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        use Tie::File;
10
11        tie @array, 'Tie::File', filename or die ...;
12
13        $array[0] = 'blah';      # first line of the file is now 'blah'
14                                   # (line numbering starts at 0)
15        print $array[42];        # display line 43 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
296       Tie::File calls "binmode" on filehandles that it opens internally, but
297       not on filehandles passed in by the user. For consistency, especially
298       if using the tied files cross-platform, you may wish to call "binmode"
299       on the filehandle prior to tying the file.
300

Deferred Writing

302       (This is an advanced feature.  Skip this section on first reading.)
303
304       Normally, modifying a "Tie::File" array writes to the underlying file
305       immediately.  Every assignment like "$a[3] = ..." rewrites as much of
306       the file as is necessary; typically, everything from line 3 through the
307       end will need to be rewritten.  This is the simplest and most
308       transparent behavior.  Performance even for large files is reasonably
309       good.
310
311       However, under some circumstances, this behavior may be excessively
312       slow.  For example, suppose you have a million-record file, and you
313       want to do:
314
315               for (@FILE) {
316                 $_ = "> $_";
317               }
318
319       The first time through the loop, you will rewrite the entire file, from
320       line 0 through the end.  The second time through the loop, you will
321       rewrite the entire file from line 1 through the end.  The third time
322       through the loop, you will rewrite the entire file from line 2 to the
323       end.  And so on.
324
325       If the performance in such cases is unacceptable, you may defer the
326       actual writing, and then have it done all at once.  The following loop
327       will perform much better for large files:
328
329               (tied @a)->defer;
330               for (@a) {
331                 $_ = "> $_";
332               }
333               (tied @a)->flush;
334
335       If "Tie::File"'s memory limit is large enough, all the writing will
336       done in memory.  Then, when you call "->flush", the entire file will be
337       rewritten in a single pass.
338
339       (Actually, the preceding discussion is something of a fib.  You don't
340       need to enable deferred writing to get good performance for this common
341       case, because "Tie::File" will do it for you automatically unless you
342       specifically tell it not to.  See "Autodeferring", below.)
343
344       Calling "->flush" returns the array to immediate-write mode.  If you
345       wish to discard the deferred writes, you may call "->discard" instead
346       of "->flush".  Note that in some cases, some of the data will have been
347       written already, and it will be too late for "->discard" to discard all
348       the changes.  Support for "->discard" may be withdrawn in a future
349       version of "Tie::File".
350
351       Deferred writes are cached in memory up to the limit specified by the
352       "dw_size" option (see above).  If the deferred-write buffer is full and
353       you try to write still more deferred data, the buffer will be flushed.
354       All buffered data will be written immediately, the buffer will be
355       emptied, and the now-empty space will be used for future deferred
356       writes.
357
358       If the deferred-write buffer isn't yet full, but the total size of the
359       buffer and the read cache would exceed the "memory" limit, the oldest
360       records will be expired from the read cache until the total size is
361       under the limit.
362
363       "push", "pop", "shift", "unshift", and "splice" cannot be deferred.
364       When you perform one of these operations, any deferred data is written
365       to the file and the operation is performed immediately.  This may
366       change in a future version.
367
368       If you resize the array with deferred writing enabled, the file will be
369       resized immediately, but deferred records will not be written.  This
370       has a surprising consequence: "@a = (...)" erases the file immediately,
371       but the writing of the actual data is deferred.  This might be a bug.
372       If it is a bug, it will be fixed in a future version.
373
374   Autodeferring
375       "Tie::File" tries to guess when deferred writing might be helpful, and
376       to turn it on and off automatically.
377
378               for (@a) {
379                 $_ = "> $_";
380               }
381
382       In this example, only the first two assignments will be done
383       immediately; after this, all the changes to the file will be deferred
384       up to the user-specified memory limit.
385
386       You should usually be able to ignore this and just use the module
387       without thinking about deferring.  However, special applications may
388       require fine control over which writes are deferred, or may require
389       that all writes be immediate.  To disable the autodeferment feature,
390       use
391
392               (tied @o)->autodefer(0);
393
394       or
395
396               tie @array, 'Tie::File', $file, autodefer => 0;
397
398       Similarly, "->autodefer(1)" re-enables autodeferment, and
399       "->autodefer()" recovers the current value of the autodefer setting.
400

CONCURRENT ACCESS TO FILES

402       Caching and deferred writing are inappropriate if you want the same
403       file to be accessed simultaneously from more than one process.  Other
404       optimizations performed internally by this module are also incompatible
405       with concurrent access.  A future version of this module will support a
406       "concurrent => 1" option that enables safe concurrent access.
407
408       Previous versions of this documentation suggested using "memory => 0"
409       for safe concurrent access.  This was mistaken.  Tie::File will not
410       support safe concurrent access before version 0.96.
411

CAVEATS

413       (That's Latin for 'warnings'.)
414
415       •   Reasonable effort was made to make this module efficient.
416           Nevertheless, changing the size of a record in the middle of a
417           large file will always be fairly slow, because everything after the
418           new record must be moved.
419
420       •   The behavior of tied arrays is not precisely the same as for
421           regular arrays.  For example:
422
423                   # This DOES print "How unusual!"
424                   undef $a[10];  print "How unusual!\n" if defined $a[10];
425
426           "undef"-ing a "Tie::File" array element just blanks out the
427           corresponding record in the file.  When you read it back again,
428           you'll get the empty string, so the supposedly-"undef"'ed value
429           will be defined.  Similarly, if you have "autochomp" disabled, then
430
431                   # This DOES print "How unusual!" if 'autochomp' is disabled
432                   undef $a[10];
433                   print "How unusual!\n" if $a[10];
434
435           Because when "autochomp" is disabled, $a[10] will read back as "\n"
436           (or whatever the record separator string is.)
437
438           There are other minor differences, particularly regarding "exists"
439           and "delete", but in general, the correspondence is extremely
440           close.
441
442       •   I have supposed that since this module is concerned with file I/O,
443           almost all normal use of it will be heavily I/O bound.  This means
444           that the time to maintain complicated data structures inside the
445           module will be dominated by the time to actually perform the I/O.
446           When there was an opportunity to spend CPU time to avoid doing I/O,
447           I usually tried to take it.
448
449       •   You might be tempted to think that deferred writing is like
450           transactions, with "flush" as "commit" and "discard" as "rollback",
451           but it isn't, so don't.
452
453       •   There is a large memory overhead for each record offset and for
454           each cache entry: about 310 bytes per cached data record, and about
455           21 bytes per offset table entry.
456
457           The per-record overhead will limit the maximum number of records
458           you can access per file. Note that accessing the length of the
459           array via "$x = scalar @tied_file" accesses all records and stores
460           their offsets.  The same for "foreach (@tied_file)", even if you
461           exit the loop early.
462

SUBCLASSING

464       This version promises absolutely nothing about the internals, which may
465       change without notice.  A future version of the module will have a
466       well-defined and stable subclassing API.
467

WHAT ABOUT "DB_File"?

469       People sometimes point out that DB_File will do something similar, and
470       ask why "Tie::File" module is necessary.
471
472       There are a number of reasons that you might prefer "Tie::File".  A
473       list is available at
474       "<http://perl.plover.com/TieFile/why-not-DB_File>".
475

AUTHOR

477       Mark Jason Dominus
478
479       To contact the author, send email to: "mjd-perl-tiefile+@plover.com"
480
481       To receive an announcement whenever a new version of this module is
482       released, send a blank email message to
483       "mjd-perl-tiefile-subscribe@plover.com".
484
485       The most recent version of this module, including documentation and any
486       news of importance, will be available at
487
488               http://perl.plover.com/TieFile/
489

LICENSE

491       "Tie::File" version 0.96 is copyright (C) 2003 Mark Jason Dominus.
492
493       This library is free software; you may redistribute it and/or modify it
494       under the same terms as Perl itself.
495
496       These terms are your choice of any of (1) the Perl Artistic Licence, or
497       (2) version 2 of the GNU General Public License as published by the
498       Free Software Foundation, or (3) any later version of the GNU General
499       Public License.
500
501       This library is distributed in the hope that it will be useful, but
502       WITHOUT ANY WARRANTY; without even the implied warranty of
503       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
504       General Public License for more details.
505
506       You should have received a copy of the GNU General Public License along
507       with this library program; it should be in the file "COPYING".  If not,
508       write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
509       Floor, Boston, MA  02110-1301, USA
510
511       For licensing inquiries, contact the author at:
512
513               Mark Jason Dominus
514               255 S. Warnock St.
515               Philadelphia, PA 19107
516

WARRANTY

518       "Tie::File" version 0.98 comes with ABSOLUTELY NO WARRANTY.  For
519       details, see the license.
520

THANKS

522       Gigantic thanks to Jarkko Hietaniemi, for agreeing to put this in the
523       core when I hadn't written it yet, and for generally being helpful,
524       supportive, and competent.  (Usually the rule is "choose any one.")
525       Also big thanks to Abhijit Menon-Sen for all of the same things.
526
527       Special thanks to Craig Berry and Peter Prymmer (for VMS portability
528       help), Randy Kobes (for Win32 portability help), Clinton Pierce and
529       Autrijus Tang (for heroic eleventh-hour Win32 testing above and beyond
530       the call of duty), Michael G Schwern (for testing advice), and the rest
531       of the CPAN testers (for testing generally).
532
533       Special thanks to Tels for suggesting several speed and memory
534       optimizations.
535
536       Additional thanks to: Edward Avis / Mattia Barbon / Tom Christiansen /
537       Gerrit Haase / Gurusamy Sarathy / Jarkko Hietaniemi (again) / Nikola
538       Knezevic / John Kominetz / Nick Ing-Simmons / Tassilo von Parseval / H.
539       Dieter Pearcey / Slaven Rezic / Eric Roode / Peter Scott / Peter Somu /
540       Autrijus Tang (again) / Tels (again) / Juerd Waalboer / Todd Rinaldo
541

TODO

543       More tests.  (Stuff I didn't think of yet.)
544
545       Paragraph mode?
546
547       Fixed-length mode.  Leave-blanks mode.
548
549       Maybe an autolocking mode?
550
551       For many common uses of the module, the read cache is a liability.  For
552       example, a program that inserts a single record, or that scans the file
553       once, will have a cache hit rate of zero.  This suggests a major
554       optimization: The cache should be initially disabled.  Here's a hybrid
555       approach: Initially, the cache is disabled, but the cache code
556       maintains statistics about how high the hit rate would be *if* it were
557       enabled.  When it sees the hit rate get high enough, it enables itself.
558       The STAT comments in this code are the beginning of an implementation
559       of this.
560
561       Record locking with fcntl()?  Then the module might support an undo log
562       and get real transactions.  What a tour de force that would be.
563
564       Keeping track of the highest cached record. This would allow reads-in-
565       a-row to skip the cache lookup faster (if reading from 1..N with empty
566       cache at start, the last cached value will be always N-1).
567
568       More tests.
569
570
571
572perl v5.34.0                      2021-10-18                    Tie::File(3pm)
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