The Windows API uses this scheme for one of its 50 string types [1].
I"m not very fond of this design as it's easy to pass a "normal" C string, which compiles because BSTR is just a typedef to it.
You can allocate the exact same data structure, but store a pointer to the size prefix, instead of the first byte - you avoid that issue, and can still pass the data field to anything expecting a zero-terminated string:
struct WeirdString { int size; char data[0]; };
struct WeirdString* ws = ...;
fopen(ws->data);