O

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



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 compila-
       tion.  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 them-
       selves ("Deparse", "Concise" etc), so that their output is not con-
       fused 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 gen-
       erates 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 "import" function which that calls loads in the appropriate
       "B::Backend" module and calls the "compile" function in that package,
       passing it OPTIONS. That function is expected to return a sub refer-
       ence 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 redi-
       rected to $O::BEGIN_output.

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



perl v5.8.8			  2001-09-21				 O(3)