MirOS Manual: Tie::File(3p)


Tie::File(3p)   Perl Programmers Reference Guide    Tie::File(3p)

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:

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             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.

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     "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.

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     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.)

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     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 = ()".)

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     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, ...;

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     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.

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     (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.

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     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];

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         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"

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     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

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     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.

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