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




NAME

       Tie::File - Access the lines of a disk file via a Perl array


SYNOPSIS

        # This file documents Tie::File version 0.98
        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 0.96.


CAVEATS

       (That's Latin for 'warnings'.)

       o   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.

       o   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.

       o   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.

       o   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.

       o   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 0.96 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.



perl v5.24.0                      2016-03-19                    Tie::File(3pm)

perl 5.24 - Generated Sat Nov 26 07:39:33 CST 2016
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