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



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 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 v5.16.2                                     2012-08-26                                           O(3pm)