[petsc-dev] what does extern "C" mean on a variable?

[Barry Smith]

Jed,

In C++ land we have

#if defined(__cplusplus)
#define PETSC_EXTERN extern "C" PETSC_VISIBILITY_PUBLIC
#define PETSC_EXTERN_TYPEDEF extern "C"
#define PETSC_INTERN extern "C” PETSC_VISIBILITY_INTERNAL

what does it mean to say 

PETSC_EXTERN PetscClassId AO_CLASSID; ? There is no c++ mangling with variables (or is there) so what does the extern “C” buy us? or mean?

should we have a different PETSC_EXTERN_VARIABLE for variables?

Barry