How to detect if C code (which needs 'extern C') is compiled in C++

I have a C header as part of a C++ library.

This C header would only make sense compiled by a C compiler, or by a C++ compiler within an extern "C" { ... } block, otherwise unresolved link errors would happen.

I com/tag/thought">thought to com/tag/add">add a com/tag/block">block such as:

#ifdef __cplusplus
#error "Compiling C bindings with C++ (forgot 'extern \"C\"'?)"
#endif

in the C header, but unfortunately the __cplusplus macro is defined also within an extern "C" { ... } block.

Is there another way to detect this condition correctly?

The common practice is not to demand client code wraps your header in extern "C", but to do so conditionally yourself. For instance:

#ifdef __cplusplus
extern "C" {
#endif

  // Header content

#ifdef __cplusplus
}
#endif

That way client code is automatically correct without doing anything beyond including the header.