1O(3pm) Perl Programmers Reference Guide O(3pm)
2
6 O - Generic interface to Perl Compiler backends
7
9 perl -MO=[-q,]Backend[,OPTIONS] foo.pl
10
12 This is the module that is used as a frontend to the Perl Compiler.
13 If you pass the "-q" option to the module, then the STDOUT filehandle
15 will be redirected into the variable $O::BEGIN_output during
16 compilation. This has the effect that any output printed to STDOUT by
17 BEGIN blocks or use'd modules will be stored in this variable rather
18 than printed. It's useful with those backends which produce output
19 themselves ("Deparse", "Concise" etc), so that their output is not
20 confused with that generated by the code being compiled.
21 The "-qq" option behaves like "-q", except that it also closes STDERR
23 after deparsing has finished. This suppresses the "Syntax OK" message
24 normally produced by perl.
25
27 Most compiler backends use the following conventions: OPTIONS consists
28 of a comma-separated list of words (no white-space). The "-v" option
29 usually puts the backend into verbose mode. The "-ofile" option
30 generates output to file instead of stdout. The "-D" option followed by
31 various letters turns on various internal debugging flags. See the
32 documentation for the desired backend (named "B::Backend" for the
33 example above) to find out about that backend.
34
36 This section is only necessary for those who want to write a compiler
37 backend module that can be used via this module.
38 The command-line mentioned in the SYNOPSIS section corresponds to the
40 Perl code
41 use O ("Backend", OPTIONS);
43 The "O::import" function loads the appropriate "B::Backend" module and
45 calls its "compile" function, passing it OPTIONS. That function is
46 expected to return a sub reference which we'll call CALLBACK. Next, the
47 "compile-only" flag is switched on (equivalent to the command-line
48 option "-c") and a CHECK block is registered which calls CALLBACK. Thus
49 the main Perl program mentioned on the command-line is read in, parsed
50 and compiled into internal syntax tree form. Since the "-c" flag is
51 set, the program does not start running (excepting BEGIN blocks of
52 course) but the CALLBACK function registered by the compiler backend is
53 called.
54 In summary, a compiler backend module should be called "B::Foo" for
56 some foo and live in the appropriate directory for that name. It
57 should define a function called "compile". When the user types
58 perl -MO=Foo,OPTIONS foo.pl
60 that function is called and is passed those OPTIONS (split on commas).
62 It should return a sub ref to the main compilation function. After the
63 user's program is loaded and parsed, that returned sub ref is invoked
64 which can then go ahead and do the compilation, usually by making use
65 of the "B" module's functionality.
66
68 The "-q" and "-qq" options don't work correctly if perl isn't compiled
69 with PerlIO support : STDOUT will be closed instead of being redirected
70 to $O::BEGIN_output.
71
73 Malcolm Beattie, "mbeattie@sable.ox.ac.uk"
74perl v5.34.1 2022-03-15 O(3pm)