tie::file(3)

NAME

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

SYNOPSIS

# This file documents Tie::File version 0.93
tie @array, 'Tie::File', filename or die ...;
$array[13] = 'blah';     # line 13 of the file  is
now 'blah'
print  $array[42];        # display line 42 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 $r1 = 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 immedi
ately.

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 "0. (Minor exception: on dos and
W0.)2 Youtmay,changecthe'definitionnoft"record"dbyy "
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!

then the @array would appear to have four elements:

"Curse th"
"e p"
"ky fli"
"!0

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 "0, then
the following two lines do exactly the same thing:

$array[17] = "Cherry pie";
$array[17] = "Cherry pie0;

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 reat
tached 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", "Frankin
cense", "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;

0)".

then the array @gifts w0,l"Myrrhar to contain "("Gold0,
"Franki0,e"Frankincense)", or (on Win32 systems) "("Gold

"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 max
imum 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 struc
tures besides the read cache and the deferred write
buffer, whose sizes are not charged against "memory".

"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 con
stants are provided by the "use Fcntl ':flock'" declara
tion.)

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

"Tie::File" promises that the following sequence of opera
tions 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 guar
antees to you that the idiot does not also have a green
light at the same time.

"autochomp"

my $old_value = $o->autochomp(0); # disable au

tochomp option
my $old_value = $o->autochomp(1); # enable au

tochomp option

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

ue

See "autochomp", above.

"defer", "flush", "discard", and "autodefer"

See "Deferred Writing", below.

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', H, ...;

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.)

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; typi
cally, 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 mil
lion-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 per
formance 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 spec
ified 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 ver
sion.

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 autode
fer setting.

CAVEATS

(That's Latin for 'warnings'.)

· This is BETA RELEASE SOFTWARE. It may have bugs. See

the discussion below about the (lack of any) warranty.

In particular, this means that the interface may
change in incompatible ways from one version to the
next, without warning. That has happened at least
once already. The interface will freeze before Perl
5.8 is released, probably sometime in April 2002.

· Reasonable effort was made to make this module effi

cient. 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.

· 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!0 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. Sim
ilarly, if you have "autochomp" disabled, then

# This DOES print "How unusual!" if 'auto

chomp' is disabled
undef $a[10];
print "How unusual!0 if $a[10];

Because when "autochomp" is disabled, $a[10] will read
back as "0 (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.

· Not quite every effort was made to make this module as

efficient as possible. "FETCHSIZE" should use binary
search instead of linear search.

The performance of the "flush" method could be
improved. At present, it still rewrites the tail of
the file once for each block of contiguous lines to be
changed. In the typical case, this will result in
only one rewrite, but in peculiar cases it might be
bad. It should be possible to perform all deferred
writing with a single rewrite.

Profiling suggests that these defects are probably
minor; in any event, they will be fixed in a future
version of the module.

· 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 main
tain 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 tried to take it.

· You might be tempted to think that deferred writing is

like transactions, with "flush" as "commit" and "dis
card" as "rollback", but it isn't, so don't.

SUBCLASSING

This version promises absolutely nothing about the inter
nals, which may change without notice. A future version
of the module will have a well-defined and stable sub
classing 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 documen
tation and any news of importance, will be available at

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

LICENSE

"Tie::File" version 0.93 is copyright (C) 2002 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 Artis
tic 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., 59 Temple Place, Suite 330, Boston, MA
02111 USA

For licensing inquiries, contact the author at:

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

WARRANTY

"Tie::File" version 0.93 comes with ABSOLUTELY NO WAR
RANTY. 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. (Usu
ally 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).

Additional thanks to: Edward Avis / Gerrit Haase / Nikola
Knezevic / Nick Ing-Simmons / Tassilo von Parseval / H.
Dieter Pearcey / Slaven Rezic / Peter Scott / Peter Somu /
Autrijus Tang (again) / Tels / Juerd Wallboer

TODO

More tests. (The cache and heap modules need more unit
tests.)

Improve SPLICE algorithm to use deferred writing machin
ery.

Cleverer strategy for flushing deferred writes.

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

Paragraph mode?

Fixed-length mode. Leave-blanks mode.

Maybe an autolocking mode?

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

More tests.

perl v5.8.0 2002-06-01 Tie::File(3)