DBI::DBD(3) User Contributed Perl Documentation DBI::DBD(3)
NAME
DBI::DBD - Perl DBI Database Driver Writer's Guide
SYNOPSIS
perldoc DBI::DBD
Version and volatility
This document is still a minimal draft which is in need of further work.
The changes will occur both because the DBI specification is changing and hence the requirements on
DBD drivers change, and because feedback from people reading this document will suggest improvements
to it.
Please read the DBI documentation first and fully, including the DBI FAQ. Then reread the DBI
specification again as you're reading this. It'll help.
This document is a patchwork of contributions from various authors. More contributions (preferably
as patches) are very welcome.
DESCRIPTION
This document is primarily intended to help people writing new database drivers for the Perl Database
Interface (Perl DBI). It may also help others interested in discovering why the internals of a DBD
driver are written the way they are.
This is a guide. Few (if any) of the statements in it are completely authoritative under all
possible circumstances. This means you will need to use judgement in applying the guidelines in this
document. If in any doubt at all, please do contact the dbi-dev mailing list (details given below)
where Tim Bunce and other driver authors can help.
CREATING A NEW DRIVER
The first rule for creating a new database driver for the Perl DBI is very simple: "DON'T!"
There is usually a driver already available for the database you want to use, almost regardless of
which database you choose. And very often, the database will provide an ODBC driver interface, so
you can often use DBD::ODBC to access the database. This is typically less convenient on a Unix box
than on a Microsoft Windows box, but there are numerous options for ODBC driver managers on Unix too,
and very often the ODBC driver is provided by the database supplier. Before deciding that you need
to write a driver, do your homework to ensure that you are not wasting your energies.
[As of December 2002, the consensus is that if you need an ODBC driver manager on Unix, then the
unixODBC driver (available from <http://www.unixodbc.org/) is the way to go.]
The second rule for creating a new database driver for the Perl DBI is also very simple: "Don't --get -get
get someone else to do it for you!"
Nevertheless, there are occasions when it is necessary to write a new driver, often to use a
proprietary language or API to access the database more swiftly, or more comprehensively, than an
ODBC driver can. Then you should read this document very carefully, but with a suitably sceptical
eye. If there is something in here that does not make any sense, question it. You might be right
that the information is bogus. But don't come to that conclusion too quickly.
URLs and mailing lists
The primary web-site for locating DBI software and information is
http://dbi.perl.org/
There are two main and one auxilliary mailing lists for people working with DBI. The primary lists
are dbi-users@perl.org for general users of DBI and DBD drivers, and dbi-dev@perl.org mainly for DBD
driver writers (don't join the dbi-dev list unless you have a good reason). The auxilliary list is
dbi-announce@perl.org for announcing new releases of DBI or DBD drivers.
You can join these lists by accessing the web-site <http://dbi.perl.org/. The lists are closed so
you cannot send email to any of the lists unless you join the list first.
You should also consider monitoring the comp.lang.perl.* newsgroups, especially
comp.lang.perl.modules.
The Cheetah book
The definitive book on Perl DBI is the Cheetah book, so called because of the picture on the cover.
Its proper title is 'Programming the Perl DBI: Database programming with Perl' by Alligator Descartes
and Tim Bunce, published by O'Reilly Associates, February 2000, ISBN 1-56592-699-4. Buy it now if
you have not already done so, and read it.
Locating drivers
Before writing a new driver, it is in your interests to find out whether there already is a driver
for your database. If there is such a driver, it would be much easier to make use of it than to
write your own!
The primary web-site for locating Perl software is <http://search.cpan.org/. You should look under
the various modules listings for the software you are after. For example:
http://search.cpan.org/modlist/Database_Interfaces
Follow the DBD:: and DBIx:: links at the top to see those subsets.
See the DBI docs for information on DBI web sites and mailing lists.
Registering a new driver
Before going through any official registration process, you will need to establish that there is no
driver already in the works. You'll do that by asking the DBI mailing lists whether there is such a
driver available, or whether anybody is working on one.
When you get the go ahead, you will need to establish the name of the driver and a prefix for the
driver. Typically, the name is based on the name of the database software it uses, and the prefix is
a contraction of that. Hence, DBD::Oracle has the name Oracle and the prefix 'ora_'. This
information will be recorded in the DBI module. Apart from documentation purposes, registration is a
prerequisite for installing private methods.
This document assumes you are writing a driver called DBD::Driver, and that the prefix 'drv_' is
assigned to the driver.
Two styles of database driver
There are two distinct styles of database driver that can be written to work with the Perl DBI.
Your driver can be written in pure Perl, requiring no C compiler. When feasible, this is the best
solution, but most databases are not written in such a way that this can be done. Some example pure
Perl drivers are DBD::File and DBD::CSV.
Alternatively, and most commonly, your driver will need to use some C code to gain access to the
database. This will be classified as a C/XS driver.
What code will you write?
There are a number of files that need to be written for either a pure Perl driver or a C/XS driver.
There are no extra files needed only by a pure Perl driver, but there are several extra files needed
only by a C/XS driver.
Files common to pure Perl and C/XS drivers
Assuming that your driver is called DBD::Driver, these files are:
Makefile.PL
README
MANIFEST
Driver.pm
lib/Bundle/DBD/Driver.pm
lib/DBD/Driver/Summary.pm
t/*.t
The first four files are mandatory. Makefile.PL is used to control how the driver is built and
installed. The README file tells people who download the file about how to build the module and any
prerequisite software that must be installed. The MANIFEST file is used by the standard Perl module
distribution mechanism. It lists all the source files that need to be distributed with your module.
Driver.pm is what is loaded by the DBI code; it contains the methods peculiar to your driver.
The lib/Bundle/DBD/Driver.pm file allows you to specify other Perl modules on which yours depends in
a format that allows someone to type a simple command and ensure that all the pre-requisites are in
place as well as building your driver. The lib/DBD/Driver/Summary.pm file contains (an updated
version of) the information that was included - or that would have been included - in the appendices
of the Cheetah book as a summary of the abilities of your driver and the associated database.
The files in the t subdirectory are unit tests for your driver. You should write your tests as
stringently as possible, while taking into account the diversity of installations that you can
encounter. Your tests should not casually modify operational databases. You should never damage
existing tables in a database. You should code your tests to use a constrained name space within the
database. For example, the tables (and all other named objects) that are created could all begin
with 'dbd_drv_'. At the end of a test run, there should be no testing objects left behind in the
database. If you create any databases, you should remove them. If your database supports temporary
tables that are automatically removed at the end of a session, then exploit them as often as
possible. Try to make your tests independent of each other. If you have a test t/t11dowhat.t that
depends upon the successful running of t/t10thingamy.t, people cannot run the single test case
t/t11dowhat.t. Further, running t/t11dowhat.t twice in a row is likely to fail (at least, if
t/t11dowhat.t modifies the database at all) because the database at the start of the second run is
not what you saw at the start of the first run. Document in your README file what you do, and what
privileges people need to do it. You can, and probably should, sequence your tests by including a
test number before an abbreviated version of the test name; the tests are run in the order in which
the names are expanded by shell-style globbing.
Many drivers also install sub-modules DBD::Driver::SubModule for any of a variety of different
reasons, such as to support the metadata methods (see the discussion of "METADATA METHODS" below).
Such sub-modules are conventionally stored in the directory lib/DBD/Driver. The module itself would
usually be in a file SubModule.pm. All such sub-modules should themselves be version stamped (see
the discussions far below).
Extra files needed by C/XS drivers
The software for a C/XS driver will typically contain at least four extra files that are not relevant
to a pure Perl driver.
Driver.xs
Driver.h
dbdimp.h
dbdimp.c
The Driver.xs file is used to generate C code that Perl can call to gain access to the C functions
you write that will, in turn, call down onto your database software.
The Driver.h header is a stylized header that ensures you can access the necessary Perl and DBI
macros, types, and function declarations.
The dbdimp.h is used to specify which functions have been implemented by your driver.
The dbdimp.c file is where you write the C code that does the real work of translating between Perl-
ish data types and what the database expects to use and return.
There are some (mainly small, but very important) differences between the contents of Makefile.PL and
Driver.pm for pure Perl and C/XS drivers, so those files are described both in the section on
creating a pure Perl driver and in the section on creating a C/XS driver.
Obviously, you can add extra source code files to the list.
Requirements on a driver and driver writer
To be remotely useful, your driver must be implemented in a format that allows it to be distributed
via CPAN, the Comprehensive Perl Archive Network (http://www.cpan.org/ and http://search.cpan.org)
Of course, it is easier if you do not have to meet this criterion, but you will not be able to ask
for much help if you do not do so, and no-one is likely to want to install your module if they have
to learn a new installation mechanism.
CREATING A PURE PERL DRIVER
Writing a pure Perl driver is surprisingly simple. However, there are some problems you should be
aware of. The best option is of course picking up an existing driver and carefully modifying one
method after the other.
Also look carefully at DBD::AnyData and DBD::Template.
As an example we take a look at the DBD::File driver, a driver for accessing plain files as tables,
which is part of the DBD::CSV package. In what follows I assume the name "Driver" for your new
package and the prefix 'drv_'. The minimal set of files we have to implement are Makefile.PL,
README, MANIFEST and Driver.pm.
Pure Perl version of Makefile.PL
You typically start with writing "Makefile.PL", a Makefile generator. The contents of this file are
described in detail in the "ExtUtils::MakeMaker" man pages. It is definitely a good idea if you
start reading them. At least you should know about the variables CONFIGURE, DEFINED, PM, DIR,
EXE_FILES, INC, LIBS, LINKTYPE, NAME, OPTIMIZE, PL_FILES, VERSION, VERSION_FROM, clean, depend,
realclean from the "ExtUtils::MakeMaker" man page: These are used in almost any Makefile.PL.
Additionally read the section on Overriding MakeMaker Methods and the descriptions of the distcheck,
disttest and dist targets: They will definitely be useful for you.
Of special importance for DBI drivers is the postamble method from the "ExtUtils::MM_Unix" man page.
And, for Emacs users, I recommend the libscan method, which removes Emacs backup files (file names
which end with a tilde '~') from lists of files.
Now an example, I use the word "Driver" wherever you should insert your driver's name:
# -*- perl -*-
use DBI 1.03;
use DBI::DBD;
use ExtUtils::MakeMaker;
WriteMakefile(
dbd_edit_mm_attribs( {
'NAME' => 'DBD::Driver',
'VERSION_FROM' => 'Driver.pm',
'INC' => $DBI_INC_DIR,
'dist' => { 'SUFFIX' => '.gz',
'COMPRESS' => 'gzip -9f' },
'realclean' => { FILES => '*.xsi' },
},
{ create_pp_tests => 1})
);
package MY;
sub postamble { return main::dbd_postamble(@_); }
sub libscan {
my ($self, $path) = @_;
($path =~ m/\~$/) ? undef : $path;
}
Note the calls to "dbd_edit_mm_attribs"() and "dbd_postamble"(). The second hash reference in the
call to "dbd_edit_mm_attribs" (containing "create_pp_tests") is optional; you should not use it
unless your driver is a pure Perl driver (that is, it does not use C and XS code). Therefore, the
call to "dbd_edit_mm_attribs" is not relevant for C/XS drivers and may be omitted; simply use the
(single) hash reference containing NAME etc as the only argument to "WriteMakefile"(). Note that the
"dbd_edit_mm_attribs" code will fail if you do not have a "t" sub-directory containing at least one
test case. All drivers must use "dbd_postamble" or risk running into problems.
Note the specification of VERSION_FROM; the named file (Driver.pm) will be scanned for the first line
that looks like an assignment to $VERSION, and the subsequent text will be used to determine the
version number. Note the commentary in ExtUtils::MakeMaker on the subject of correctly formatted
version numbers.
If your driver depends upon external software (it usually will), you will need to add code to ensure
that your environment is workable before the call to "WriteMakefile"(). A full-fledged Makefile.PL
can be quite large (for example, the files for DBD::Oracle and DBD::Informix are both over 1000 lines
long, and the Informix one uses - and creates - auxilliary modules too).
See also ExtUtils::MakeMaker and ExtUtils::MM_Unix. Consider using CPAN::MakeMaker in place of
ExtUtils::MakeMaker.
README
The README file should describe what the driver is for, the pre-requisites for the build process, the
actual build process, how to report errors, and who to report them to. Users will find ways of
breaking the driver build and test process which you would never even have dreamed to be possible in
your worst nightmares. Therefore, you need to write this document defensively, precisely and
concisely. Also, it is in your interests to ensure that your tests work as widely as possible. As
always, use the README from one of the established drivers as a basis for your own; the version in
DBD::Informix is worth a look as it has been quite successful in heading off problems.
Note that users will have versions of Perl and DBI that are both older and newer than you expected,
but this will seldom cause much trouble. When it does, it will be because you are using features
of DBI that are not supported in the version they are using.
Note that users will have versions of the database software that are both older and newer than you
expected. You will save yourself time in the long run if you can identify the range of versions
which have been tested and warn about versions which are not known to be OK.
Note that many people trying to install your driver will not be experts in the database software.
Note that many people trying to install your driver will not be experts in C or Perl.
MANIFEST
The MANIFEST will be used by the Makefile's dist target to build the distribution tar file that is
uploaded to CPAN. It should list every file that you want to include in your distribution, one per
line.
lib/Bundle/DBD/Driver.pm
The CPAN module provides an extremely powerful bundle mechanism that allows you to specify pre-
requisites for your driver. The primary pre-requisite is Bundle::DBI; you may want or need to add
some more. With the bundle set up correctly, the user can type:
perl -MCPAN -e 'install Bundle::DBD::Driver'
and Perl will download, compile, test and install all the Perl modules needed to build your driver.
A suitable skeleton for this file is shown below. The prerequisite modules are listed in the
"CONTENTS" section, with the official name of the module followed by a dash and an informal name or
description. Listing Bundle::DBI as the main pre-requisite simplifies life. Don't forget to list
your driver. Note that unless the DBMS is itself a Perl module, you cannot list it as a pre-
requisite in this file. You should keep the version of the bundle the same as the version of your
driver. You should add configuration management, copyright, and licencing information at the top.
package Bundle::DBD::Driver;
$VERSION = '0.01';
1;
__END__
=head1 NAME
Bundle::DBD::Driver - A bundle to install all DBD::Driver related modules
=head1 SYNOPSIS
C<perl -MCPAN -e 'install Bundle::DBD::Driver'>
=head1 CONTENTS
Bundle::DBI - Bundle for DBI by TIMB (Tim Bunce)
DBD::Driver - DBD::Driver by YOU (Your Name)
=head1 DESCRIPTION
This bundle includes all the modules used by the Perl Database
Interface (DBI) driver for Driver (DBD::Driver), assuming the
use of DBI version 1.13 or later, created by Tim Bunce.
If you've not previously used the CPAN module to install any
bundles, you will be interrogated during its setup phase.
But when you've done it once, it remembers what you told it.
You could start by running:
C<perl -MCPAN -e 'install Bundle::CPAN'>
=head1 SEE ALSO
Bundle::DBI
=head1 AUTHOR
Your Name E<lt>F<you@yourdomain.com>E<gt>
=head1 THANKS
This bundle was created by ripping off Bundle::libnet created by
Graham Barr E<lt>F<gbarr@ti.com>E<gt>, and radically simplified
with some information from Jochen Wiedmann E<lt>F<joe@ispsoft.de>E<gt>.
The template was then included in the DBI::DBD documentation by
Jonathan Leffler E<lt>F<jleffler@informix.com>E<gt>.
=cut
lib/DBD/Driver/Summary.pm
There is no substitute for taking the summary file from a driver that was documented in the Perl book
(such as DBD::Oracle or DBD::Informix or DBD::ODBC, to name but three), and adapting it to describe
the facilities available via DBD::Driver when accessing the Driver database.
Pure Perl version of Driver.pm
The "Driver.pm" file defines the Perl module DBD::Driver for your driver. It will define a package
DBD::Driver along with some version information, some variable definitions, and a function driver()
which will have a more or less standard structure.
It will also define three sub-packages of DBD::Driver:
DBD::Driver::dr
with methods connect(), data_sources() and disconnect_all();
DBD::Driver::db
with methods such as prepare();
DBD::Driver::st
with methods such as execute() and fetch().
The Driver.pm file will also contain the documentation specific to DBD::Driver in the format used by
perldoc.
In a pure Perl driver, the Driver.pm file is the core of the implementation. You will need to
provide all the key methods needed by DBI.
Now let's take a closer look at an excerpt of File.pm as an example. We ignore things that are
common to any module (even non-DBI modules) or really specific to the DBD::File package.
The DBD::Driver package
The header
package DBD::File;
use strict;
use vars qw($VERSION $drh);
$VERSION = "1.23.00" # Version number of DBD::File
This is where the version number of your driver is specified. The code in Makefile.PL is told to
look in this file for the information. It is recommended that you use a two-part (1.23) or three-
part (1.23.45) version number. Please ensure that any other modules added with your driver are also
version stamped so that CPAN does not get confused. Also consider the CPAN system, which gets
confused and considers version 1.10 to precede version 1.9, so that using a raw CVS, RCS or SCCS
version number is probably not appropriate (despite being very common). For RCS or CVS you can use
this code:
$VERSION = sprintf "%d.%02d", '$Revision: 11.21 $ ' =~ /(\d+)\.(\d+)/;
which pads out the fractional part with leading zeros so all is well (so long as you don't go past
x.99)
$drh = undef; # holds driver handle once initialized
This is where the driver handle will be stored, once created. Note that you may assume there is only
one handle for your driver.
The driver constructor
Note that the driver method is in the DBD::Driver package, not in one of the sub-packages
DBD::Driver::dr, DBD::Driver::db, or DBD::Driver::db.
sub driver
{
return $drh if $drh; # already created - return same one
my ($class, $attr) = @_;
$class .= "::dr";
# not a 'my' since we use it above to prevent multiple drivers
$drh = DBI::_new_drh($class, {
'Name' => 'File',
'Version' => $VERSION,
'Attribution' => 'DBD::File by Jochen Wiedmann',
})
or return undef;
return $drh;
}
The driver method is the driver handle constructor. It's a reasonable example of how DBI implements
its handles. There are three kinds: driver handles (typically stored in $drh; from now on called
"drh" or $drh), database handles (from now on called "dbh" or $dbh) and statement handles (from now
on called "sth" or $sth).
The prototype of DBI::_new_drh is
$drh = DBI::_new_drh($class, $public_attrs, $private_attrs);
with the following arguments:
$class
is typically the class for your driver, (for example, "DBD::File::dr"), passed as the first
argument to the driver method.
$public_attrs
is a hash ref to attributes like Name, Version, and Attribution. These are processed and used by
DBI. You had better not make any assumptions about them nor should you add private attributes
here.
$private_attrs
This is another (optional) hash ref with your private attributes. DBI will store them and
otherwise leave them alone.
The DBI::new_drh method and the driver method both return "undef" for failure (in which case you must
look at $DBI::err and $DBI::errstr for the failure information, because you have no driver handle to
use).
The CLONE special subroutine
Also needed here, in the DBD::Driver package, is a CLONE() method that will be called by perl when an
intrepreter is cloned. All your CLONE method needs to do, currently, is clear the cached $drh so the
new interpreter won't start using the cached $drh from the old interpreter:
sub CLONE {
undef $drh;
}
See <http://search.cpan.org/dist/perl/pod/perlmod.pod#Making_your_module_threadsafe for details.
The DBD::Driver::dr package
The database handle constructor
The next lines of code look as follows:
package DBD::Driver::dr; # ====== DRIVER ======
$DBD::Driver::dr::imp_data_size = 0;
Note that no @ISA is needed here, or for the other DBD::Driver::* classes, because the DBI takes care
of that for you when the driver is loaded.
The database handle constructor is a driver method, thus we have to change the namespace.
sub connect
{
my ($drh, $dr_dsn, $user, $auth, $attr) = @_;
# Some database specific verifications, default settings
# and the like can go here. This should only include
# syntax checks or similar stuff where it's legal to
# 'die' in case of errors.
# For example, many database packages requires specific
# environment variables to be set; this could be where you
# validate that they are set, or default them if they are not set.
my $driver_prefix = "drv_"; # the assigned prefix for this driver
# Process attributes from the DSN; we assume ODBC syntax
# here, that is, the DSN looks like var1=val1;...;varN=valN
foreach my $var ( split /;/, $dr_dsn ) {
my ($attr_name, $attr_value) = split '=', $var, 2;
return $drh->set_err(1, "Can't parse DSN part '$var'")
unless defined $attr_value;
# add driver prefix to attribute name if it doesn't have it already
$attr_name = $driver_prefix.$attr_name
unless $attr_name =~ /^$driver_prefix/o;
# Store attribute into %$attr, replacing any existing value.
# The DBI will STORE() these into $dbh after we've connected
$attr->{$attr_name} = $attr_value;
}
# Get the attributes we'll use to connect.
# We use delete here because these no need to STORE them
my $db = delete $attr->{drv_database} || delete $attr->{drv_db}
or return $drh->set_err(1, "No database name given in DSN '$dr_dsn'");
my $host = delete $attr->{drv_host} || 'localhost';
my $port = delete $attr->{drv_port} || 123456;
# Assume you can attach to your database via drv_connect:
my $connection = drv_connect($db, $host, $port, $user, $auth)
or return $drh->set_err(1, "Can't connect to $dr_dsn: ...");
# create a 'blank' dbh (call superclass constructor)
my ($outer, $dbh) = DBI::_new_dbh($drh, { Name => $dr_dsn });
$dbh->STORE('Active', 1 );
$dbh->{drv_connection} = $connection;
return $outer;
}
The Name attribute is a standard DBI attribute.
This is mostly the same as in the driver handle constructor above. The arguments are described in
the DBI man page. See DBI. The constructor _new_dbh is called, returning a database handle. The
constructor's prototype is:
($outer, $inner) = DBI::_new_dbh($drh, $public_attr, $private_attr);
with similar arguments to those in the driver handle constructor, except that the $class is replaced
by $drh.
In scalar context, only the outer handle is returned.
Note the use of the STORE method for setting the dbh attributes. That's because within the driver
code, the handle object you have is the 'inner' handle of a tied hash, not the outer handle that the
users of your driver have.
Because you have the inner handle, tie magic doesn't get invoked when you get or set values in the
hash. This is often very handy for speed when you want to get or set simple non-special driver-specific driverspecific
specific attributes.
However, some attribute values, such as those handled by the DBI like PrintError, don't actually
exist in the hash and must be read via $h->FETCH($attrib) and set via $h->STORE($attrib, $value). If
in any doubt, use these methods.
The data_sources method
The data_sources method must populate and return a list of valid data sources, prefixed with the
"dbi:Driver" incantation that allows them to be used in the first argument of the "DBI->connect"
method. An example of this might be scanning the $HOME/.odbcini file on Unix for ODBC data sources
(DSNs). As a trivial example, consider a fixed list of data sources:
sub data_sources
{
my($drh, $attr) = @_;
my(@list) = ();
# You need more sophisticated code than this to set @list...
push @list, "dbi:Driver:abc";
push @list, "dbi:Driver:def";
push @list, "dbi:Driver:ghi";
# End of code to set @list
return @list;
}
Error handling
It is quite likely that something fails in the connect method. With DBD::File for example, you might
catch an error when setting the current directory to something not existent by using the (driver-specific) (driverspecific)
specific) f_dir attribute.
To report an error, you use the "set_err" method:
$h->set_err($err, $errmsg, $state);
This will ensure that the error is recorded correctly and that RaiseError and PrintError etc are
handled correctly. Typically you'll always use the method instance, aka your method's first
argument.
As set_err always returns undef your error handling code can usually be simplified to something like
this:
return $h->set_err($err, $errmsg, $state) if ...;
The disconnect_all method
If you need to release any resources when the driver is unloaded, you can provide a disconnect_all
method.
Other driver handle methods
If you need any other driver handle methods, they can follow here.
The DBD::Driver::db package
The statement handle constructor
There's nothing much new in the statement handle constructor.
package DBD::Driver::db; # ====== DATABASE ======
$DBD::Driver::db::imp_data_size = 0;
sub prepare
{
my ($dbh, $statement, @attribs) = @_;
# create a 'blank' sth
my ($outer, $sth) = DBI::_new_sth($dbh, { Statement => $statement });
$sth->STORE('NUM_OF_PARAMS', ($statement =~ tr/?//));
$sth->{drv_params} = [];
return $outer;
}
This is still the same: check the arguments and call the super class constructor DBI::_new_sth.
Again, in scalar context, only the outer handle is returned. The "Statement" attribute should be
cached as shown.
Note the prefix drv_ in the attribute names: it is required that all your private attributes use a
lowercase prefix unique to your driver. The DBI contains a registry of known driver prefixes and may
one day warn about unknown attributes that don't have a registered prefix.
Note that we parse the statement here in order to set the attribute NUM_OF_PARAMS. The technique
illustrated is not very reliable; it can be confused by question marks appearing in quoted strings,
delimited identifiers or in SQL comments that are part of the SQL statement. We could set
NUM_OF_PARAMS in the execute method instead because the DBI specification explicitly allows a driver
to defer this, but then the user could not call bind_param.
Transaction handling
Pure Perl drivers will rarely support transactions. Thus your commit and rollback methods will
typically be quite simple:
sub commit
{
my ($dbh) = @_;
if ($dbh->FETCH('Warn')) {
warn("Commit ineffective while AutoCommit is on");
}
0;
}
sub rollback {
my ($dbh) = @_;
if ($dbh->FETCH('Warn')) {
warn("Rollback ineffective while AutoCommit is on");
}
0;
}
Or even simpler, just use the default methods provided by the DBI that do nothing except return
undef.
The DBI's default begin_work method can be used by inheritance.
The STORE and FETCH methods
These methods (that we have already used, see above) are called for you, whenever the user does a:
$dbh->{$attr} = $val;
or, respectively,
$val = $dbh->{$attr};
See perltie for details on tied hash refs to understand why these methods are required.
The DBI will handle most attributes for you, in particular attributes like RaiseError or PrintError.
All you have to do is handle your driver's private attributes and any attributes, like AutoCommit and
ChopBlanks, that the DBI can't handle for you. A good example might look like this:
sub STORE
{
my ($dbh, $attr, $val) = @_;
if ($attr eq 'AutoCommit') {
# AutoCommit is currently the only standard attribute we have
# to consider.
if (!$val) { die "Can't disable AutoCommit"; }
return 1;
}
if ($attr =~ m/^drv_/) {
# Handle only our private attributes here
# Note that we could trigger arbitrary actions.
# Ideally we should warn about unknown attributes.
$dbh->{$attr} = $val; # Yes, we are allowed to do this,
return 1; # but only for our private attributes
}
# Else pass up to DBI to handle for us
$dbh->SUPER::STORE($attr, $val);
}
sub FETCH
{
my ($dbh, $attr) = @_;
if ($attr eq 'AutoCommit') { return 1; }
if ($attr =~ m/^drv_/) {
# Handle only our private attributes here
# Note that we could trigger arbitrary actions.
return $dbh->{$attr}; # Yes, we are allowed to do this,
# but only for our private attributes
}
# Else pass up to DBI to handle
$dbh->SUPER::FETCH($attr);
}
The DBI will actually store and fetch driver-specific attributes (with all lowercase names) without
warning or error, so there's actually no need to implement driver-specific any code in your FETCH and
STORE methods unless you need extra logic/checks, beyond getting or setting the value.
Unless your driver documentation indicates otherwise, the return value of the STORE method is
unspecified and the caller shouldn't use that value.
Other database handle methods
As with the driver package, other database handle methods may follow here. In particular you should
consider a (possibly empty) disconnect method and possibly a quote method if DBI's default isn't
correct for you.
Where reasonable use $h->SUPER::foo() to call the DBI's method in some or all cases and just wrap
your custom behavior around that.
If you want to use private trace flags you'll probably want to be able to set them by name. To do
that you'll need to define a parse_trace_flag() method (note that's parse_trace_flag not
parse_trace_flags).
sub parse_trace_flag {
my ($h, $name) = @_;
return 0x01000000 if $name eq 'foo';
return 0x02000000 if $name eq 'bar';
return 0x04000000 if $name eq 'baz';
return 0x08000000 if $name eq 'boo';
return 0x10000000 if $name eq 'bop';
return $h->SUPER::parse_trace_flag($name);
}
All private flag names must be lowercase, and all private flags must be in the top 8 of the 32 bits.
The DBD::Driver::st package
The execute method
This is perhaps the most difficult method because we have to consider parameter bindings here. We
present a simplified implementation by using the drv_params attribute from above:
package DBD::Driver::st;
$DBD::Driver::st::imp_data_size = 0;
sub bind_param
{
my ($sth, $pNum, $val, $attr) = @_;
my $type = (ref $attr) ? $attr->{TYPE} : $attr;
if ($type) {
my $dbh = $sth->{Database};
$val = $dbh->quote($sth, $type);
}
my $params = $sth->{drv_params};
$params->[$pNum-1] = $val;
1;
}
sub execute
{
my ($sth, @bind_values) = @_;
# start of by finishing any previous execution if still active
$sth->finish if $sth->FETCH('Active');
my $params = (@bind_values) ?
\@bind_values : $sth->{drv_params};
my $numParam = $sth->FETCH('NUM_OF_PARAMS');
return $sth->set_err(1, "Wrong number of parameters")
if @$params != $numParam;
my $statement = $sth->{'Statement'};
for (my $i = 0; $i < $numParam; $i++) {
$statement =~ s/?/$params->[$i]/; # XXX doesn't deal with quoting etc!
}
# Do anything ... we assume that an array ref of rows is
# created and store it:
$sth->{'drv_data'} = $data;
$sth->{'drv_rows'} = @$data; # number of rows
$sth->STORE('NUM_OF_FIELDS') = $numFields;
@$data || '0E0';
}
There are a number of things you should note here. We setup the NUM_OF_FIELDS attribute here,
because this is essential for bind_columns to work. We use attribute "$sth->{Statement}" which we
created within prepare. The attribute "$sth->{Database}", which is nothing else than the dbh, was
automatically created by DBI.
Finally note that (as specified in the DBI specification) we return the string '0E0' instead of the
number 0, so that the result tests true but equal to zero.
$sth->execute() or die $sth->errstr;
Fetching data
We should not implement the methods fetchrow_array, fetchall_arrayref, ... because these are already
part of DBI. All we need is the method fetchrow_arrayref:
sub fetchrow_arrayref
{
my ($sth) = @_;
my $data = $sth->{drv_data};
my $row = shift @$data;
if (!$row) {
$sth->STORE(Active => 0); # mark as no longer active
return undef;
}
if ($sth->FETCH('ChopBlanks')) {
map { $_ =~ s/\s+$//; } @$row;
}
return $sth->_set_fbav($row);
}
*fetch = \&fetchrow_arrayref; # required alias for fetchrow_arrayref
Note the use of the method _set_fbav: This is required so that bind_col and bind_columns work.
If an error occurs which leaves the $sth in a state where remaining rows can't be fetched then Active
should be turned off before the method returns.
The rows method for this driver can be implemented like this:
sub rows { shift->{drv_rows} }
because it knows in advance how many rows it has fetched. Alternatively you could delete that method
and so fallback to the DBI's own method which does the right thing based on the number of calls to
_set_fbav().
Statement attributes
The main difference between dbh and sth attributes is, that you should implement a lot of attributes
here that are required by the DBI, such as NAME, NULLABLE, TYPE, ...
Besides that the STORE and FETCH methods are mainly the same as above for dbh's.
Other statement methods
A trivial "finish" method to discard the stored data and do $sth->SUPER::finish;
If you've defined a parse_trace_flag() method in ::db you'll also want it in ::st, so just alias it
in:
*parse_trace_flag = \&DBD::foo:db::parse_trace_flag;
And perhaps some other methods that are not part of the DBI specification, in particular to make
metadata available. Remember that they must have names that begin with your drivers registered
prefix so they can be installed using install_method().
If DESTROY() is called on a statement handle that's still active ($sth->{Active} is true) then it
should effectively call finish().
sub DESTROY {
my $sth = shift;
$sth->finish if $sth->FETCH('Active');
}
Tests
The test process should conform as closely as possibly to the Perl standard test harness.
In particular, most (all) of the tests should be run in the t sub-directory, and should simply
produce an 'ok' when run under 'make test'. For details on how this is done, see the Camel book and
the section in Chapter 7, "The Standard Perl Library" on Test::Harness.
The tests may need to adapt to the type of database which is being used for testing, and to the
privileges of the user testing the driver.
The DBD::Informix test code has to adapt in a number of places to the type of database to which it is
connected as different Informix databases have different capabilities. For example, some of the
tests are for databases without transaction logs; others are for databases with a transaction log.
Some versions of the server have support for blobs, or stored procedures, or user-defined data types,
and others do not. When a complete file of tests must be skipped, you can provide a reason in a
pseudo-comment:
if ($no_transactions_available)
{
print "1..0 # Skip: No transactions available\n";
exit 0;
}
Consider downloading the DBD::Informix code and look at the code in DBD/Informix/TestHarness.pm which
is used throughout the DBD::Informix tests in the t sub-directory.
CREATING A C/XS DRIVER
Creating a new C/XS driver from scratch will always be a daunting task. You can and should greatly
simplify your task by taking a good reference driver implementation and modifying that to match the
database product for which you are writing a driver.
The de facto reference driver has been the one for DBD::Oracle written by Tim Bunce, who is also the
author of the DBI package. The DBD::Oracle module is a good example of a driver implemented around a
C-level API.
Nowadays it it seems better to base on DBD::ODBC, another driver maintained by Tim and Jeff Urlwin,
because it offers a lot of metadata and seems to become the guideline for the future development.
(Also as DBD::Oracle digs deeper into the Oracle 8 OCI interface it'll get even more hairy than it is
now.)
The DBD::Informix driver is one driver implemented using embedded SQL instead of a function-based
API. DBD::Ingres may also be worth a look.
C/XS version of Driver.pm
A lot of the code in the Driver.pm file is very similar to the code for pure Perl modules - see
above. However, there are also some subtle (and not so subtle) differences, including:
The variables $DBD::File::{dr|db|st}::imp_data_size are not defined here, but in the XS code,
because they declare the size of certain C structures.
Some methods are typically moved to the XS code, in particular prepare, execute, disconnect,
disconnect_all and the STORE and FETCH methods.
Other methods are still part of "Driver.pm", but have callbacks to the XS code.
If the driver-specific parts of the imp_drh_t structure need to be formally initialized
(which does not seem to be a common requirement), then you need to add a call to an
appropriate XS function in the driver method of DBD::Driver::driver, and you define the
corresponding function in Driver.xs, and you define the C code in dbdimp.c and the prototype
in dbdimp.h.
For example, DBD::Informix has such a requirement, and adds the following call after the call
to _new_drh in Informix.pm:
DBD::Informix::dr::driver_init($drh);
and the following code in Informix.xs:
# Initialize the DBD::Informix driver data structure
void
driver_init(drh)
SV *drh
CODE:
ST(0) = dbd_ix_dr_driver_init(drh) ? &sv_yes : &sv_no;
and the code in dbdimp.h declares:
extern int dbd_ix_dr_driver_init(SV *drh);
and the code in dbdimp.ec (equivalent to dbdimp.c) defines:
/* Formally initialize the DBD::Informix driver structure */
int
dbd_ix_dr_driver(SV *drh)
{
D_imp_drh(drh);
imp_drh->n_connections = 0; /* No active connections */
imp_drh->current_connection = 0; /* No current connection */
imp_drh->multipleconnections = (ESQLC_VERSION >= 600) ? True : False;
dbd_ix_link_newhead(&imp_drh->head); /* Empty linked list of connections */
return 1;
}
DBD::Oracle has a similar requirement but gets around it by checking whether the private data
part of the driver handle is all zeroed out, rather than add extra functions.
Now let's take a closer look at an excerpt from Oracle.pm (revised heavily to remove idiosyncrasies)
as an example. We also ignore things that are already discussed for pure Perl drivers.
The connect method
The connect method is the database handle constructor. You could write either of two versions of
this method: either one which takes connection attributes (new code) and one which ignores them (old
code only). If you ignore the connection attributes, then you omit all mention of the $auth variable
(which is a reference to a hash of attributes), and the XS system manages the differences for you.
sub connect
{
my ($drh, $dbname, $user, $auth, $attr) = @_;
# Some database specific verifications, default settings
# and the like following here. This should only include
# syntax checks or similar stuff where it's legal to
# 'die' in case of errors.
my $dbh = DBI::_new_dbh($drh, {
'Name' => $dbname,
})
or return undef;
# Call the driver-specific function _login in Driver.xs file which
# calls the DBMS-specific function(s) to connect to the database,
# and populate internal handle data.
DBD::Driver::db::_login($dbh, $dbname, $user, $auth, $attr)
or return undef;
$dbh;
}
This is mostly the same as in the pure Perl case, the exception being the use of the private _login
callback, which is the function that will really connect to the database. It is implemented in
Driver.xst (you should not implement it) and calls dbd_db_login6 from dbdimp.c. See below for
details.
*FIX ME* Discuss removing attributes from hash reference as an optimization
to skip later calls to $dbh->STORE made by DBI->connect.
*FIX ME* Discuss removing attributes in Perl code.
*FIX ME* Discuss removing attributes in C code.
The disconnect_all method
*FIX ME* T.B.S
The data_sources method
If your data_sources method can be implemented in pure Perl, then do so because it is easier than
doing it in XS code (see the section above for pure Perl drivers). If your data_sources method must
call onto compiled functions, then you will need to define dbd_dr_data_sources in your dbdimp.h file,
which will trigger Driver.xst (in DBI v1.33 or greater) to generate the XS code that calls your
actual C function (see the discussion below for details) and you do not code anything in Driver.pm to
handle it.
The prepare method
The prepare method is the statement handle constructor, and most of it is not new. Like the connect
method, it now has a C callback:
package DBD::Driver::db; # ====== DATABASE ======
use strict;
sub prepare
{
my ($dbh, $statement, $attribs) = @_;
# create a 'blank' sth
my $sth = DBI::_new_sth($dbh, {
'Statement' => $statement,
})
or return undef;
# Call the driver-specific function _prepare in Driver.xs file
# which calls the DBMS-specific function(s) to prepare a statement
# and populate internal handle data.
DBD::Driver::st::_prepare($sth, $statement, $attribs)
or return undef;
$sth;
}
The execute method
*FIX ME* T.B.S
The fetchrow_arrayref method
*FIX ME* T.B.S
Other methods?
*FIX ME* T.B.S
Driver.xs
Driver.xs should look something like this:
#include "Driver.h"
DBISTATE_DECLARE;
INCLUDE: Driver.xsi
MODULE = DBD::Driver PACKAGE = DBD::Driver::dr
/* Non-standard drh XS methods following here, if any. */
/* If none (the usual case), omit the MODULE line above too. */
MODULE = DBD::Driver PACKAGE = DBD::Driver::db
/* Non-standard dbh XS methods following here, if any. */
/* Currently this includes things like _list_tables from */
/* DBD::mSQL and DBD::mysql. */
MODULE = DBD::Driver PACKAGE = DBD::Driver::st
/* Non-standard sth XS methods following here, if any. */
/* In particular this includes things like _list_fields from */
/* DBD::mSQL and DBD::mysql for accessing metadata. */
Note especially the include of Driver.xsi here: DBI inserts stub functions for almost all private
methods here which will typically do much work for you. Wherever you really have to implement
something, it will call a private function in dbdimp.c, and this is what you have to implement.
You need to set up an extra routine if your driver needs to export constants of its own, analogous to
the SQL types available when you say:
use DBI qw(:sql_types);
*FIX ME* T.B.S
Driver.h
Driver.h is very simple and the operational contents should look like this:
#ifndef DRIVER_H_INCLUDED
#define DRIVER_H_INCLUDED
#define NEED_DBIXS_VERSION 93 /* 93 for DBI versions 1.00 to 1.51+ */
#define PERL_NO_GET_CONTEXT /* if used require DBI 1.51+ */
#include <DBIXS.h> /* installed by the DBI module */
#include "dbdimp.h"
#include "dbivport.h" /* see below */
#include <dbd_xsh.h> /* installed by the DBI module */
#endif /* DRIVER_H_INCLUDED */
The "DBIXS.h" header defines most of the interesting information that the writer of a driver needs.
The file "dbd_xsh.h" header provides prototype declarations for the C functions that you might decide
to implement. Note that you should normally only define one of dbd_db_login and dbd_db_login6 unless
you are intent on supporting really old versions of DBI (prior to DBI 1.06) as well as modern
versions. The only standard, DBI-mandated functions that you need write are those specified in the
dbd_xsh.h header. You might also add extra driver-specific functions in Driver.xs.
The dbivport.h file should be copied from the latest DBI release into your distribution each time you
enhance your driver to use new features for which the DBI is offering backwards compatibility via
dbivport.h.
Its job is to allow you to enhance your code to work with the latest DBI API while still allowing
your driver to be compiled and used with older versions of the DBI. For example, when the
DBIh_SET_ERR_CHAR macro was added to DBI 1.41 in an emulation of it was added to dbivport.h.
Copying dbivport.h into your driver distribution and #including it in Driver.h, as shown above, lets
you enhance your driver to use the new DBIh_SET_ERR_CHAR macro even with versions of the DBI earlier
than 1.41. This makes users happy and your life easier.
Always read the notes in dbivport.h to check for any limitations in the emulation that you should be
aware of.
With DBI v1.51 or better I recommend that the driver defines PERL_NO_GET_CONTEXT before DBIXS.h is
included. This can significantly improve efficiency when running under a thread enabled perl.
(Remember that the standard perl in most Linux distributions is built with threads enabled. So is
ActiveState perl for Windows, and perl built for Apache mod_perl2.) If you do this there are some
things to keep in mind:
* If PERL_NO_GET_CONTEXT is defined, then every function that calls the Perl API will need to start
out with a "dTHX;" declaration.
* You'll know which functions need this, because the C compiler will complain that the undeclared
identifier "my_perl" is used if and only if the perl you are using to develop and test your driver
has threads enabled.
* So if you don't remember to test with a thread-enabled perl before making a release it's likely
that you'll get failure reports from users who are.
* For driver private functions it is possible to gain even more efficiency by replacing "dTHX;" with
"pTHX_" prepended to the parameter list and then "aTHX_" prepended to the argument list where the
function is called.
See "How multiple interpreters and concurrency are supported" in perlguts for additional information
about PERL_NO_GET_CONTEXT.
Implementation header dbdimp.h
This header file has two jobs:
First it defines data structures for your private part of the handles.
Second it defines macros that rename the generic names like dbd_db_login to database specific names
like ora_db_login. This avoids name clashes and enables use of different drivers when you work with a
statically linked perl.
It also will have the important task of disabling XS methods that you don't want to implement.
Finally, the macros will also be used to select alternate implementations of some functions. For
example, the dbd_db_login function is not passed the attribute hash. Since DBI v1.06, if a
dbd_db_login6 macro is defined (for a function with 6 arguments), it will be used instead with the
attribute hash passed as the sixth argument.
People used to just pick Oracle's dbdimp.c and use the same names, structures and types. I strongly
recommend against that. At first glance this saves time, but your implementation will be less
readable. It was just hell when I had to separate DBI specific parts, Oracle specific parts, mSQL
specific parts and mysql specific parts in DBD::mysql's dbdimp.h and dbdimp.c. (DBD::mysql was a
port of DBD::mSQL which was based on DBD::Oracle.) [Seconded, based on the experience taking
DBD::Informix apart, even though the version inherited in 1996 was only based on DBD::Oracle.]
This part of the driver is your exclusive part. Rewrite it from scratch, so it will be clean and
short: in other words, a better piece of code. (Of course keep an eye on other people's work.)
struct imp_drh_st {
dbih_drc_t com; /* MUST be first element in structure */
/* Insert your driver handle attributes here */
};
struct imp_dbh_st {
dbih_dbc_t com; /* MUST be first element in structure */
/* Insert your database handle attributes here */
};
struct imp_sth_st {
dbih_stc_t com; /* MUST be first element in structure */
/* Insert your statement handle attributes here */
};
/* Rename functions for avoiding name clashes; prototypes are */
/* in dbd_xst.h */
#define dbd_init drv_dr_init
#define dbd_db_login6 drv_db_login
#define dbd_db_do drv_db_do
... many more here ...
These structures implement your private part of the handles. You have to use the name
imp_dbh_{dr|db|st} and the first field must be of type dbih_drc_t|_dbc_t|_stc_t and must be called
"com". You should never access these fields directly, except by using the DBIc_xxx macros below.
Implementation source dbdimp.c
Conventionally, dbdimp.c is the main implementation file (but DBD::Informix calls the file
dbdimp.ec). This section includes a short note on each function that is used in the Driver.xsi
template and thus has to be implemented.
Of course, you will probably also need to implement other support functions, which should usually be
file static if the are placed in dbdimp.c. If they are placed in other files, you need to list those
files in Makefile.PL (and MANIFEST) to handle them correctly.
It is wise to adhere to a namespace convention for your functions to avoid conflicts. For example,
for a driver with prefix "drv", you might call externally visible functions "dbd_drv_xxxx". You
should also avoid non-constant global variables as much as possible to improve the support for
threading.
Since Perl requires support for function prototypes (ANSI or ISO or Standard C), you should write
your code using function prototypes too.
It is possible to use either the unmapped names such as "dbd_init" or the mapped names such as
"dbd_ix_dr_init" in the "dbdimp.c" file. DBD::Informix uses the mapped names which makes it easier
to identify where to look for linkage problems at runtime (which will report errors using the mapped
names). Most other drivers, and in particular DBD::Oracle, use the unmapped names in the source code
which makes it a little easier to compare code between drivers and eases discussions on the dbi-dev
mailing list. The majority of the code fragments here will use the unmapped names.
Ultimately, you should provide implementations for most fo the functions listed in the dbd_xsh.h
header. The exceptions are optional functions (such as dbd_st_rows) and those functions with
alternative signatures, such as dbd_db_login6 and dbd_db_login. Then you should only implement one
of the alternatives, and generally the newer one of the alternatives.
The dbd_init method
#include "Driver.h"
DBISTATE_DECLARE;
void dbd_init(dbistate_t* dbistate)
{
DBISTATE_INIT; /* Initialize the DBI macros */
}
The "dbd_init" function will be called when your driver is first loaded; the bootstrap command in
DBD::Driver::dr::driver triggers this, and the call is generated in the BOOT section of Driver.xst.
These statements are needed to allow your driver to use the DBI macros. They will include your
private header file dbdimp.h in turn. Note that DBISTATE_INIT requires the name of the argument to
dbd_init to be called dbistate.
The dbd_drv_error method
You need a function to record errors so DBI can access them properly. You can call it whatever you
like, but we'll call it "dbd_drv_error" here. The argument list depends on your database software;
different systems provide different ways to get at error information.
static void dbd_drv_error(SV *h, int rc, const char *what)
{
Note that h is a generic handle, may it be a driver handle, a database or a statement handle.
D_imp_xxh(h);
This macro will declare and initialize a variable imp_xxh with a pointer to your private handle
pointer. You may cast this to to imp_drh_t, imp_dbh_t or imp_sth_t.
To record the error correctly, equivalent to the set_err() method, use one of the
DBIh_SET_ERR_CHAR(...) or DBIh_SET_ERR_SV(...) macros, which were added in DBI 1.41:
DBIh_SET_ERR_SV(h, imp_xxh, err, errstr, state, method);
DBIh_SET_ERR_CHAR(h, imp_xxh, err_c, err_i, errstr, state, method);
For DBIh_SET_ERR_SV the err, errstr, state, and method parameters are SV*. For DBIh_SET_ERR_CHAR the
err_c, errstr, state, method are char*. The err_i parameter is an IV that's used instead of err_c is
err_c is Null. The method parameter can be ignored.
The DBIh_SET_ERR_CHAR macro is usually the simplest to use when you just have an integer error code
and an error message string:
DBIh_SET_ERR_CHAR(h, imp_xxh, Nullch, rc, what, Nullch, Nullch);
As you can see, any parameters that aren't relevant to you can be Null.
To make drivers compatible with DBI < 1.41 you should be using dbivport.h as described in "Driver.h"
above.
The (obsolete) macros such as DBIh_EVENT2 should be removed from drivers.
The names dbis and DBIS, which were used in previous versions of this document, should be replaced
with the "DBIc_STATE(imp_xxh)" macro.
The name DBILOGFP, which was also used in previous versions of this document, should be replaced by
DBIc_LOGPIO(imp_xxh).
Your code should not call the C "<stdio.h>" I/O functions; you should use "PerlIO_printf"() as shown:
if (DBIc_TRACE_LEVEL(imp_xxh) >= 2)
PerlIO_printf(DBIc_LOGPIO(imp_xxh), "foobar %s: %s\n",
foo, neatsvpv(errstr,0));
That's the first time we see how tracing works within a DBI driver. Make use of this as often as you
can! But don't output anything at a trace level less than 3. Levels 1 and 2 are reserved for the DBI.
You can define up to 8 private trace flags using the top 8 bits of DBIc_TRACE_FLAGS(imp), that is:
0xFF000000. See the parse_trace_flag() method elsewhere in this document.
The dbd_dr_data_sources method
This method is optional; the support for it was added in DBI v1.33.
As noted in the discussion of Driver.pm, if the data sources can be determined by pure Perl code, do
it that way. If, as in DBD::Informix, the information is obtained by a C function call, then you
need to define a function that matches the prototype:
extern AV *dbd_dr_data_sources(SV *drh, imp_drh_t *imp_drh, SV *attrs);
An outline implementation for DBD::Informix follows, assuming that the sqgetdbs() function call shown
will return up to 100 databases names, with the pointers to each name in the array dbsname and the
name strings themselves being stores in dbsarea. The actual DBD::Informix implementation has a
number of extra lines of code, logs function entry and exit, reports the error from sqgetdbs(), and
uses #define'd constatnts for the array sizes.
AV *dbd_dr_data_sources(SV *drh, imp_drh_t *imp_drh, SV *attr)
{
int ndbs;
int i;
char *dbsname[100];
char dbsarea[10000];
AV *av = Nullav;
if (sqgetdbs(&ndbs, dbsname, 100, dbsarea, sizeof(dbsarea)) == 0)
{
av = NewAV();
av_extend(av, (I32)ndbs);
sv_2mortal((SV *)av);
for (i = 0; i < ndbs; i++)
av_store(av, i, newSVpvf("dbi:Informix:%s", dbsname[i]));
}
return(av);
}
The dbd_db_login6 method
int dbd_db_login6(SV* dbh, imp_dbh_t* imp_dbh, char* dbname,
char* user, char* auth, SV *attr);
This function will really connect to the database. The argument dbh is the database handle. imp_dbh
is the pointer to the handles private data, as is imp_xxx in dbd_drv_error above. The arguments
dbname, user, auth and attr correspond to the arguments of the driver handle's connect method.
You will quite often use database specific attributes here, that are specified in the DSN. I
recommend you parse the DSN (using Perl) within the connect method and pass the segments of the DSN
via the attributes parameter through _login to dbd_db_login6. Here's how you fetch them; as an
example we use hostname attribute, which can be up to 12 characters long excluding null terminator:
SV** svp;
STRLEN len;
char* hostname;
if ( (svp = DBD_ATTRIB_GET_SVP(attr, "drv_hostname", 12)) && SvTRUE(*svp)) {
hostname = SvPV(*svp, len);
DBD__ATTRIB_DELETE(attr, "drv_hostname", 12); /* avoid later STORE */
} else {
hostname = "localhost";
}
Note that you can also obtain standard attributes such as AutoCommit and ChopBlanks from the
attributes parameter, using DBD_ATTRIB_GET_IV for integer attributes. If, for example, your database
does not support transactions but AutoCommit is set off (requesting transaction support), then you
can emulate a 'failure to connect'.
Now you should really connect to the database. In general, if the connection fails, it is best to
ensure that all allocated resources are released so that the handle does not need to be destroyed
separately. If you are successful (and possibly even if you fail but you have allocated some
resources), you should use the following macros:
DBIc_IMPSET_on(imp_dbh);
This indicates that the driver (implementor) has allocated resources in the imp_dbh structure and
that the implementors private dbd_db_destroy function should be called when the handle is destroyed.
DBIc_ACTIVE_on(imp_dbh);
This indicates that the handle has an active connection to the server and that the dbd_db_disconnect
function should be called before the handle is destroyed.
Note that if you do need to fail, you should report errors via the drh or imp_drh rather than via dbh
or imp_dbh because imp_dbh will be destroyed by the failure, so errors recorded in that handle will
not be visible to DBI, and hence not the user either. Note to that the function is passed dbh and
imp_dbh, and there is a macro D_imp_drh_from_dbh which can recover the imp_drh from the imp_dbh, but
there is no DBI macro to provide you with the drh given either the imp_dbh or the dbh or the imp_drh
(and there's no way to recover the dbh given just the imp_dbh). This suggests that despite the notes
about dbd_drv_error above taking an SV *, it may be better to have two error routines, one taking
imp_dbh and one taking imp_drh instead. With care, you can factor most of the formatting code out so
that these are small routines calling onto a common error formatter. See the code in DBD::Informix
1.05.00 for more information.
The dbd_db_login6 function should return TRUE for success, FALSE otherwise.
Drivers implemented long ago may define the five-argument function dbd_db_login instead of
dbd_db_login6. The missing argument is the attributes. There are ways to work around the missing
attributes, but they are ungainly; it is much better to use the 6-argument form.
The dbd_db_commit and dbd_db_rollback methods
int dbd_db_commit(SV *dbh, imp_dbh_t *imp_dbh);
int dbd_db_rollback(SV* dbh, imp_dbh_t* imp_dbh);
These are used for commit and rollback. They should return TRUE for success, FALSE for error.
The arguments dbh and imp_dbh are the same as for dbd_db_login6 above; I will omit describing them in
what follows, as they appear always.
These functions should return TRUE for success, FALSE otherwise.
The dbd_db_disconnect method
This is your private part of the disconnect method. Any dbh with the ACTIVE flag on must be
disconnected. (Note that you have to set it in dbd_db_connect above.)
int dbd_db_disconnect(SV* dbh, imp_dbh_t* imp_dbh);
The database handle will return TRUE for success, FALSE otherwise. In any case it should do a:
DBIc_ACTIVE_off(imp_dbh);
before returning so DBI knows that dbd_db_disconnect was executed.
Note that there's nothing to stop a dbh being disconnected while it still have active children. If
your database API reacts badly to trying to use an sth in this situation then you'll need to add code
like this to all sth methods:
if (!DBIc_ACTIVE(DBIc_PARENT_COM(imp_sth)))
return 0;
Alternatively, you can add code to your driver to keep explicit track of the statement handles that
exist for each database handle and arrange to destroy those handles before disconnecting from the
database. There is code to do this in DBD::Informix. Similar comments apply to the driver handle
keeping track of all the database handles. Note that the code which destroys the subordinate handles
should only release the associated database resources and mark the handles inactive; it does not
attempt to free the actual handle structures.
This function should return TRUE for success, FALSE otherwise, but it is not clear what anything can
do about a failure.
The dbd_db_discon_all method
int dbd_discon_all (SV *drh, imp_drh_t *imp_drh);
This function may be called at shutdown time. It should make best-efforts to disconnect all database
handles - if possible. Some databases don't support that, in which case you can do nothing but return
'success'.
This function should return TRUE for success, FALSE otherwise, but it is not clear what anything can
do about a failure.
The dbd_db_destroy method
This is your private part of the database handle destructor. Any dbh with the IMPSET flag on must be
destroyed, so that you can safely free resources. (Note that you have to set it in dbd_db_connect
above.)
void dbd_db_destroy(SV* dbh, imp_dbh_t* imp_dbh)
{
DBIc_IMPSET_off(imp_dbh);
}
The DBI Driver.xst code will have called dbd_db_disconnect for you, if the handle is still 'active',
before calling dbd_db_destroy.
Before returning the function must switch IMPSET to off, so DBI knows that the destructor was called.
A DBI handle doesn't keep references to its children. But children do keep references to their
parents. So a database handle won't be DESTROY'd until all its children have been DESTROY'd.
The dbd_db_STORE_attrib method
This function handles
$dbh->{$key} = $value;
Its prototype is:
int dbd_db_STORE_attrib(SV* dbh, imp_dbh_t* imp_dbh, SV* keysv,
SV* valuesv);
You do not handle all attributes; on the contrary, you should not handle DBI attributes here: leave
this to DBI. (There are two exceptions, AutoCommit and ChopBlanks, which you should care about.)
The return value is TRUE if you have handled the attribute or FALSE otherwise. If you are handling an
attribute and something fails, you should call dbd_drv_error, so DBI can raise exceptions, if
desired. If dbd_drv_error returns, however, you have a problem: the user will never know about the
error, because he typically will not check "$dbh->errstr".
I cannot recommend a general way of going on, if dbd_drv_error returns, but there are examples where
even the DBI specification expects that you croak(). (See the AutoCommit method in DBI.)
If you have to store attributes, you should either use your private data structure imp_xxx, the
handle hash (via (HV*)SvRV(dbh)), or use the private imp_data.
The first is best for internal C values like integers or pointers and where speed is important within
the driver. The handle hash is best for values the user may want to get/set via driver-specific
attributes. The private imp_data is an additional SV attached to the handle. You could think of it
as an unnamed handle attribute. It's not normally used.
The dbd_db_FETCH_attrib method
This is the counterpart of dbd_db_STORE_attrib, needed for:
$value = $dbh->{$key};
Its prototype is:
SV* dbd_db_FETCH_attrib(SV* dbh, imp_dbh_t* imp_dbh, SV* keysv);
Unlike all previous methods this returns an SV with the value. Note that you should normally execute
sv_2mortal, if you return a nonconstant value. (Constant values are &sv_undef, &sv_no and &sv_yes.)
Note, that DBI implements a caching algorithm for attribute values. If you think, that an attribute
may be fetched, you store it in the dbh itself:
if (cacheit) /* cache value for later DBI 'quick' fetch? */
hv_store((HV*)SvRV(dbh), key, kl, cachesv, 0);
The dbd_st_prepare method
This is the private part of the prepare method. Note that you must not really execute the statement
here. You may, for example, preparse and validate the statement or do similar things.
int dbd_st_prepare(SV* sth, imp_sth_t* imp_sth, char* statement,
SV* attribs);
A typical, simple, possibility is to do nothing and rely on the perl perpare() code that set the
Statement attribute on the handle. This attribute can then be used by dbd_st_execute.
If the driver supports placeholders then the NUM_OF_PARAMS attribute must be set correctly by
dbd_st_prepare:
DBIc_NUM_PARAMS(imp_sth) = ...
If you can, you should also setup attributes like NUM_OF_FIELDS, NAME, ... here, but DBI doesn't
require that. However, if you do, document it.
In any case you should set the IMPSET flag, as you did in dbd_db_connect above:
DBIc_IMPSET_on(imp_sth);
The dbd_st_execute method
This is where a statement will really be executed.
int dbd_st_execute(SV* sth, imp_sth_t* imp_sth);
Note, that you must be aware, that a statement may be executed repeatedly. Also, you should not
expect, that finish will be called between two executions, so you'll might need code like the
following near the start of the function:
if (DBIc_ACTIVE(imp_sth))
dbd_st_finish(h, imp_sth);
If your driver supports the binding of parameters (it should!), but the database doesn't, you must do
it here. This can be done as follows:
SV *svp;
char* statement = DBD_ATTRIB_GET_PV(h, "Statement", 9, svp, "");
int numParam = DBIc_NUM_PARAMS(imp_sth);
int i;
for (i = 0; i < numParam; i++)
{
char* value = dbd_db_get_param(sth, imp_sth, i);
/* It is your drivers task to implement dbd_db_get_param, */
/* it must be setup as a counterpart of dbd_bind_ph. */
/* Look for '?' and replace it with 'value'. Difficult */
/* task, note that you may have question marks inside */
/* quotes and comments the like ... :-( */
/* See DBD::mysql for an example. (Don't look too deep into */
/* the example, you will notice where I was lazy ...) */
}
The next thing is you really execute the statement. Note that you must set the attributes
NUM_OF_FIELDS, NAME, etc when the statement is successfully executed if the driver has not already
done so. They may be used even before a potential fetchrow. In particular you have to tell DBI the
number of fields, that the statement has, because it will be used by DBI internally. Thus the
function will typically ends with:
if (isSelectStatement) {
DBIc_NUM_FIELDS(imp_sth) = numFields;
DBIc_ACTIVE_on(imp_sth);
}
It is important that the ACTIVE flag only be set for "SELECT" statements (or any other statements
that can return multiple sets of values from the database using a cursor-like mechanism). See
dbd_db_connect above for more explanations.
There plans for a preparse function to be provided by DBI, but this has not reached fruition yet.
Meantime, if you want to know how ugly it can get, try looking at the dbd_ix_preparse in
DBD::Informix dbdimp.ec and the related functions in iustoken.c and sqltoken.c.
The dbd_st_fetch method
This function fetches a row of data. The row is stored in in an array, of SV's that DBI prepares for
you. This has two advantages: it is fast (you even reuse the SV's, so they don't have to be created
after the first fetchrow), and it guarantees that DBI handles bind_cols for you.
What you do is the following:
AV* av;
int numFields = DBIc_NUM_FIELDS(imp_sth); /* Correct, if NUM_FIELDS
is constant for this statement. There are drivers where this is
not the case! */
int chopBlanks = DBIc_is(imp_sth, DBIcf_ChopBlanks);
int i;
if (!fetch_new_row_of_data(...)) {
... /* check for error or end-of-data */
DBIc_ACTIVE_off(imp_sth); /* turn off Active flag automatically */
return Nullav;
}
/* get the fbav (field buffer array value) for this row */
/* it is very important to only call this after you know */
/* that you have a row of data to return. */
av = DBIc_DBISTATE(imp_sth)->get_fbav(imp_sth);
for (i = 0; i < numFields; i++) {
SV* sv = fetch_a_field(..., i);
if (chopBlanks && SvOK(sv) && type_is_blank_padded(field_type[i])) {
/* Remove white space from end (only) of sv */
}
sv_setsv(AvARRAY(av)[i], sv); /* Note: (re)use! */
}
return av;
There's no need to use a fetch_a_field function returning an SV*. It's more common to use your
database API functions to fetch the data as character strings and use code like this:
sv_setpvn(AvARRAY(av)[i], char_ptr, char_count);
NULL values must be returned as undef. You can use code like this:
SvOK_off(AvARRAY(av)[i]);
The function returns the AV prepared by DBI for success or "Nullav" otherwise.
*FIX ME* Discuss what happens when there's no more data to fetch.
Are errors permitted if another fetch occurs after the first fetch
that reports no more data. (Permitted, not required.)
If an error occurs which leaves the $sth in a state where remaining rows can't be fetched then Active
should be turned off before the method returns.
The dbd_st_finish3 method
The "$sth->finish" method can be called if the user wishes to indicate that no more rows will be
fetched even if the database has more rows to offer, and the DBI code can call the function when
handles are being destroyed. See the DBI specification for more background details. In both
circumstances, the DBI code ends up calling the "dbd_st_finish3" method (if you provide a mapping for
dbd_st_finish3 in dbdimp.h), or dbd_st_finish otherwise. The difference is that dbd_st_finish3 takes
a third argument which is an "int" with the value 1 if it is being called from a destroy method and 0
otherwise.
Note that DBI v1.32 and earlier test on dbd_db_finish3 to call dbd_st_finish3; if you provide
dbd_st_finish3, either define dbd_db_finish3 too, or insist on DBI v1.33 or later.
All it needs to do is turn off the Active flag for the sth. It will only be called by Driver.xst
code, if the driver has set ACTIVE to on for the sth.
Outline example:
int dbd_st_finish3(SV* sth, imp_sth_t* imp_sth, int from_destroy) {
if (DBIc_ACTIVE(imp_sth))
{
/* close cursor or equivalent action */
DBIc_ACTIVE_off(imp_sth);
}
return 1;
}
The from_destroy parameter is true if dbd_st_finish3 is being called from DESTROY - and so the
statement is about to be destroyed. For many drivers there's no point in doing anything more than
turing of the Active flag in this case.
The function returns TRUE for success, FALSE otherwise, but there isn't a lot anyone can do to
recover if there is an error.
The dbd_st_destroy method
This function is the private part of the statement handle destructor.
void dbd_st_destroy(SV* sth, imp_sth_t* imp_sth) {
... /* any clean-up that's needed */
DBIc_IMPSET_off(imp_sth); /* let DBI know we've done it */
}
The DBI Driver.xst code will call dbd_st_finish for you, if the sth has the ACTIVE flag set, before
calling dbd_st_destroy.
The dbd_st_STORE_attrib and dbd_st_FETCH_attrib methods
These functions correspond to dbd_db_STORE and dbd_db_FETCH attrib above, except that they are for
statement handles. See above.
int dbd_st_STORE_attrib(SV* sth, imp_sth_t* imp_sth, SV* keysv,
SV* valuesv);
SV* dbd_st_FETCH_attrib(SV* sth, imp_sth_t* imp_sth, SV* keysv);
The dbd_bind_ph method
This function is internally used by the bind_param method, the bind_param_inout method and by the DBI
Driver.xst code if "execute" is called with any bind parameters.
int dbd_bind_ph (SV *sth, imp_sth_t *imp_sth, SV *param,
SV *value, IV sql_type, SV *attribs,
int is_inout, IV maxlen);
The param argument holds an IV with the parameter number (1, 2, ...). The value argument is the
parameter value and sql_type is its type.
If your driver does not support bind_param_inout then you should ignore maxlen and croak if is_inout
is TRUE.
If your driver does support bind_param_inout then you should note that value is the SV after
dereferencing the reference passed to bind_param_inout.
In drivers of simple databases the function will, for example, store the value in a parameter array
and use it later in dbd_st_execute. See the DBD::mysql driver for an example.
Implementing bind_param_inout support
To provide support for parameters bound by reference rather than by value, the driver must do a
number of things. First, and most importantly, it must note the references and stash them in its own
driver structure. Secondly, when a value is bound to a column, the driver must discard any previous
reference bound to the column. On each execute, the driver must evaluate the references and
internally bind the values resulting from the references. This is only applicable if the user
writes:
$sth->execute;
If the user writes:
$sth->execute(@values);
then DBI automatically calls the binding code for each element of @values. These calls are
indistinguishable from explicit user calls to bind_param.
C/XS version of Makefile.PL
The Makefile.PL file for a C/XS driver is similar to the code needed for a pure Perl driver, but
there are a number of extra bits of information needed by the build system. For example, the
attributes list passed to "WriteMakefile" needs to specify the object files that need to be compiled
and built into the shared object (DLL). This is often, but not necessarily, just dbdimp.o (unless
that should be dbdimp.obj because you're building on MS Windows). Note that you can reliably
determine the extension of the object files from the $Config{obj_ext} values, and there are many
other useful pieces of configuration information lurking in that hash. You get access to it with:
use Config;
Methods which do not need to be written
The DBI code implements the majority of the methods which are accessed using the notation
DBI->function(), the only exceptions being DBI->connect() and DBI->data_sources() which require
support from the driver.
The DBI code implements the following documented driver, database and statement functions which do
not need to be written by the DBD driver writer.
$dbh->do()
The default implementation of this function prepares, executes and destroys the statement. This
can be replaced if there is a better way to implement this, such as EXECUTE IMMEDIATE which can
sometimes be used if there are no parameters.
$h->errstr()
$h->err()
$h->state()
$h->trace()
The DBD driver does not need to worry about these routines at all.
$h->{ChopBlanks}
This attribute needs to be honured during fetch operations, but does not need to be handled by
the attribute handling code.
$h->{RaiseError}
The DBD driver does not need to worry about this attribute at all.
$h->{PrintError}
The DBD driver does not need to worry about this attribute at all.
$sth->bind_col()
Assuming the driver uses the DBIc_DBISTATE(imp_xxh)->get_fbav() function (C drivers, see below),
or the $sth->_set_fbav($data) method (Perl drivers) the driver does not need to do anything about
this routine.
$sth->bind_columns()
Regardless of whether the driver uses DBIc_DBISTATE(imp_xxh)->get_fbav(), the driver does not
need to do anything about this routine as it simply iteratively calls $sth->bind_col().
The DBI code implements a default implementation of the following functions which do not need to be
written by the DBD driver writer unless the default implementation is incorrect for the Driver.
$dbh->quote()
This should only be written if the database does not accept the ANSI SQL standard for quoting
strings, with the string enclosed in single quotes and any embedded single quotes replaced by two
consecutive single quotes.
For the two argument form of quote, you need to implement the "type_info" method to provide the
information that quote needs.
$dbh->ping()
This should be implemented as a simple efficient way to determine whether the connection to the
database is still alive. Typically code like this:
sub ping {
my $dbh = shift;
$sth = $dbh->prepare_cached(q{
select * from A_TABLE_NAME where 1=0
}) or return 0;
$sth->execute or return 0;
$sth->finish;
return 1;
}
where A_TABLE_NAME is the name of a table that always exists (such as a database system
catalogue).
METADATA METHODS
The exposition above ignores the DBI MetaData methods. The metadata methods are all associated with
a database handle.
Using DBI::DBD::Metadata
The DBI::DBD::Metadata module is a good semi-automatic way for the developer of a DBD module to write
the get_info and type_info functions quickly and accurately.
Generating the get_info method
Prior to DBI v1.33, this existed as the method write_getinfo_pm in the DBI::DBD module. From DBI
v1.33, it exists as the method write_getinfo_pm in the DBI::DBD::Metadata module. This discussion
assumes you have DBI v1.33 or later.
You examine the documentation for write_getinfo_pm using:
perldoc DBI::DBD::Metadata
To use it, you need a Perl DBI driver for your database which implements the get_info method. In
practice, this means you need to install DBD::ODBC, an ODBC driver manager, and an ODBC driver for
your database. With the pre-requisites in place, you might type:
perl -MDBI::DBD::Metadata -e write_getinfo_pm \
dbi:ODBC:foo_db username password Driver
The procedure writes to standard output the code that should be added to your Driver.pm file and the
code that should be written to lib/DBD/Driver/GetInfo.pm. You should review the output to ensure
that it is sensible.
Generating the type_info method
Given the idea of the write_getinfo_pm method, it was not hard to devise a parallel method,
write_typeinfo_pm, which does the analogous job for the DBI type_info_all metadata method. The the
write_typeinfo_pm method was added to DBI v1.33.
You examine the documentation for write_typeinfo_pm using:
perldoc DBI::DBD::Metadata
The setup is exactly analogous to the mechanism descibed in "Generating the get_info method" With the
pre-requisites in place, you might type:
perl -MDBI::DBD::Metadata -e write_typeinfo \
dbi:ODBC:foo_db username password Driver
The procedure writes to standard output the code that should be added to your Driver.pm file and the
code that should be written to lib/DBD/Driver/TypeInfo.pm. You should review the output to ensure
that it is sensible.
Writing DBD::Driver::db::get_info
If you use the DBI::DBD::Metadata module, then the code you need is generated for you.
If you decide not to use the DBI::DBD::Metadata module, you should probably borrow the code from a
driver that has done so (eg DBD::Informix from version 1.05 onwards) and crib the code from there, or
look at the code that generates that module and follow that. The method in Driver.pm will be very
simple; the method in lib/DBD/Driver/GetInfo.pm is not very much more complex unless your DBMS itself
is much more complex.
Note that some of the DBI utility methods rely on information from the get_info method to perform
their operations correctly. See, for example, the quote_identifier and quote methods, discussed
below.
Writing DBD::Driver::db::type_info_all
If you use the DBI::DBD::Metadata module, then the code you need is generated for you.
If you decide not to use the DBI::DBD::Metadata module, you should probably borrow the code from a
driver that has done so (eg DBD::Informix from version 1.05 onwards) and crib the code from there, or
look at the code that generates that module and follow that. The method in Driver.pm will be very
simple; the method in lib/DBD/Driver/TypeInfo.pm is not very much more complex unless your DBMS
itself is much more complex.
Writing DBD::Driver::db::type_info
The guidelines on writing this method are still not really clear. No sample implementation is
available.
Writing DBD::Driver::db::table_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::column_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::primary_key_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::primary_key
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::foreign_key_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::tables
This method generates an array of names in a format suitable for being embedded in SQL statements in
places where a table name is expected.
If your database hews close enough to the SQL standard or if you have implemented an appropriate
table_info function and and the appropriate quote_identifier function, then the DBI default version
of this method will work for your driver too.
Otherwise, you have to write a function yourself, such as:
sub tables
{
my($dbh, $cat, $sch, $tab, $typ) = @_;
my(@res);
my($sth) = $dbh->table_info($cat, $sch, $tab, $typ);
my(@arr);
while (@arr = $sth->fetchrow_array)
{
push @res, $dbh->quote_identifier($arr[0], $arr[1], $arr[2]);
}
return @res;
}
See also the default implementation in DBI.pm.
Writing DBD::Driver::db::quote
This method takes a value and converts it into a string suitable for embedding in an SQL statement as
a string literal.
If your DBMS accepts the SQL standard notation for strings (single quotes around the string as a
whole with any embedded single quotes doubled up), then you do not need to write this method as DBI
provides a default method that does it for you. If your DBMS uses an alternative notation or escape
mechanism, then you need to provide an equivalent function. For example, suppose your DBMS used C
notation with double quotes around the string and backslashes escaping both double quotes and
backslashes themselves. Then you might write the function as:
sub quote
{
my($dbh, $str) = @_;
$str =~ s/["\\]/\\$&/gmo;
return qq{"$str"};
}
Handling newlines and other control characters is left as an exercise for the reader.
This sample method ignores the $data_type indicator which is the optional second argument to the
method.
Writing DBD::Driver::db::quote_identifier
This method is called to ensure that the name of the given table (or other database object) can be
embedded into an SQL statement without danger of misinterpretation. The result string should be
usable in the text of an SQL statement as the identifier for a table.
If your DBMS accepts the SQL standard notation for quoted identifiers (which uses double quotes
around the identifier as a whole, with any embedded double quotes doubled up) and accepts
"schema"."identifier" (and "catalog"."schema"."identifier" when a catalog is specified), then you do
not need to write this method as DBI provides a default method that does it for you. In fact, even
if your DBMS does not handle exactly that notation but you have implemented the get_info method and
it gives the correct responses, then it will work for you. If your database is fussier, then you
need to implement your own version of the function.
For example, DBD::Informix has to deal with an environment variable DELIMIDENT. If it is not set,
then the DBMS treats names enclosed in double quotes as strings rather than names, which is usually a
syntax error. Additionally, the catalog portion of the name is separated from the schema and table
by a different delimiter (colon instead of dot), and the catalog portion is never enclosed in quotes.
(Fortunately, valid strings for the catalog will never contain weird characters that might need to be
escaped, unless you count dots, dashes, slashes and at-signs as weird.) Finally, an Informix
database can contain objects that cannot be accessed because they were created by a user with the
DELIMIDENT environment variable set, but the current user does not have it set. By design choice,
the quote_identifier method encloses those identifiers in double quotes anyway, which generally
triggers a syntax error, and the metadata methods which generate lists of tables etc omit those
identifiers from the result sets.
sub quote_identifier
{
my($dbh, $cat, $sch, $obj) = @_;
my($rv) = "";
my($qq) = (defined $ENV{DELIMIDENT}) ? '"' : '';
$rv .= qq{$cat:} if (defined $cat);
if (defined $sch)
{
if ($sch !~ m/^\w+$/o)
{
$qq = '"';
$sch =~ s/$qq/$qq$qq/gm;
}
$rv .= qq{$qq$sch$qq.};
}
if (defined $obj)
{
if ($obj !~ m/^\w+$/o)
{
$qq = '"';
$obj =~ s/$qq/$qq$qq/gm;
}
$rv .= qq{$qq$obj$qq};
}
return $rv;
}
Handling newlines and other control characters is left as an exercise for the reader.
Note that there is an optional fourth parameter to this function which is a reference to a hash of
attributes; this sample implementation ignores that. This sample implementation also ignores the
single-argument variant of the method.
WRITING AN EMULATION LAYER FOR AN OLD PERL INTERFACE
Study Oraperl.pm (supplied with DBD::Oracle) and Ingperl.pm (supplied with DBD::Ingres) and the
corresponding dbdimp.c files for ideas.
Note that the emulation code sets $dbh->{CompatMode} = 1; for each connection so that the internals
of the driver can implement behaviour compatible with the old interface when dealing with those
handles.
Setting emulation perl variables
For example, ingperl has a $sql_rowcount variable. Rather than try to manually update this in
Ingperl.pm it can be done faster in C code. In dbd_init():
sql_rowcount = perl_get_sv("Ingperl::sql_rowcount", GV_ADDMULTI);
In the relevant places do:
if (DBIc_COMPAT(imp_sth)) /* only do this for compatibility mode handles */
sv_setiv(sql_rowcount, the_row_count);
OTHER MISCELLANEOUS INFORMATION
The imp_xyz_t types
Any handle has a corresponding C structure filled with private data. Some of this data is reserved
for use by DBI (except for using the DBIc macros below), some is for you. See the description of the
dbdimp.h file above for examples. The most functions in dbdimp.c are passed both the handle "xyz" and
a pointer to "imp_xyz". In rare cases, however, you may use the following macros:
D_imp_dbh(dbh)
Given a function argument dbh, declare a variable imp_dbh and initialize it with a pointer to the
handles private data. Note: This must be a part of the function header, because it declares a
variable.
D_imp_sth(sth)
Likewise for statement handles.
D_imp_xxx(h)
Given any handle, declare a variable imp_xxx and initialize it with a pointer to the handles
private data. It is safe, for example, to cast imp_xxx to "imp_dbh_t*", if DBIc_TYPE(imp_xxx) ==
DBIt_DB. (You can also call sv_derived_from(h, "DBI::db"), but that's much slower.)
D_imp_dbh_from_sth
Given a imp_sth, declare a variable imp_dbh and initialize it with a pointer to the parent database
handle's implementors structure.
Using DBIc_IMPSET_on
The driver code which initializes a handle should use DBIc_IMPSET_on() as soon as its state is such
that the cleanup code must be called. When this happens is determined by your driver code.
Failure to call this can lead to corruption of data structures. For example, DBD::Informix maintains
a linked list of database handles in the driver, and within each handle, a linked list of statements.
Once a statement is added to the linked list, it is crucial that it is cleaned up (removed from the
list). When DBIc_IMPSET_on() was being called too late, it was able to cause all sorts of problems.
Using DBIc_is(), DBIc_has(), DBIc_on() and DBIc_off()
Once upon a long time ago, the only way of handling the internal DBI boolean flags/attributes was
through macros such as:
DBIc_WARN DBIc_WARN_on DBIc_WARN_off
DBIc_COMPAT DBIc_COMPAT_on DBIc_COMPAT_off
Each of these took an imp_xxh pointer as an argument.
Since then, new attributes have been added such as ChopBlanks, RaiseError and PrintError, and these
do not have the full set of macros. The approved method for handling these is now the four macros:
DBIc_is(imp, flag)
DBIc_has(imp, flag) an alias for DBIc_is
DBIc_on(imp, flag)
DBIc_off(imp, flag)
DBIc_set(imp, flag, on) set if on is true, else clear
Consequently, the DBIc_XXXXX family of macros is now mostly deprecated and new drivers should avoid
using them, even though the older drivers will probably continue to do so for quite a while yet.
However...
There is an important exception to that. The ACTIVE and IMPSET flags should be set via the
DBIc_ACTIVE_on and DBIc_IMPSET_on macros, and unset via the DBIc_ACTIVE_off and DBIc_IMPSET_off
macros.
Using the get_fbav() method
THIS IS CRITICAL for C/XS drivers.
The $sth->bind_col() and $sth->bind_columns() documented in the DBI specification do not have to be
implemented by the driver writer because DBI takes care of the details for you. However, the key to
ensuring that bound columns work is to call the function DBIc_DBISTATE(imp_xxh)->get_fbav() in the
code which fetches a row of data. This returns an AV, and each element of the AV contains the SV
which should be set to contain the returned data.
The pure Perl equivalent is the $sth->_set_fbav($data) method, as described in the part on pure Perl
drivers.
SUBCLASSING DBI DRIVERS
This is definitely an open subject. It can be done, as demonstrated by the DBD::File driver, but it
is not as simple as one might think.
(Note that this topic is different from subclassing the DBI. For an example of that, see the
t/subclass.t file supplied with the DBI.)
The main problem is that the dbh's and sth's that your connect and prepare methods return are not
instances of your DBD::Driver::db or DBD::Driver::st packages, they are not even derived from it.
Instead they are instances of the DBI::db or DBI::st classes or a derived subclass. Thus, if you
write a method mymethod and do a
$dbh->mymethod()
then the autoloader will search for that method in the package DBI::db. Of course you can instead to
a
$dbh->func('mymethod')
and that will indeed work, even if mymethod is inherited, but not without additional work. Setting
@ISA is not sufficient.
Overwriting methods
The first problem is, that the connect method has no idea of subclasses. For example, you cannot
implement base class and subclass in the same file: The install_driver method wants to do a
require DBD::Driver;
In particular, your subclass has to be a separate driver, from the view of DBI, and you cannot share
driver handles.
Of course that's not much of a problem. You should even be able to inherit the base classes connect
method. But you cannot simply overwrite the method, unless you do something like this, quoted from
DBD::CSV:
sub connect ($$;$$$) {
my ($drh, $dbname, $user, $auth, $attr) = @_;
my $this = $drh->DBD::File::dr::connect($dbname, $user, $auth, $attr);
if (!exists($this->{csv_tables})) {
$this->{csv_tables} = {};
}
$this;
}
Note that we cannot do a
$drh->SUPER::connect($dbname, $user, $auth, $attr);
as we would usually do in a an OO environment, because $drh is an instance of DBI::dr. And note, that
the connect method of DBD::File is able to handle subclass attributes. See the description of Pure
Perl drivers above.
It is essential that you always call superclass method in the above manner. However, that should do.
Attribute handling
Fortunately the DBI specifications allow a simple, but still performant way of handling attributes.
The idea is based on the convention that any driver uses a prefix driver_ for its private methods.
Thus it's always clear whether to pass attributes to the super class or not. For example, consider
this STORE method from the DBD::CSV class:
sub STORE {
my ($dbh, $attr, $val) = @_;
if ($attr !~ /^driver_/) {
return $dbh->DBD::File::db::STORE($attr, $val);
}
if ($attr eq 'driver_foo') {
...
}
AUTHORS
Jonathan Leffler <jleffler@us.ibm.com> (previously <jleffler@informix.com>), Jochen Wiedmann
<joe@ispsoft.de>, Steffen Goeldner <sgoeldner@cpan.org>, and Tim Bunce <dbi-users@perl.org>.
perl v5.8.8 2006-05-10 DBI::DBD(3)
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