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.
"-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., 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 0.97 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.