typedef struct
{
    fract16 *h;     /*  filter coefficients            */
    fract16 *d;     /*  start of delay line            */
    fract16 *p;     /*  read/write pointer             */
    int k;          /*  number of coefficients         */
    int l;          /*  interpolation/decimation index */
} _fir_fr16_state;

void fir_fr16 (const fract16 _input[],
                       fract16 _output[], int _length,
                       fir_state_fr16 *_filter_state);

这个函数怎么用 参数什么意思