O - Generic interface to Perl Compiler backends
perl -MO=[-q,]Backend[,OPTIONS] foo.pl
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.
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.
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
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.
Malcolm Beattie, "email@example.com"