Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)NAMETie::File - Access the lines of a disk file via a Perl array
SYNOPSIS
# This file documents Tie::File version 0.97
use Tie::File;
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 $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:
perl v5.8.8 2005-02-05 1
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
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 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.
perl v5.8.8 2005-02-05 2
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
"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;
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.
perl v5.8.8 2005-02-05 3
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
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 speci-
fied 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 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.)
perl v5.8.8 2005-02-05 4
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
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; simi-
larly 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 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 = ()".)
perl v5.8.8 2005-02-05 5
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
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', $fh, ...;
Similarly if you opened that handle "FH" with regular "open"
or "sysopen", you may use:
tie @array, 'Tie::File', \*FH, ...;
perl v5.8.8 2005-02-05 6
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
Handles that were opened write-only won't work. Handles
that were opened read-only will work as long as you don't
try to modify the array. Handles must be attached to seek-
able sources of data---that means no pipes or sockets. If
"Tie::File" can detect that you supplied a non-seekable han-
dle, the "tie" call will throw an exception. (On Unix sys-
tems, it can detect this.)
Note that Tie::File will only close any filehandles that it
opened internally. If you passed it a filehandle as above,
you "own" the filehandle, and are responsible for closing it
after you have untied the @array.
Deferred Writing
(This is an advanced feature. Skip this section on first
reading.)
Normally, modifying a "Tie::File" array writes to the under-
lying file immediately. Every assignment like "$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 writ-
ing will done in memory. Then, when you call "->flush", the
entire file will be rewritten in a single pass.
perl v5.8.8 2005-02-05 7
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
(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 speci-
fied 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 emp-
tied, 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.
perl v5.8.8 2005-02-05 8
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
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 mis-
taken. Tie::File will not support safe concurrent access
before version 0.98.
CAVEATS
(That's Latin for 'warnings'.)
+ 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!\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];
perl v5.8.8 2005-02-05 9
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
Because when "autochomp" is disabled, $a[10] will read
back as "\n" (or whatever the record separator string
is.)
There are other minor differences, particularly regard-
ing "exists" and "delete", but in general, the
correspondence is extremely close.
+ I have supposed that since this module is concerned with
file I/O, almost all normal use of it will be heavily
I/O bound. This means that the time to maintain compli-
cated data structures inside the module will be dom-
inated by the time to actually perform the I/O. When
there was an opportunity to spend CPU time to avoid
doing I/O, I usually tried to take it.
+ You might be tempted to think that deferred writing is
like transactions, with "flush" as "commit" and "dis-
card" as "rollback", but it isn't, so don't.
+ There is a large memory overhead for each record offset
and for each cache entry: about 310 bytes per cached
data record, and about 21 bytes per offset table entry.
The per-record overhead will limit the maximum number of
records you can access per file. Note that accessing the
length of the array via "$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 inter-
nals, 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"
perl v5.8.8 2005-02-05 10
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
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 documenta-
tion and any news of importance, will be available at
http://perl.plover.com/TieFile/
LICENSE
"Tie::File" version 0.97 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 use-
ful, but WITHOUT ANY WARRANTY; without even the implied war-
ranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR-
POSE. 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 Founda-
tion, 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 0.97 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 gen-
erally 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
perl v5.8.8 2005-02-05 11
Tie::File(3p) Perl Programmers Reference Guide Tie::File(3p)
Schwern (for testing advice), and the rest of the CPAN tes-
ters (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
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 lia-
bility. 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 statis-
tics 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 v5.8.8 2005-02-05 12
