MULTIPLICITY和PERL_IMPLICIT_CONTEXT之间的关系

Question

What is the relationship between the Perl API macros MULTIPLICITY and PERL_IMPLICIT_CONTEXT ?

According to perlguts :

One macro controls the major Perl build flavor: MULTIPLICITY . The MULTIPLICITY build has a C structure that packages all the interpreter state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures.

(by the way, which "three data structures" are referred to here?)

I have noticed that when I build perl with usethreads :

./Configure -des -Dusethreads

the macros PERL_IMPLICIT_CONTEXT and MULTIPLICITY will both be set (defined). Also, in embedvar.h there is a comment that may be relevant:

The following combinations of MULTIPLICITY and PERL_IMPLICIT_CONTEXT are supported:
1) none
2) MULTIPLICITY # supported for compatibility
3) MULTIPLICITY && PERL_IMPLICIT_CONTEXT

All other combinations of these flags are errors.

only #3 is supported directly, while #2 is a special case of #3 (supported by redefining vTHX appropriately).

• So, when writing XS code is there any difference in writing #ifdef MULTIPLICITY versus writing #ifdef PERL_IMPLICIT_CONTEXT ?

• What is the history behind the two variables? It seems like they today could be reduced to a single. For example, what would happen if all occurences of MULTIPLICITY was replaced with PERL_IMPLICIT_CONTEXT in the perl source? What would it break?

Answer 1

Here is what I have found so far. Running sh Configure -des creates the header config.h . This header file will:

• define USE_ITHREADS if and only if Configure was given the flag -Dusethreads , eg:

   sh Configure -des -Dusethreads 

• define MULTIPLICITY if and only if Configure was given the flag -Dusemultiplicity :

   sh Configure -des -Dusemultiplicity 

• Setting MULTIPLICITY through ccflags will not set MULTIPLICITY in config.h , eg:

   sh Configure -des -Accflags="-DMULTIPLICITY" 

• Configure has no -D flag for PERL_IMPLICIT_CONTEXT , and defining it through ccflags will not define it in config.h .

The generated config.h header is #include d by perl.h . Note, that the latter header is also usually included by Perl XS extension files ( .xs -files). At line 59 in perl.h we have:

#ifdef USE_ITHREADS  
#  if !defined(MULTIPLICITY)
#    define MULTIPLICITY
#  endif
#endif

#ifdef PERL_GLOBAL_STRUCT_PRIVATE
#  ifndef PERL_GLOBAL_STRUCT
#    define PERL_GLOBAL_STRUCT
#  endif
#endif

#ifdef PERL_GLOBAL_STRUCT
#  ifndef MULTIPLICITY
#    define MULTIPLICITY
#  endif
#endif

#ifdef MULTIPLICITY
#  ifndef PERL_IMPLICIT_CONTEXT
#    define PERL_IMPLICIT_CONTEXT
#  endif
#endif

This means that:

• if -Dusethreads is given, USE_ITHREADS , MULTIPLICITY , and PERL_IMPLICIT_CONTEXT will all be defined.

• if -Dusemultiplicity is given, MULTIPLICITY and PERL_IMPLICIT_CONTEXT will be defined, whereas USE_ITHREADS will be undefined.

• if none of -Dusethreads or -Dusemultiplicity is given USE_ITHREADS , MULTIPLICITY , and PERL_IMPLICIT_CONTEXT will all be undefined.

• it is not possible to have MULTIPLICITY defined and PERL_IMPLICIT_CONTEXT undefined (unless one uses ccflags , but then this will only be during the perl build. XS extension modules that include perl.h will not see this)

So extension modules can usually assume that either:

• MULTIPLICITY and PERL_IMPLICIT_CONTEXT are both defined, or
• MULTIPLICITY and PERL_IMPLICIT_CONTEXT are both undefined.