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 CW-q option to the module, then the STDOUT filehandle will be redirected into the variable CW$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 (CWDeparse, CWConcise etc), so that their output is not confused with that generated by the code being compiled.

The CW-qq option behaves like CW-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 CW-v option usually puts the backend into verbose mode. The CW-ofile option generates output to file instead of stdout. The CW-D option followed by various letters turns on various internal debugging flags. See the documentation for the desired backend (named CWB::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 CWimport function which that calls loads in the appropriate CWB::Backend module and calls the CWcompile function in that package, 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 CW-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 CW-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 CWcompile. 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 CWB module's functionality.

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

Malcolm Beattie, CWmbeattie@sable.ox.ac.uk