On GNU systems, if you want to generalize printf, all you need is vfprintf - because there is:

"fmemopen(3)" that creates FILE* that writes to pre-allocate dbuffer

"open_memstream(3)" that creates FILE* that writes to auto-allocated buffer;

and if that's not sufficient, there is "fopencookie(3)" which takes general callbacks and creates FILE* that redirects all operations to those callbacks.

If that does not work for some reason, then having custom callback with user-passed 3 parameters is too much. Why add dedicated FILE* or "size" parameters which are only ever used in one specific case? Do a generic "void * context" argument ("int (write)(char data, void * context)" + "void * context") and let user figure out how to use it.

Yeah

Pretty sure a vfprintf-like function sits at the bottom of the printf stack in all of the libc's I've surveyed (which includes BSDs). And yeah, BSDs also support memstream APIs, for example https://man.openbsd.org/fmemopen.3