VMS Help CC, Run-time functions, strnlen  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns the number of bytes in the string pointed to by s.  The
  string length value does not include the terminating null
  character.  The strnlen function counts bytes until the first null
  byte or until n bytes have been examined.

  Syntax:

       #include <string.h>

       size_t strnlen(const char *s, size_t n);