


O(3p)            Perl Programmers Reference Guide           O(3p)


NAME
       O - Generic interface to Perl Compiler backends

SYNOPSIS
               perl -MO=[-q,]Backend[,OPTIONS] foo.pl

DESCRIPTION
       This is the module that is used as a frontend to the Perl
       Compiler.

       If you pass the "-q" option to the module, then the STDOUT
       filehandle will be redirected into the variable
       $O::BEGIN_output during compilation.  This has the effect
       that any output printed to STDOUT by BEGIN blocks or use'd
       modules will be stored in this variable rather than
       printed. It's useful with those backends which produce
       output themselves ("Deparse", "Concise" etc), so that
       their output is not confused with that generated by the
       code being compiled.

       The "-qq" option behaves like "-q", except that it also
       closes STDERR after deparsing has finished. This
       suppresses the "Syntax OK" message normally produced by
       perl.

CONVENTIONS
       Most compiler backends use the following conventions:
       OPTIONS consists of a comma-separated list of words (no
       white-space).  The "-v" option usually puts the backend
       into verbose mode.  The "-ofile" option generates output
       to file instead of stdout. The "-D" option followed by
       various letters turns on various internal debugging flags.
       See the documentation for the desired backend (named
       "B::Backend" for the example above) to find out about that
       backend.

IMPLEMENTATION
       This section is only necessary for those who want to write
       a compiler backend module that can be used via this
       module.

       The command-line mentioned in the SYNOPSIS section
       corresponds to the Perl code

           use O ("Backend", OPTIONS);

       The "O::import" function loads the appropriate
       "B::Backend" module and calls its "compile" function,
       passing it OPTIONS. That function is expected to return a
       sub reference which we'll call CALLBACK. Next, the
       "compile-only" flag is switched on (equivalent to the
       command-line option "-c") and a CHECK block is registered
       which calls CALLBACK. Thus the main Perl program mentioned
       on the command-line is read in, parsed and compiled into



perl v5.10.1                2009-10-12                          1





O(3p)            Perl Programmers Reference Guide           O(3p)


       internal syntax tree form. Since the "-c" flag is set, the
       program does not start running (excepting BEGIN blocks of
       course) but the CALLBACK function registered by the
       compiler backend is called.

       In summary, a compiler backend module should be called
       "B::Foo" for some foo and live in the appropriate
       directory for that name.  It should define a function
       called "compile". When the user types

           perl -MO=Foo,OPTIONS foo.pl

       that function is called and is passed those OPTIONS (split
       on commas). It should return a sub ref to the main
       compilation function.  After the user's program is loaded
       and parsed, that returned sub ref is invoked which can
       then go ahead and do the compilation, usually by making
       use of the "B" module's functionality.

BUGS
       The "-q" and "-qq" options don't work correctly if perl
       isn't compiled with PerlIO support : STDOUT will be closed
       instead of being redirected to $O::BEGIN_output.

AUTHOR
       Malcolm Beattie, "mbeattie@sable.ox.ac.uk"































perl v5.10.1                2009-10-12                          2


