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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 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 to $O::BEGIN_output.
Malcolm Beattie, mbeattie@sable.ox.ac.uk
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