C ++ 'substitute' for a flexible array member structure

Question

Consider the following C99 structure ending with a flexible array element:

struct hdr
{
  size_t len;   
  size_t free;  
  char buf[];
};

len, for example, gets access using the built-in function (which should be placed in the header file), for example, having bufas an argument:

static inline size_t slen(const char *s)
{
  struct hdr *h = (struct hdr*)(s - (int)offsetof(struct hdr, buf));
  return h->len;
}

This is the part of the library that needs to be compiled using the C compiler. However, I would like to access this library from C ++; this essentially means that the corresponding header file (with the correct extern "C" {...}guard) must be valid C ++ code. A possible solution is to define a function slenin the body of the source code, completely avoiding inline code, but this is not optimal.

, ++, , - hdr, .

struct cpp_hdr
{
  size_t len;
  size_t free;
  char buf[1];
}

, () len free; buf.

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static inline size_t slen(const char *s)
{
  struct cpp_hdr *h = (struct cpp_hdr*)(s - (int)offsetof(struct cpp_hdr, buf));
  return h->len;
}

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