As it is known, declaring extern "C" to C++ function makes its name have C linkage, enabling C code to link.
My question is - are there other programming languages we can make C++ function names have linkage to, something like extern "Lisp" or extern "FORTRAN"?
If not, why? What is the internal structure behind the "C", that makes the limitations?
What are the alternatives?
The C++ standard, 7.5.2 dcl.link, says:
Linkage between C++ and non-C++ code fragments can be achieved using a linkage-specification:
linkage-specification: extern string-literal { declaration-seqopt} extern string-literal declarationThe string-literal indicates the required language linkage. This International Standard specifies the semantics for the string-literals "C" and "C++". Use of a string-literal other than "C" or "C++" is conditionally supported, with implementation-defined semantics. [ Note: Therefore, a linkage-specification with a string literal that is unknown to the implementation requires a diagnostic. —end note ] [ Note: It is recommended that the spelling of the string-literal be taken from the document defining that language. For example, Ada (not ADA) and Fortran or FORTRAN, depending on the vintage. —end note ]
So in principle, implementers can choose to support other linkage specifications than C and C++.
In practise however, on all modern platforms, C linkage is the lowest common denominator. As a general rule, these days, binary interop for languages other than C uses C linkage. On the widely used modern platforms, you will not see anything other than C and C++ linkage.
extern "Pascal" exists (although it's a non-standard extension) - it is used to differentiate between the different parameter passing conventions between C and Pascal.
Most contemporary C++ compilers don't support anything other the extern "C" and extern "C++" required by the ISO C++ standard. Integration with other languages most commonly uses C calling conventions and extern "C".
And even when non-C calling conventions were supported, they often weren't selected by an extern string literal, they still used extern "C" but in conjunction with some non-standard calling convention declaration: for example, Microsoft C++ for 16-bit platforms (DOS, Windows 3.x, OS/2 1.x) used __pascal for the Pascal calling convention and __fortran for Fortran, instead of something like extern "Pascal" or extern "Fortran".
That said, IBM's contemporary C++ compilers for z/OS support extern "FORTRAN" for calling Fortran, extern "COBOL" for calling COBOL, and extern "PLI" for calling PL/I.
Similarly, IBM's ILE C++ compiler on IBM i supports a whole list of different extern string literals, see PDF page 338 of IBM i: ILE C/C++ Programmer's Guide, like extern "RPG", extern "C nowiden" (which is like extern "C", but turns off integer promotion, so a short will actually be passed as a 2 bytes instead of 4), extern "OS" (which enables IBM's legacy pre-ILE calling convention).
It appears, nowadays, IBM is the only major C++ vendor who supports non-"C"/"C++" extern, and even for them only on their mainframe and midrange platforms; but, historically, there probably were others.
