XSLoader - Dynamically load C libraries into Perl code

- NAME
- SYNOPSIS
- DESCRIPTION

- Migration from `DynaLoader`
- Backward compatible boilerplate

- Order of initialization: early `load()`

- The most hairy case

- LIMITATIONS
- AUTHOR

# NAME

XSLoader - Dynamically load C libraries into Perl code

# SYNOPSIS

        package YourPackage;
        use XSLoader;

        XSLoader::load 'YourPackage', $YourPackage::VERSION;

# DESCRIPTION

This module defines a standard simplified interface to the dynamic linking mechanisms available on many platforms. Its primary purpose is to implement cheap automatic dynamic loading of Perl modules.

For more complicated interface see the DynaLoader manpage. Many (most) features of DynaLoader are not implemented in XSLoader, like for example the dl_load_flags is not honored by XSLoader.

## Migration from `DynaLoader`

A typical module using DynaLoader starts like this:

        package YourPackage;
        require DynaLoader;

        our @ISA = qw( OnePackage OtherPackage DynaLoader );
        our $VERSION = '0.01';
        bootstrap YourPackage $VERSION;

Change this to

        package YourPackage;
        use XSLoader;

        our @ISA = qw( OnePackage OtherPackage );
        our $VERSION = '0.01';
        XSLoader::load 'YourPackage', $VERSION;

In other words: replace `require DynaLoader` by `use XSLoader`, remove `DynaLoader` from @ISA, change `bootstrap` by `XSLoader::load`. Do not forget to quote the name of your package on the `XSLoader::load` line, and add comma (`,`) before the arguments ($VERSION above).

Of course, if @ISA contained only `DynaLoader`, there is no need to have the @ISA assignment at all; moreover, if instead of `our` one uses backward-compatible

        use vars qw($VERSION @ISA);

one can remove this reference to @ISA together with the @ISA assignment

If no $VERSION was specified on the `bootstrap` line, the last line becomes

        XSLoader::load 'YourPackage';

## Backward compatible boilerplate

If you want to have your cake and eat it too, you need a more complicated boilerplate.

        package YourPackage;
        use vars qw($VERSION @ISA);

        @ISA = qw( OnePackage OtherPackage );
        $VERSION = '0.01';
        eval {
           require XSLoader;
           XSLoader::load('YourPackage', $VERSION);
           1;
        } or do {
           require DynaLoader;
           push @ISA, 'DynaLoader';
           bootstrap YourPackage $VERSION;
        };

The parentheses about XSLoader::load() arguments are needed since we replaced `use XSLoader` by `require`, so the compiler does not know that a function XSLoader::load() is present.

This boilerplate uses the low-overhead `XSLoader` if present; if used with an antic Perl which has no `XSLoader`, it falls back to using `DynaLoader`.

# Order of initialization: early `load()`

Skip this section if the XSUB functions are supposed to be called from other modules only; read it only if you call your XSUBs from the code in your module, or have a `BOOT:` section in your XS file (see The BOOT: Keyword in the perlxs manpage). What is described here is equally applicable to DynaLoader interface.

A sufficiently complicated module using XS would have both Perl code (defined in YourPackage.pm) and XS code (defined in YourPackage.xs). If this Perl code makes calls into this XS code, and/or this XS code makes calls to the Perl code, one should be careful with the order of initialization.

The call to XSLoader::load() (or `bootstrap())` has three side effects:

if $VERSION was specified, a sanity check is done to insure that the versions of the .pm and the (compiled) .xs parts are compatible;

The XSUBs are made accessible from Perl;

If the `BOOT:` section was present in .xs file, the code there is called.

Consequently, if the code in .pm file makes calls to these XSUBs, it is convenient to have XSUBs installed before the Perl code is defined; for example, this makes prototypes for XSUBs visible to this Perl code. Alternatively, if the `BOOT:` section makes calls to Perl functions (or uses Perl variables) defined in .pm file, they must be defined prior to the call to XSLoader::load() (or bootstrap()).

The first situation being much more frequent, it makes sense to rewrite the boilerplate as

        package YourPackage;
        use XSLoader;
        use vars qw($VERSION @ISA);

        BEGIN {
           @ISA = qw( OnePackage OtherPackage );
           $VERSION = '0.01';

           # Put Perl code used in the BOOT: section here

           XSLoader::load 'YourPackage', $VERSION;
        }

        # Put Perl code making calls into XSUBs here

## The most hairy case

If the interdependence of your `BOOT:` section and Perl code is more complicated than this (e.g., the `BOOT:` section makes calls to Perl functions which make calls to XSUBs with prototypes), get rid of the `BOOT:` section altogether. Replace it with a function onBOOT(), and call it like this:

        package YourPackage;
        use XSLoader;
        use vars qw($VERSION @ISA);

        BEGIN {
           @ISA = qw( OnePackage OtherPackage );
           $VERSION = '0.01';
           XSLoader::load 'YourPackage', $VERSION;
        }

        # Put Perl code used in onBOOT() function here; calls to XSUBs are
        # prototype-checked.

        onBOOT;

        # Put Perl initialization code assuming that XS is initialized here

# LIMITATIONS

To reduce the overhead as much as possible, only one possible location is checked to find the extension DLL (this location is where `make install` would put the DLL). If not found, the search for the DLL is transparently delegated to `DynaLoader`, which looks for the DLL along the @INC list.

In particular, this is applicable to the structure of @INC used for testing not-yet-installed extensions. This means that the overhead of running uninstalled extension may be much more than running the same extension after `make install`.

# AUTHOR

Ilya Zakharevich: extraction from DynaLoader.

XSLoader - Dynamically load C libraries into Perl code