C++ allows a degree of interoperability with other languages through the use of language linkage specifications. These specifications effect affect the way in which functions are called or data is accessed. By default, all function types, as well as function and variable names, with external linkage have C++ language linkage, though a different language linkage may be specified. Implementations are required to support "C" and as a language linkage, but other language linkages exist with implementation-defined semantics, such as "C++""java", "Ada", and "FORTRAN".
Language linkage is specified to be part of the function type, according to the C++ Standard, [dcl.link], paragraph 1 [ISO/IEC 14882-2014], which states, in part, states the following:
Two function types with different language linkages are distinct types even if they are otherwise identical.
When calling a function, it is undefined behavior if the language linkage of the function type used in the call does not match the language linkage of the function definition. For instance, a mismatch in language linkage specification may corrupt the call stack due to calling convention conventions or other ABI mismatches.
Do not call a function through a type whose language linkage does not match the language linkage of the called function's definition. This restriction applies both to functions called within a C++ program as well as function pointers used to make a function call from outside of the C++ program.
...
| Code Block |
|---|
typedef void (*cpp_func)(void);
extern "C" typedef void (*c_func)(void);
void f(cpp_func fp) {}
void f(c_func fp) {} |
Some compilers do conform to the C++ Standard, but only in their strictest conformance mode, such as EDG 4.11. This implementation divergence from the C++ Standard is a matter of practical design trade-offs. Compilers are only required to support the only the "C" and "C++" language linkages, and interoperability between these two languages often does not require significant code generation differences beyond the mangling of function types for most common architectures such as x86, x86-64, and ARM. There are extant Standard Template Library implementations for which language linkage specifications being correctly implemented as part of the function type would break existing code on common platforms where the language linkage has no effect on the runtime implementation of a function call.
...
In this noncompliant code example, the call_java_fn_ptr() function expects to receive a function pointer with "java" language linkage because that function pointer will be used by a Java interpreter to call back into the C++ code. However, the function is given a pointer with "C++" language linkage instead, resulting in undefined behavior when the interpreter attempts to call the function pointer. This code should be ill-formed because the type of callback_func() is different than the type java_callback. However, but due to common implementation divergence from the C++ Standard, some compilers may incorrectly accept this code without issuing a diagnostic.
...