Question about rtlsdr read_sync()

Just trying to figure out how to use the rtlsdr_read_sync() function, and I'm not understanding just by reading the source. Am I supposed to call this function continuously in a loop in a thread separate from the sample processing, or can I do some processing in between calls to rtlsdr_read_sync()? In other words, are the samples being buffered on the rtlsdr device in between calls? If so, what's the size of that buffer? Also, what functions need to be called before I can use the read_sync or read_async functions? I'm calling rtlsdr_open, rtlsdr_set_center_freq, and rtlsdr_set_sample_rate before calling rtlsdr_read_sync, but I'm not getting any samples (and subsequent calls to rtlsdr_read_sync or read_async fail, e.g. from running the rtlsdr_fm program). Thanks, Allen