Manpages - Tie_File.3perl
Table of Contents
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;
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:
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.
@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 =$n=th 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’.)
- 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 aTie::Filearray 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 haveautochompdisabled, then # This DOES print “How unusual!” if autochomp is disabled undef $a[10]; print “How unusual!\n” if $a[10]; Because whenautochompis disabled,$a[10]will read back as"\n"(or whatever the record separator string is.) There are other minor differences, particularly regardingexistsanddelete, 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
flushascommitanddiscardasrollback, 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_fileaccesses all records and stores their offsets. The same forforeach (@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
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.