From the libsndfile docs:
On success, the sf_open function returns a non-NULL pointer which should be passed as the first parameter to all subsequent libsndfile calls dealing with that audio file. On fail, the sf_open function returns a NULL pointer. An explanation of the error can obtained by passing NULL to sf_strerror.
If outfile is 0x2194000 and not null, then you probably opened the file correctly.
Try using sf_strerror() to see what error you had when you provided a full file path and not just the file name.