1Tie::File(3pm)         Perl Programmers Reference Guide         Tie::File(3pm)
2
3
4

NAME

6       Tie::File - Access the lines of a disk file via a Perl array
7

SYNOPSIS

9        # This file documents Tie::File version 0.98
10        use Tie::File;
11
12        tie @array, 'Tie::File', filename or die ...;
13
14        $array[13] = 'blah';     # line 13 of the file is now 'blah'
15        print $array[42];        # display line 42 of the file
16
17        $n_recs = @array;        # how many records are in the file?
18        $#array -= 2;            # chop two records off the end
19
20
21        for (@array) {
22          s/PERL/Perl/g;        # Replace PERL with Perl everywhere in the file
23        }
24
25        # These are just like regular push, pop, unshift, shift, and splice
26        # Except that they modify the file in the way you would expect
27
28        push @array, new recs...;
29        my $r1 = pop @array;
30        unshift @array, new recs...;
31        my $r2 = shift @array;
32        @old_recs = splice @array, 3, 7, new recs...;
33
34        untie @array;            # all finished
35

DESCRIPTION

37       "Tie::File" represents a regular text file as a Perl array.  Each
38       element in the array corresponds to a record in the file.  The first
39       line of the file is element 0 of the array; the second line is element
40       1, and so on.
41
42       The file is not loaded into memory, so this will work even for gigantic
43       files.
44
45       Changes to the array are reflected in the file immediately.
46
47       Lazy people and beginners may now stop reading the manual.
48
49   "recsep"
50       What is a 'record'?  By default, the meaning is the same as for the
51       "<...>" operator: It's a string terminated by $/, which is probably
52       "\n".  (Minor exception: on DOS and Win32 systems, a 'record' is a
53       string terminated by "\r\n".)  You may change the definition of
54       "record" by supplying the "recsep" option in the "tie" call:
55
56               tie @array, 'Tie::File', $file, recsep => 'es';
57
58       This says that records are delimited by the string "es".  If the file
59       contained the following data:
60
61               Curse these pesky flies!\n
62
63       then the @array would appear to have four elements:
64
65               "Curse th"
66               "e p"
67               "ky fli"
68               "!\n"
69
70       An undefined value is not permitted as a record separator.  Perl's
71       special "paragraph mode" semantics (a la "$/ = """) are not emulated.
72
73       Records read from the tied array do not have the record separator
74       string on the end; this is to allow
75
76               $array[17] .= "extra";
77
78       to work as expected.
79
80       (See "autochomp", below.)  Records stored into the array will have the
81       record separator string appended before they are written to the file,
82       if they don't have one already.  For example, if the record separator
83       string is "\n", then the following two lines do exactly the same thing:
84
85               $array[17] = "Cherry pie";
86               $array[17] = "Cherry pie\n";
87
88       The result is that the contents of line 17 of the file will be replaced
89       with "Cherry pie"; a newline character will separate line 17 from line
90       18.  This means that this code will do nothing:
91
92               chomp $array[17];
93
94       Because the "chomp"ed value will have the separator reattached when it
95       is written back to the file.  There is no way to create a file whose
96       trailing record separator string is missing.
97
98       Inserting records that contain the record separator string is not
99       supported by this module.  It will probably produce a reasonable
100       result, but what this result will be may change in a future version.
101       Use 'splice' to insert records or to replace one record with several.
102
103   "autochomp"
104       Normally, array elements have the record separator removed, so that if
105       the file contains the text
106
107               Gold
108               Frankincense
109               Myrrh
110
111       the tied array will appear to contain "("Gold", "Frankincense",
112       "Myrrh")".  If you set "autochomp" to a false value, the record
113       separator will not be removed.  If the file above was tied with
114
115               tie @gifts, "Tie::File", $gifts, autochomp => 0;
116
117       then the array @gifts would appear to contain "("Gold\n",
118       "Frankincense\n", "Myrrh\n")", or (on Win32 systems) "("Gold\r\n",
119       "Frankincense\r\n", "Myrrh\r\n")".
120
121   "mode"
122       Normally, the specified file will be opened for read and write access,
123       and will be created if it does not exist.  (That is, the flags "O_RDWR
124       | O_CREAT" are supplied in the "open" call.)  If you want to change
125       this, you may supply alternative flags in the "mode" option.  See Fcntl
126       for a listing of available flags.  For example:
127
128               # open the file if it exists, but fail if it does not exist
129               use Fcntl 'O_RDWR';
130               tie @array, 'Tie::File', $file, mode => O_RDWR;
131
132               # create the file if it does not exist
133               use Fcntl 'O_RDWR', 'O_CREAT';
134               tie @array, 'Tie::File', $file, mode => O_RDWR | O_CREAT;
135
136               # open an existing file in read-only mode
137               use Fcntl 'O_RDONLY';
138               tie @array, 'Tie::File', $file, mode => O_RDONLY;
139
140       Opening the data file in write-only or append mode is not supported.
141
142   "memory"
143       This is an upper limit on the amount of memory that "Tie::File" will
144       consume at any time while managing the file.  This is used for two
145       things: managing the read cache and managing the deferred write buffer.
146
147       Records read in from the file are cached, to avoid having to re-read
148       them repeatedly.  If you read the same record twice, the first time it
149       will be stored in memory, and the second time it will be fetched from
150       the read cache.  The amount of data in the read cache will not exceed
151       the value you specified for "memory".  If "Tie::File" wants to cache a
152       new record, but the read cache is full, it will make room by expiring
153       the least-recently visited records from the read cache.
154
155       The default memory limit is 2Mib.  You can adjust the maximum read
156       cache size by supplying the "memory" option.  The argument is the
157       desired cache size, in bytes.
158
159        # I have a lot of memory, so use a large cache to speed up access
160        tie @array, 'Tie::File', $file, memory => 20_000_000;
161
162       Setting the memory limit to 0 will inhibit caching; records will be
163       fetched from disk every time you examine them.
164
165       The "memory" value is not an absolute or exact limit on the memory
166       used.  "Tie::File" objects contains some structures besides the read
167       cache and the deferred write buffer, whose sizes are not charged
168       against "memory".
169
170       The cache itself consumes about 310 bytes per cached record, so if your
171       file has many short records, you may want to decrease the cache memory
172       limit, or else the cache overhead may exceed the size of the cached
173       data.
174
175   "dw_size"
176       (This is an advanced feature.  Skip this section on first reading.)
177
178       If you use deferred writing (See "Deferred Writing", below) then data
179       you write into the array will not be written directly to the file;
180       instead, it will be saved in the deferred write buffer to be written
181       out later.  Data in the deferred write buffer is also charged against
182       the memory limit you set with the "memory" option.
183
184       You may set the "dw_size" option to limit the amount of data that can
185       be saved in the deferred write buffer.  This limit may not exceed the
186       total memory limit.  For example, if you set "dw_size" to 1000 and
187       "memory" to 2500, that means that no more than 1000 bytes of deferred
188       writes will be saved up.  The space available for the read cache will
189       vary, but it will always be at least 1500 bytes (if the deferred write
190       buffer is full) and it could grow as large as 2500 bytes (if the
191       deferred write buffer is empty.)
192
193       If you don't specify a "dw_size", it defaults to the entire memory
194       limit.
195
196   Option Format
197       "-mode" is a synonym for "mode".  "-recsep" is a synonym for "recsep".
198       "-memory" is a synonym for "memory".  You get the idea.
199

Public Methods

201       The "tie" call returns an object, say $o.  You may call
202
203               $rec = $o->FETCH($n);
204               $o->STORE($n, $rec);
205
206       to fetch or store the record at line $n, respectively; similarly the
207       other tied array methods.  (See perltie for details.)  You may also
208       call the following methods on this object:
209
210   "flock"
211               $o->flock(MODE)
212
213       will lock the tied file.  "MODE" has the same meaning as the second
214       argument to the Perl built-in "flock" function; for example "LOCK_SH"
215       or "LOCK_EX | LOCK_NB".  (These constants are provided by the "use
216       Fcntl ':flock'" declaration.)
217
218       "MODE" is optional; the default is "LOCK_EX".
219
220       "Tie::File" maintains an internal table of the byte offset of each
221       record it has seen in the file.
222
223       When you use "flock" to lock the file, "Tie::File" assumes that the
224       read cache is no longer trustworthy, because another process might have
225       modified the file since the last time it was read.  Therefore, a
226       successful call to "flock" discards the contents of the read cache and
227       the internal record offset table.
228
229       "Tie::File" promises that the following sequence of operations will be
230       safe:
231
232               my $o = tie @array, "Tie::File", $filename;
233               $o->flock;
234
235       In particular, "Tie::File" will not read or write the file during the
236       "tie" call.  (Exception: Using "mode => O_TRUNC" will, of course, erase
237       the file during the "tie" call.  If you want to do this safely, then
238       open the file without "O_TRUNC", lock the file, and use "@array = ()".)
239
240       The best way to unlock a file is to discard the object and untie the
241       array.  It is probably unsafe to unlock the file without also untying
242       it, because if you do, changes may remain unwritten inside the object.
243       That is why there is no shortcut for unlocking.  If you really want to
244       unlock the file prematurely, you know what to do; if you don't know
245       what to do, then don't do it.
246
247       All the usual warnings about file locking apply here.  In particular,
248       note that file locking in Perl is advisory, which means that holding a
249       lock will not prevent anyone else from reading, writing, or erasing the
250       file; it only prevents them from getting another lock at the same time.
251       Locks are analogous to green traffic lights: If you have a green light,
252       that does not prevent the idiot coming the other way from plowing into
253       you sideways; it merely guarantees to you that the idiot does not also
254       have a green light at the same time.
255
256   "autochomp"
257               my $old_value = $o->autochomp(0);    # disable autochomp option
258               my $old_value = $o->autochomp(1);    #  enable autochomp option
259
260               my $ac = $o->autochomp();   # recover current value
261
262       See "autochomp", above.
263
264   "defer", "flush", "discard", and "autodefer"
265       See "Deferred Writing", below.
266
267   "offset"
268               $off = $o->offset($n);
269
270       This method returns the byte offset of the start of the $nth record in
271       the file.  If there is no such record, it returns an undefined value.
272

Tying to an already-opened filehandle

274       If $fh is a filehandle, such as is returned by "IO::File" or one of the
275       other "IO" modules, you may use:
276
277               tie @array, 'Tie::File', $fh, ...;
278
279       Similarly if you opened that handle "FH" with regular "open" or
280       "sysopen", you may use:
281
282               tie @array, 'Tie::File', \*FH, ...;
283
284       Handles that were opened write-only won't work.  Handles that were
285       opened read-only will work as long as you don't try to modify the
286       array.  Handles must be attached to seekable sources of data---that
287       means no pipes or sockets.  If "Tie::File" can detect that you supplied
288       a non-seekable handle, the "tie" call will throw an exception.  (On
289       Unix systems, it can detect this.)
290
291       Note that Tie::File will only close any filehandles that it opened
292       internally.  If you passed it a filehandle as above, you "own" the
293       filehandle, and are responsible for closing it after you have untied
294       the @array.
295

Deferred Writing

297       (This is an advanced feature.  Skip this section on first reading.)
298
299       Normally, modifying a "Tie::File" array writes to the underlying file
300       immediately.  Every assignment like "$a[3] = ..." rewrites as much of
301       the file as is necessary; typically, everything from line 3 through the
302       end will need to be rewritten.  This is the simplest and most
303       transparent behavior.  Performance even for large files is reasonably
304       good.
305
306       However, under some circumstances, this behavior may be excessively
307       slow.  For example, suppose you have a million-record file, and you
308       want to do:
309
310               for (@FILE) {
311                 $_ = "> $_";
312               }
313
314       The first time through the loop, you will rewrite the entire file, from
315       line 0 through the end.  The second time through the loop, you will
316       rewrite the entire file from line 1 through the end.  The third time
317       through the loop, you will rewrite the entire file from line 2 to the
318       end.  And so on.
319
320       If the performance in such cases is unacceptable, you may defer the
321       actual writing, and then have it done all at once.  The following loop
322       will perform much better for large files:
323
324               (tied @a)->defer;
325               for (@a) {
326                 $_ = "> $_";
327               }
328               (tied @a)->flush;
329
330       If "Tie::File"'s memory limit is large enough, all the writing will
331       done in memory.  Then, when you call "->flush", the entire file will be
332       rewritten in a single pass.
333
334       (Actually, the preceding discussion is something of a fib.  You don't
335       need to enable deferred writing to get good performance for this common
336       case, because "Tie::File" will do it for you automatically unless you
337       specifically tell it not to.  See "Autodeferring", below.)
338
339       Calling "->flush" returns the array to immediate-write mode.  If you
340       wish to discard the deferred writes, you may call "->discard" instead
341       of "->flush".  Note that in some cases, some of the data will have been
342       written already, and it will be too late for "->discard" to discard all
343       the changes.  Support for "->discard" may be withdrawn in a future
344       version of "Tie::File".
345
346       Deferred writes are cached in memory up to the limit specified by the
347       "dw_size" option (see above).  If the deferred-write buffer is full and
348       you try to write still more deferred data, the buffer will be flushed.
349       All buffered data will be written immediately, the buffer will be
350       emptied, and the now-empty space will be used for future deferred
351       writes.
352
353       If the deferred-write buffer isn't yet full, but the total size of the
354       buffer and the read cache would exceed the "memory" limit, the oldest
355       records will be expired from the read cache until the total size is
356       under the limit.
357
358       "push", "pop", "shift", "unshift", and "splice" cannot be deferred.
359       When you perform one of these operations, any deferred data is written
360       to the file and the operation is performed immediately.  This may
361       change in a future version.
362
363       If you resize the array with deferred writing enabled, the file will be
364       resized immediately, but deferred records will not be written.  This
365       has a surprising consequence: "@a = (...)" erases the file immediately,
366       but the writing of the actual data is deferred.  This might be a bug.
367       If it is a bug, it will be fixed in a future version.
368
369   Autodeferring
370       "Tie::File" tries to guess when deferred writing might be helpful, and
371       to turn it on and off automatically.
372
373               for (@a) {
374                 $_ = "> $_";
375               }
376
377       In this example, only the first two assignments will be done
378       immediately; after this, all the changes to the file will be deferred
379       up to the user-specified memory limit.
380
381       You should usually be able to ignore this and just use the module
382       without thinking about deferring.  However, special applications may
383       require fine control over which writes are deferred, or may require
384       that all writes be immediate.  To disable the autodeferment feature,
385       use
386
387               (tied @o)->autodefer(0);
388
389       or
390
391               tie @array, 'Tie::File', $file, autodefer => 0;
392
393       Similarly, "->autodefer(1)" re-enables autodeferment, and
394       "->autodefer()" recovers the current value of the autodefer setting.
395

CONCURRENT ACCESS TO FILES

397       Caching and deferred writing are inappropriate if you want the same
398       file to be accessed simultaneously from more than one process.  Other
399       optimizations performed internally by this module are also incompatible
400       with concurrent access.  A future version of this module will support a
401       "concurrent => 1" option that enables safe concurrent access.
402
403       Previous versions of this documentation suggested using "memory => 0"
404       for safe concurrent access.  This was mistaken.  Tie::File will not
405       support safe concurrent access before version 0.96.
406

CAVEATS

408       (That's Latin for 'warnings'.)
409
410       ·   Reasonable effort was made to make this module efficient.
411           Nevertheless, changing the size of a record in the middle of a
412           large file will always be fairly slow, because everything after the
413           new record must be moved.
414
415       ·   The behavior of tied arrays is not precisely the same as for
416           regular arrays.  For example:
417
418                   # This DOES print "How unusual!"
419                   undef $a[10];  print "How unusual!\n" if defined $a[10];
420
421           "undef"-ing a "Tie::File" array element just blanks out the
422           corresponding record in the file.  When you read it back again,
423           you'll get the empty string, so the supposedly-"undef"'ed value
424           will be defined.  Similarly, if you have "autochomp" disabled, then
425
426                   # This DOES print "How unusual!" if 'autochomp' is disabled
427                   undef $a[10];
428                   print "How unusual!\n" if $a[10];
429
430           Because when "autochomp" is disabled, $a[10] will read back as "\n"
431           (or whatever the record separator string is.)
432
433           There are other minor differences, particularly regarding "exists"
434           and "delete", but in general, the correspondence is extremely
435           close.
436
437       ·   I have supposed that since this module is concerned with file I/O,
438           almost all normal use of it will be heavily I/O bound.  This means
439           that the time to maintain complicated data structures inside the
440           module will be dominated by the time to actually perform the I/O.
441           When there was an opportunity to spend CPU time to avoid doing I/O,
442           I usually tried to take it.
443
444       ·   You might be tempted to think that deferred writing is like
445           transactions, with "flush" as "commit" and "discard" as "rollback",
446           but it isn't, so don't.
447
448       ·   There is a large memory overhead for each record offset and for
449           each cache entry: about 310 bytes per cached data record, and about
450           21 bytes per offset table entry.
451
452           The per-record overhead will limit the maximum number of records
453           you can access per file. Note that accessing the length of the
454           array via "$x = scalar @tied_file" accesses all records and stores
455           their offsets.  The same for "foreach (@tied_file)", even if you
456           exit the loop early.
457

SUBCLASSING

459       This version promises absolutely nothing about the internals, which may
460       change without notice.  A future version of the module will have a
461       well-defined and stable subclassing API.
462

WHAT ABOUT "DB_File"?

464       People sometimes point out that DB_File will do something similar, and
465       ask why "Tie::File" module is necessary.
466
467       There are a number of reasons that you might prefer "Tie::File".  A
468       list is available at "http://perl.plover.com/TieFile/why-not-DB_File".
469

AUTHOR

471       Mark Jason Dominus
472
473       To contact the author, send email to: "mjd-perl-tiefile+@plover.com"
474
475       To receive an announcement whenever a new version of this module is
476       released, send a blank email message to
477       "mjd-perl-tiefile-subscribe@plover.com".
478
479       The most recent version of this module, including documentation and any
480       news of importance, will be available at
481
482               http://perl.plover.com/TieFile/
483

LICENSE

485       "Tie::File" version 0.96 is copyright (C) 2003 Mark Jason Dominus.
486
487       This library is free software; you may redistribute it and/or modify it
488       under the same terms as Perl itself.
489
490       These terms are your choice of any of (1) the Perl Artistic Licence, or
491       (2) version 2 of the GNU General Public License as published by the
492       Free Software Foundation, or (3) any later version of the GNU General
493       Public License.
494
495       This library is distributed in the hope that it will be useful, but
496       WITHOUT ANY WARRANTY; without even the implied warranty of
497       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
498       General Public License for more details.
499
500       You should have received a copy of the GNU General Public License along
501       with this library program; it should be in the file "COPYING".  If not,
502       write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
503       Floor, Boston, MA  02110-1301, USA
504
505       For licensing inquiries, contact the author at:
506
507               Mark Jason Dominus
508               255 S. Warnock St.
509               Philadelphia, PA 19107
510

WARRANTY

512       "Tie::File" version 0.98 comes with ABSOLUTELY NO WARRANTY.  For
513       details, see the license.
514

THANKS

516       Gigantic thanks to Jarkko Hietaniemi, for agreeing to put this in the
517       core when I hadn't written it yet, and for generally being helpful,
518       supportive, and competent.  (Usually the rule is "choose any one.")
519       Also big thanks to Abhijit Menon-Sen for all of the same things.
520
521       Special thanks to Craig Berry and Peter Prymmer (for VMS portability
522       help), Randy Kobes (for Win32 portability help), Clinton Pierce and
523       Autrijus Tang (for heroic eleventh-hour Win32 testing above and beyond
524       the call of duty), Michael G Schwern (for testing advice), and the rest
525       of the CPAN testers (for testing generally).
526
527       Special thanks to Tels for suggesting several speed and memory
528       optimizations.
529
530       Additional thanks to: Edward Avis / Mattia Barbon / Tom Christiansen /
531       Gerrit Haase / Gurusamy Sarathy / Jarkko Hietaniemi (again) / Nikola
532       Knezevic / John Kominetz / Nick Ing-Simmons / Tassilo von Parseval / H.
533       Dieter Pearcey / Slaven Rezic / Eric Roode / Peter Scott / Peter Somu /
534       Autrijus Tang (again) / Tels (again) / Juerd Waalboer / Todd Rinaldo
535

TODO

537       More tests.  (Stuff I didn't think of yet.)
538
539       Paragraph mode?
540
541       Fixed-length mode.  Leave-blanks mode.
542
543       Maybe an autolocking mode?
544
545       For many common uses of the module, the read cache is a liability.  For
546       example, a program that inserts a single record, or that scans the file
547       once, will have a cache hit rate of zero.  This suggests a major
548       optimization: The cache should be initially disabled.  Here's a hybrid
549       approach: Initially, the cache is disabled, but the cache code
550       maintains statistics about how high the hit rate would be *if* it were
551       enabled.  When it sees the hit rate get high enough, it enables itself.
552       The STAT comments in this code are the beginning of an implementation
553       of this.
554
555       Record locking with fcntl()?  Then the module might support an undo log
556       and get real transactions.  What a tour de force that would be.
557
558       Keeping track of the highest cached record. This would allow reads-in-
559       a-row to skip the cache lookup faster (if reading from 1..N with empty
560       cache at start, the last cached value will be always N-1).
561
562       More tests.
563
564
565
566perl v5.28.2                      2018-03-01                    Tie::File(3pm)
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