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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 v5.24.0                      2015-10-14                            O(3pm)

perl 5.24 - Generated Fri Nov 18 18:51:18 CST 2016
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