O(3perl) Perl Programmers Reference Guide O(3perl)NAMEO - 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
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
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.
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
Malcolm Beattie, "firstname.lastname@example.org"
perl v5.18.2 2013-11-04 O(3perl)