wcstombs_s, _wcstombs_s_l
The function is supposedly declared in stdlib.h
However, it may have some conditional compilation guards, which prevent those declarations being visible in the project you're trying to build.
Eg.
Code:
#ifdef FOOBAR
errno_t wcstombs_s(
size_t *pReturnValue,
char *mbstr,
size_t sizeInBytes,
const wchar_t *wcstr,
size_t count
);
#endif
Now, if FOOBAR isn't set for your project, or some other header file before stdlib.h has #undef FOOBAR, then you won't see the declaration of wcstombs_s