tie::file.3

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.\" ========================================================================
.\"
.IX Title "Tie::File 3"
.TH Tie::File 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Tie::File \- Access the lines of a disk file via a Perl array
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&        # This file documents Tie::File version 0.97
\&        use Tie::File;
\&
\&        tie @array, \*(AqTie::File\*(Aq, filename or die ...;
\&
\&        $array[13] = \*(Aqblah\*(Aq;     # line 13 of the file is now \*(Aqblah\*(Aq
\&        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 $r2 = shift @array;
\&        @old_recs = splice @array, 3, 7, new recs...;
\&
\&        untie @array;            # all finished
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`Tie::File\*(C'\fR represents a regular text file as a Perl array.  Each
element in the array corresponds to a record in the file.  The first
line of the file is element 0 of the array; the second line is element
1, and so on.
.PP
The file is \fInot\fR loaded into memory, so this will work even for
gigantic files.
.PP
Changes to the array are reflected in the file immediately.
.PP
Lazy people and beginners may now stop reading the manual.
.ie n .Sh """recsep"""
.el .Sh "\f(CWrecsep\fP"
.IX Subsection "recsep"
What is a 'record'?  By default, the meaning is the same as for the
\&\f(CW\*(C`<...>\*(C'\fR operator: It's a string terminated by \f(CW$/\fR, which is
probably \f(CW"\en"\fR.  (Minor exception: on \s-1DOS\s0 and Win32 systems, a
\&'record' is a string terminated by \f(CW"\er\en"\fR.)  You may change the
definition of \*(L"record\*(R" by supplying the \f(CW\*(C`recsep\*(C'\fR option in the \f(CW\*(C`tie\*(C'\fR
call:
.PP
.Vb 1
\&        tie @array, \*(AqTie::File\*(Aq, $file, recsep => \*(Aqes\*(Aq;
.Ve
.PP
This says that records are delimited by the string \f(CW\*(C`es\*(C'\fR.  If the file
contained the following data:
.PP
.Vb 1
\&        Curse these pesky flies!\en
.Ve
.PP
then the \f(CW@array\fR would appear to have four elements:
.PP
.Vb 4
\&        "Curse th"
\&        "e p"
\&        "ky fli"
\&        "!\en"
.Ve
.PP
An undefined value is not permitted as a record separator.  Perl's
special \*(L"paragraph mode\*(R" semantics (a\*` la \f(CW\*(C`$/ = ""\*(C'\fR) are not
emulated.
.PP
Records read from the tied array do not have the record separator
string on the end; this is to allow
.PP
.Vb 1
\&        $array[17] .= "extra";
.Ve
.PP
to work as expected.
.PP
(See \*(L"autochomp\*(R", 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 \f(CW"\en"\fR, then the following two lines do exactly
the same thing:
.PP
.Vb 2
\&        $array[17] = "Cherry pie";
\&        $array[17] = "Cherry pie\en";
.Ve
.PP
The result is that the contents of line 17 of the file will be
replaced with \*(L"Cherry pie\*(R"; a newline character will separate line 17
from line 18.  This means that this code will do nothing:
.PP
.Vb 1
\&        chomp $array[17];
.Ve
.PP
Because the \f(CW\*(C`chomp\*(C'\fRed 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.
.PP
Inserting records that \fIcontain\fR 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.
.ie n .Sh """autochomp"""
.el .Sh "\f(CWautochomp\fP"
.IX Subsection "autochomp"
Normally, array elements have the record separator removed, so that if
the file contains the text
.PP
.Vb 3
\&        Gold
\&        Frankincense
\&        Myrrh
.Ve
.PP
the tied array will appear to contain \f(CW\*(C`("Gold", "Frankincense",
"Myrrh")\*(C'\fR.  If you set \f(CW\*(C`autochomp\*(C'\fR to a false value, the record
separator will not be removed.  If the file above was tied with
.PP
.Vb 1
\&        tie @gifts, "Tie::File", $gifts, autochomp => 0;
.Ve
.PP
then the array \f(CW@gifts\fR would appear to contain \f(CW\*(C`("Gold\en",
"Frankincense\en", "Myrrh\en")\*(C'\fR, or (on Win32 systems) \f(CW\*(C`("Gold\er\en",
"Frankincense\er\en", "Myrrh\er\en")\*(C'\fR.
.ie n .Sh """mode"""
.el .Sh "\f(CWmode\fP"
.IX Subsection "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
\&\f(CW\*(C`O_RDWR | O_CREAT\*(C'\fR are supplied in the \f(CW\*(C`open\*(C'\fR call.)  If you want to
change this, you may supply alternative flags in the \f(CW\*(C`mode\*(C'\fR option.
See Fcntl for a listing of available flags.
For example:
.PP
.Vb 3
\&        # open the file if it exists, but fail if it does not exist
\&        use Fcntl \*(AqO_RDWR\*(Aq;
\&        tie @array, \*(AqTie::File\*(Aq, $file, mode => O_RDWR;
\&
\&        # create the file if it does not exist
\&        use Fcntl \*(AqO_RDWR\*(Aq, \*(AqO_CREAT\*(Aq;
\&        tie @array, \*(AqTie::File\*(Aq, $file, mode => O_RDWR | O_CREAT;
\&
\&        # open an existing file in read\-only mode
\&        use Fcntl \*(AqO_RDONLY\*(Aq;
\&        tie @array, \*(AqTie::File\*(Aq, $file, mode => O_RDONLY;
.Ve
.PP
Opening the data file in write-only or append mode is not supported.
.ie n .Sh """memory"""
.el .Sh "\f(CWmemory\fP"
.IX Subsection "memory"
This is an upper limit on the amount of memory that \f(CW\*(C`Tie::File\*(C'\fR will
consume at any time while managing the file.  This is used for two
things: managing the \fIread cache\fR and managing the \fIdeferred write
buffer\fR.
.PP
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 \fIread cache\fR.  The amount of data in the read cache will not
exceed the value you specified for \f(CW\*(C`memory\*(C'\fR.  If \f(CW\*(C`Tie::File\*(C'\fR 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.
.PP
The default memory limit is 2Mib.  You can adjust the maximum read
cache size by supplying the \f(CW\*(C`memory\*(C'\fR option.  The argument is the
desired cache size, in bytes.
.PP
.Vb 2
\&        # I have a lot of memory, so use a large cache to speed up access
\&        tie @array, \*(AqTie::File\*(Aq, $file, memory => 20_000_000;
.Ve
.PP
Setting the memory limit to 0 will inhibit caching; records will be
fetched from disk every time you examine them.
.PP
The \f(CW\*(C`memory\*(C'\fR value is not an absolute or exact limit on the memory
used.  \f(CW\*(C`Tie::File\*(C'\fR objects contains some structures besides the read
cache and the deferred write buffer, whose sizes are not charged
against \f(CW\*(C`memory\*(C'\fR.
.PP
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.
.ie n .Sh """dw_size"""
.el .Sh "\f(CWdw_size\fP"
.IX Subsection "dw_size"
(This is an advanced feature.  Skip this section on first reading.)
.PP
If you use deferred writing (See \*(L"Deferred Writing\*(R", below) then
data you write into the array will not be written directly to the
file; instead, it will be saved in the \fIdeferred write buffer\fR to be
written out later.  Data in the deferred write buffer is also charged
against the memory limit you set with the \f(CW\*(C`memory\*(C'\fR option.
.PP
You may set the \f(CW\*(C`dw_size\*(C'\fR 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 \f(CW\*(C`dw_size\*(C'\fR to 1000 and
\&\f(CW\*(C`memory\*(C'\fR 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.)
.PP
If you don't specify a \f(CW\*(C`dw_size\*(C'\fR, it defaults to the entire memory
limit.
.Sh "Option Format"
.IX Subsection "Option Format"
\&\f(CW\*(C`\-mode\*(C'\fR is a synonym for \f(CW\*(C`mode\*(C'\fR.  \f(CW\*(C`\-recsep\*(C'\fR is a synonym for
\&\f(CW\*(C`recsep\*(C'\fR.  \f(CW\*(C`\-memory\*(C'\fR is a synonym for \f(CW\*(C`memory\*(C'\fR.  You get the
idea.
.SH "Public Methods"
.IX Header "Public Methods"
The \f(CW\*(C`tie\*(C'\fR call returns an object, say \f(CW$o\fR.  You may call
.PP
.Vb 2
\&        $rec = $o\->FETCH($n);
\&        $o\->STORE($n, $rec);
.Ve
.PP
to fetch or store the record at line \f(CW$n\fR, respectively; similarly
the other tied array methods.  (See perltie for details.)  You may
also call the following methods on this object:
.ie n .Sh """flock"""
.el .Sh "\f(CWflock\fP"
.IX Subsection "flock"
.Vb 1
\&        $o\->flock(MODE)
.Ve
.PP
will lock the tied file.  \f(CW\*(C`MODE\*(C'\fR has the same meaning as the second
argument to the Perl built-in \f(CW\*(C`flock\*(C'\fR function; for example
\&\f(CW\*(C`LOCK_SH\*(C'\fR or \f(CW\*(C`LOCK_EX | LOCK_NB\*(C'\fR.  (These constants are provided by
the \f(CW\*(C`use Fcntl \*(Aq:flock\*(Aq\*(C'\fR declaration.)
.PP
\&\f(CW\*(C`MODE\*(C'\fR is optional; the default is \f(CW\*(C`LOCK_EX\*(C'\fR.
.PP