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Tie::File(3)
NAME
Tie::File - Access the lines of a disk file via a Perl array
SYNOPSIS
# This file documents Tie::File version 0.97
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:
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', $fh, ...;
Similarly if you opened that handle "FH" with regular "open" or "sysopen",
you may use:
tie @array, 'Tie::File', \*FH, ...;
Handles that were opened write-only won't work. Handles that were opened
read-only will work as long as you don't try to modify the array. Handles
must be attached to seekable sources of data---that means no pipes or
sockets. If "Tie::File" can detect that you supplied a non-seekable
handle, the "tie" call will throw an exception. (On Unix systems, it can
detect this.)
Note that Tie::File will only close any filehandles that it opened
internally. If you passed it a filehandle as above, you "own" the
filehandle, and are responsible for closing it after you have untied the
@array.
Deferred Writing
(This is an advanced feature. Skip this section on first reading.)
Normally, modifying a "Tie::File" array writes to the underlying file
immediately. Every assignment like "$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 0.98.
CAVEATS
(That's Latin for 'warnings'.)
· 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.
· 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, $a[10] will read back as "\n" (or
whatever the record separator string is.)
There are other minor differences, particularly regarding "exists" and
"delete", but in general, the correspondence is extremely close.
· I have supposed that since this module is concerned with file I/O,
almost all normal use of it will be heavily I/O bound. This means that
the time to maintain complicated data structures inside the module will
be dominated by the time to actually perform the I/O. When there was
an opportunity to spend CPU time to avoid doing I/O, I usually tried to
take it.
· You might be tempted to think that deferred writing is like
transactions, with "flush" as "commit" and "discard" as "rollback", but
it isn't, so don't.
· There is a large memory overhead for each record offset and for each
cache entry: about 310 bytes per cached data record, and about 21 bytes
per offset table entry.
The per-record overhead will limit the maximum number of records you
can access per file. Note that accessing the length of the array via
"$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 0.97 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 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 0.97 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 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
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.
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