frankincense",
       "Myrrh")".  If you set "autochomp" to a false value, the	record
       separator will not be removed.  If the file above was tied with

	       tie @gifts, "Tie::File",	$gifts,	autochomp => 0;

       then the	array @gifts would appear to contain "("Gold\n",
       "Frankincense\n", "Myrrh\n")", or (on Win32 systems) "("Gold\r\n",
       "Frankincense\r\n", "Myrrh\r\n")".

   "mode"
       Normally, the specified file will be opened for read and	write access,
       and will	be created if it does not exist.  (That	is, the	flags "O_RDWR
       | O_CREAT" are supplied in the "open" call.)  If	you want to change
       this, you may supply alternative	flags in the "mode" option.  See Fcntl
       for a listing of	available flags.  For example:

	       # open the file if it exists, but fail if it does not exist
	       use Fcntl 'O_RDWR';
	       tie @array, 'Tie::File',	$file, mode => O_RDWR;

	       # create	the file if it does not	exist
	       use Fcntl 'O_RDWR', 'O_CREAT';
	       tie @array, 'Tie::File',	$file, mode => O_RDWR |	O_CREAT;

	       # open an existing file in read-only mode
	       use Fcntl 'O_RDONLY';
	       tie @array, 'Tie::File',	$file, mode => O_RDONLY;

       Opening the data	file in	write-only or append mode is not supported.

   "memory"
       This is an upper	limit on the amount of memory that "Tie::File" will
       consume at any time while managing the file.  This is used for two
       things: managing	the read cache and managing the	deferred write buffer.

       Records read in from the	file are cached, to avoid having to re-read
       them repeatedly.	 If you	read the same record twice, the	first time it
       will be stored in memory, and the second	time it	will be	fetched	from
       the read	cache.	The amount of data in the read cache will not exceed
       the value you specified for "memory".  If "Tie::File" wants to cache a
       new record, but the read	cache is full, it will make room by expiring
       the least-recently visited records from the read	cache.

       The default memory limit	is 2Mib.  You can adjust the maximum read
       cache size by supplying the "memory" option.  The argument is the
       desired cache size, in bytes.

	# I have a lot of memory, so use a large cache to speed	up access
	tie @array, 'Tie::File', $file,	memory => 20_000_000;

       Setting the memory limit	to 0 will inhibit caching; records will	be
       fetched from disk every time you	examine	them.

       The "memory" value is not an absolute or	exact limit on the memory
       used.  "Tie::File" objects contains some	structures besides the read
       cache and the deferred write buffer, whose sizes	are not	charged
       against "memory".

       The cache itself	consumes about 310 bytes per cached record, so if your
       file has	many short records, you	may want to decrease the cache memory
       limit, or else the cache	overhead may exceed the	size of	the cached
       data.

   "dw_size"
       (This is	an advanced feature.  Skip this	section	on first reading.)

       If you use deferred writing (See	"Deferred Writing", below) then	data
       you write into the array	will not be written directly to	the file;
       instead,	it will	be saved in the	deferred write buffer to be written
       out later.  Data	in the deferred	write buffer is	also charged against
       the memory limit	you set	with the "memory" option.

       You may set the "dw_size" option	to limit the amount of data that can
       be saved	in the deferred	write buffer.  This limit may not exceed the
       total memory limit.  For	example, if you	set "dw_size" to 1000 and
       "memory"	to 2500, that means that no more than 1000 bytes of deferred
       writes will be saved up.	 The space available for the read cache	will
       vary, but it will always	be at least 1500 bytes (if the deferred	write
       buffer is full) and it could grow as large as 2500 bytes	(if the
       deferred	write buffer is	empty.)

       If you don't specify a "dw_size", it defaults to	the entire memory
       limit.

   Option Format
       "-mode" is a synonym for	"mode".	 "-recsep" is a	synonym	for "recsep".
       "-memory" is a synonym for "memory".  You get the idea.

Public Methods
       The "tie" call returns an object, say $o.  You may call

	       $rec = $o->FETCH($n);
	       $o->STORE($n, $rec);

       to fetch	or store the record at line $n,	respectively; similarly	the
       other tied array	methods.  (See perltie for details.)  You may also
       call the	following methods on this object:

   "flock"
	       $o->flock(MODE)

       will lock the tied file.	 "MODE"	has the	same meaning as	the second
       argument	to the Perl built-in "flock" function; for example "LOCK_SH"
       or "LOCK_EX | LOCK_NB".	(These constants are provided by the "use
       Fcntl ':flock'" declaration.)

       "MODE" is optional; the default is "LOCK_EX".

       "Tie::File" maintains an	internal table of the byte offset of each
       record it has seen in the file.

       When you	use "flock" to lock the	file, "Tie::File" assumes that the
       read cache is no	longer trustworthy, because another process might have
       modified	the file since the last	time it	was read.  Therefore, a
       successful call to "flock" discards the contents	of the read cache and
       the internal record offset table.

       "Tie::File" promises that the following sequence	of operations will be
       safe:

	       my $o = tie @array, "Tie::File",	$filename;
	       $o->flock;

       In particular, "Tie::File" will not read	or write the file during the
       "tie" call.  (Exception:	Using "mode => O_TRUNC"	will, of course, erase
       the file	during the "tie" call.	If you want to do this safely, then
       open the	file without "O_TRUNC",	lock the file, and use "@array = ()".)

       The best	way to unlock a	file is	to discard the object and untie	the
       array.  It is probably unsafe to	unlock the file	without	also untying
       it, because if you do, changes may remain unwritten inside the object.
       That is why there is no shortcut	for unlocking.	If you really want to
       unlock the file prematurely, you	know what to do; if you	don't know
       what to do, then	don't do it.

       All the usual warnings about file locking apply here.  In particular,
       note that file locking in Perl is advisory, which means that holding a
       lock will not prevent anyone else from reading, writing,	or erasing the
       file; it	only prevents them from	getting	another	lock at	the same time.
       Locks are analogous to green traffic lights: If you have	a green	light,
       that does not prevent the idiot coming the other	way from plowing into
       you sideways; it	merely guarantees to you that the idiot	does not also
       have a green light at the same time.

   "autochomp"
	       my $old_value = $o->autochomp(0);    # disable autochomp	option
	       my $old_value = $o->autochomp(1);    #  enable autochomp	option

	       my $ac =	$o->autochomp();   # recover current value

       See "autochomp",	above.

   "defer", "flush", "discard",	and "autodefer"
       See "Deferred Writing", below.

   "offset"
	       $off = $o->offset($n);

       This method returns the byte offset of the start	of the $nth record in
       the file.  If there is no such record, it returns an undefined value.

Tying to an already-opened filehandle
       If $fh is a filehandle, such as is returned by "IO::File" or one	of the
       other "IO" modules, you may use:

	       tie @array, 'Tie::File',	$fh, ...;

       Similarly if you	opened that handle "FH"	with regular "open" or
       "sysopen", you may use:

	       tie @array, 'Tie::File',	\*FH, ...;

       Handles that were opened	write-only won't work.	Handles	that were
       opened read-only	will work as long as you don't try to modify the
       array.  Handles must be attached	to seekable sources of data---that
       means no	pipes or sockets.  If "Tie::File" can detect that you supplied
       a non-seekable handle, the "tie"	call will throw	an exception.  (On
       Unix systems, it	can detect this.)

       Note that Tie::File will	only close any filehandles that	it opened
       internally.  If you passed it a filehandle as above, you	"own" the
       filehandle, and are responsible for closing it after you	have untied
       the @array.

       Tie::File calls "binmode" on filehandles	that it	opens internally, but
       not on filehandles passed in by the user. For consistency, especially
       if using	the tied files cross-platform, you may wish to call "binmode"
       on the filehandle prior to tying	the file.

Deferred Writing
       (This is	an advanced feature.  Skip this	section	on first reading.)

       Normally, modifying a "Tie::File" array writes to the underlying	file
       immediately.  Every assignment like "$a[3] = ..." rewrites as much of
       the file	as is necessary; typically, everything from line 3 through the
       end will	need to	be rewritten.  This is the simplest and	most
       transparent behavior.  Performance even for large files is reasonably
       good.

       However,	under some circumstances, this behavior	may be excessively
       slow.  For example, suppose you have a million-record file, and you
       want to do:

	       for (@FILE) {
		 $_ = "> $_";
	       }

       The first time through the loop,	you will rewrite the entire file, from
       line 0 through the end.	The second time	through	the loop, you will
       rewrite the entire file from line 1 through the end.  The third time
       through the loop, you will rewrite the entire file from line 2 to the
       end.  And so on.

       If the performance in such cases	is unacceptable, you may defer the
       actual writing, and then	have it	done all at once.  The following loop
       will perform much better	for large files:

	       (tied @a)->defer;
	       for (@a)	{
		 $_ = "> $_";
	       }
	       (tied @a)->flush;

       If "Tie::File"'s	memory limit is	large enough, all the writing will
       done in memory.	Then, when you call "->flush", the entire file will be
       rewritten in a single pass.

       (Actually, the preceding	discussion is something	of a fib.  You don't
       need to enable deferred writing to get good performance for this	common
       case, because "Tie::File" will do it for	you automatically unless you
       specifically tell it not	to.  See "Autodeferring", below.)

       Calling "->flush" returns the array to immediate-write mode.  If	you
       wish to discard the deferred writes, you	may call "->discard" instead
       of "->flush".  Note that	in some	cases, some of the data	will have been
       written already,	and it will be too late	for "->discard"	to discard all
       the changes.  Support for "->discard" may be withdrawn in a future
       version of "Tie::File".

       Deferred	writes are cached in memory up to the limit specified by the
       "dw_size" option	(see above).  If the deferred-write buffer is full and
       you try to write	still more deferred data, the buffer will be flushed.
       All buffered data will be written immediately, the buffer will be
       emptied,	and the	now-empty space	will be	used for future	deferred
       writes.

       If the deferred-write buffer isn't yet full, but	the total size of the
       buffer and the read cache would exceed the "memory" limit, the oldest
       records will be expired from the	read cache until the total size	is
       under the limit.

       "push", "pop", "shift", "unshift", and "splice" cannot be deferred.
       When you	perform	one of these operations, any deferred data is written
       to the file and the operation is	performed immediately.	This may
       change in a future version.

       If you resize the array with deferred writing enabled, the file will be
       resized immediately, but	deferred records will not be written.  This
       has a surprising	consequence: "@a = (...)" erases the file immediately,
       but the writing of the actual data is deferred.	This might be a	bug.
       If it is	a bug, it will be fixed	in a future version.

   Autodeferring
       "Tie::File" tries to guess when deferred	writing	might be helpful, and
       to turn it on and off automatically.

	       for (@a)	{
		 $_ = "> $_";
	       }

       In this example,	only the first two assignments will be done
       immediately; after this,	all the	changes	to the file will be deferred
       up to the user-specified	memory limit.

       You should usually be able to ignore this and just use the module
       without thinking	about deferring.  However, special applications	may
       require fine control over which writes are deferred, or may require
       that all	writes be immediate.  To disable the autodeferment feature,
       use

	       (tied @o)->autodefer(0);

       or

	       tie @array, 'Tie::File',	$file, autodefer => 0;

       Similarly, "->autodefer(1)" re-enables autodeferment, and
       "->autodefer()" recovers	the current value of the autodefer setting.

CONCURRENT ACCESS TO FILES
       Caching and deferred writing are	inappropriate if you want the same
       file to be accessed simultaneously from more than one process.  Other
       optimizations performed internally by this module are also incompatible
       with concurrent access.	A future version of this module	will support a
       "concurrent => 1" option	that enables safe concurrent access.

       Previous	versions of this documentation suggested using "memory => 0"
       for safe	concurrent access.  This was mistaken.	Tie::File will not
       support safe concurrent access before version 0.96.

CAVEATS
       (That's Latin for 'warnings'.)

       o   Reasonable effort was made to make this module efficient.
	   Nevertheless, changing the size of a	record in the middle of	a
	   large file will always be fairly slow, because everything after the
	   new record must be moved.

       o   The behavior	of tied	arrays is not precisely	the same as for
	   regular arrays.  For	example:

		   # This DOES print "How unusual!"
		   undef $a[10];  print	"How unusual!\n" if defined $a[10];

	   "undef"-ing a "Tie::File" array element just	blanks out the
	   corresponding record	in the file.  When you read it back again,
	   you'll get the empty	string,	so the supposedly-"undef"'ed value
	   will	be defined.  Similarly,	if you have "autochomp"	disabled, then

		   # This DOES print "How unusual!" if 'autochomp' is disabled
		   undef $a[10];
		   print "How unusual!\n" if $a[10];

	   Because when	"autochomp" is disabled, $a[10]	will read back as "\n"
	   (or whatever	the record separator string is.)

	   There are other minor differences, particularly regarding "exists"
	   and "delete", but in	general, the correspondence is extremely
	   close.

       o   I have supposed that	since this module is concerned with file I/O,
	   almost all normal use of it will be heavily I/O bound.  This	means
	   that	the time to maintain complicated data structures inside	the
	   module will be dominated by the time	to actually perform the	I/O.
	   When	there was an opportunity to spend CPU time to avoid doing I/O,
	   I usually tried to take it.

       o   You might be	tempted	to think that deferred writing is like
	   transactions, with "flush" as "commit" and "discard"	as "rollback",
	   but it isn't, so don't.

       o   There is a large memory overhead for	each record offset and for
	   each	cache entry: about 310 bytes per cached	data record, and about
	   21 bytes per	offset table entry.

	   The per-record overhead will	limit the maximum number of records
	   you can access per file. Note that accessing	the length of the
	   array via "$x = scalar @tied_file" accesses all records and stores
	   their offsets.  The same for	"foreach (@tied_file)",	even if	you
	   exit	the loop early.

SUBCLASSING
       This version promises absolutely	nothing	about the internals, which may
       change without notice.  A future	version	of the module will have	a
       well-defined and	stable subclassing API.

WHAT ABOUT "DB_File"?
       People sometimes	point out that DB_File will do something similar, and
       ask why "Tie::File" module is necessary.

       There are a number of reasons that you might prefer "Tie::File".	 A
       list is available at
       "<http://perl.plover.com/TieFile/why-not-DB_File>".

AUTHOR
       Mark Jason Dominus

       To contact the author, send email to: "mjd-perl-tiefile+@plover.com"

       To receive an announcement whenever a new version of this module	is
       released, send a	blank email message to
       "mjd-perl-tiefile-subscribe@plover.com".

       The most	recent version of this module, including documentation and any
       news of importance, will	be available at

	       http://perl.plover.com/TieFile/

LICENSE
       "Tie::File" version 0.96	is copyright (C) 2003 Mark Jason Dominus.

       This library is free software; you may redistribute it and/or modify it
       under the same terms as Perl itself.

       These terms are your choice of any of (1) the Perl Artistic Licence, or
       (2) version 2 of	the GNU	General	Public License as published by the
       Free Software Foundation, or (3)	any later version of the GNU General
       Public License.

       This library is distributed in the hope that it will be useful, but
       WITHOUT ANY WARRANTY; without even the implied warranty of
       MERCHANTABILITY or FITNESS FOR A	PARTICULAR PURPOSE.  See the GNU
       General Public License for more details.

       You should have received	a copy of the GNU General Public License along
       with this library program; it should be in the file "COPYING".  If not,
       write to	the Free Software Foundation, Inc., 51 Franklin	Street,	Fifth
       Floor, Boston, MA  02110-1301, USA

       For licensing inquiries,	contact	the author at:

	       Mark Jason Dominus
	       255 S. Warnock St.
	       Philadelphia, PA	19107

WARRANTY
       "Tie::File" version 0.98	comes with ABSOLUTELY NO WARRANTY.  For
       details,	see the	license.

THANKS
       Gigantic	thanks to Jarkko Hietaniemi, for agreeing to put this in the
       core when I hadn't written it yet, and for generally being helpful,
       supportive, and competent.  (Usually the	rule is	"choose	any one.")
       Also big	thanks to Abhijit Menon-Sen for	all of the same	things.

       Special thanks to Craig Berry and Peter Prymmer (for VMS	portability
       help), Randy Kobes (for Win32 portability help),	Clinton	Pierce and
       Autrijus	Tang (for heroic eleventh-hour Win32 testing above and beyond
       the call	of duty), Michael G Schwern (for testing advice), and the rest
       of the CPAN testers (for	testing	generally).

       Special thanks to Tels for suggesting several speed and memory
       optimizations.

       Additional thanks to: Edward Avis / Mattia Barbon / Tom Christiansen /
       Gerrit Haase / Gurusamy Sarathy / Jarkko	Hietaniemi (again) / Nikola
       Knezevic	/ John Kominetz	/ Nick Ing-Simmons / Tassilo von Parseval / H.
       Dieter Pearcey /	Slaven Rezic / Eric Roode / Peter Scott	/ Peter	Somu /
       Autrijus	Tang (again) / Tels (again) / Juerd Waalboer / Todd Rinaldo

TODO
       More tests.  (Stuff I didn't think of yet.)

       Paragraph mode?

       Fixed-length mode.  Leave-blanks	mode.

       Maybe an	autolocking mode?

       For many	common uses of the module, the read cache is a liability.  For
       example,	a program that inserts a single	record,	or that	scans the file
       once, will have a cache hit rate	of zero.  This suggests	a major
       optimization: The cache should be initially disabled.  Here's a hybrid
       approach: Initially, the	cache is disabled, but the cache code
       maintains statistics about how high the hit rate	would be *if* it were
       enabled.	 When it sees the hit rate get high enough, it enables itself.
       The STAT	comments in this code are the beginning	of an implementation
       of this.

       Record locking with fcntl()?  Then the module might support an undo log
       and get real transactions.  What	a tour de force	that would be.

       Keeping track of	the highest cached record. This	would allow reads-in-
       a-row to	skip the cache lookup faster (if reading from 1..N with	empty
       cache at	start, the last	cached value will be always N-1).

       More tests.

perl v5.32.0			  2020-06-14			  Tie::File(3)

NAME | SYNOPSIS | DESCRIPTION | Public Methods | Tying to an already-opened filehandle | Deferred Writing | CONCURRENT ACCESS TO FILES | CAVEATS | SUBCLASSING | WHAT ABOUT "DB_File"? | AUTHOR | LICENSE | WARRANTY | THANKS | TODO

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