## f_forward

The f_forward function reads the file data and forward it to the data streaming device.

    FRESULT f_forward (
      FIL* fp,                        /* [IN] File object */
      UINT (*func)(const BYTE*,UINT), /* [IN] Data streaming function */
      UINT btf,                       /* [IN] Number of bytes to forward */
      UINT* bf                        /* [OUT] Number of bytes forwarded */
    );

#### Parameters

fp
  Pointer to the open file object. If a null pointer is given, the function fails with `FR_INVALID_OBJECT`.
func
  Pointer to the user-defined data streaming function. For details, refer to the sample code.
btf
  Number of bytes to forward in range of `UINT`.
bf
  Pointer to the variable in `UINT` type to return number of bytes forwarded.

#### Return Values

FR_OK, FR_DISK_ERR, FR_INT_ERR, FR_INVALID_OBJECT, FR_DENIED, FR_TIMEOUT

#### Description

The `f_forward` function reads the data from the file and forward it to the outgoing stream. This function is suitable for small memory system, because it does not require any data buffer in the application module. The file pointer of the file object advances in number of bytes forwarded. In case of `*bf` is less than `btf` without error, it means the requested size of data could not be transferred due to end of file or stream goes busy during data transfer.

#### QuickInfo

Available when `FF_USE_FORWARD == 1`.

#### Example

    /*------------------------------------------------------------------------*/
    /* Sample code of data transfer function to be called back from f_forward */
    /*------------------------------------------------------------------------*/

    UINT out_stream (   /* Returns number of bytes sent or stream status */
        const BYTE *p,  /* Pointer to the data block to be sent */
        UINT btf        /* >0: Transfer call (Number of bytes to be sent). 0: Sense call */
    )
    {
        UINT cnt = 0;

        if (btf == 0) {     /* Sense call */
            /* Return stream status (0: Busy, 1: Ready) */
            /* When once it returned ready to sense call, it must accept a byte at least */
            /* at subsequent transfer call, or f_forward will fail with FR_INT_ERR. */
            if (FIFO_READY) cnt = 1;
        }
        else {              /* Transfer call */
            do {    /* Repeat while there is any data to be sent and the stream is ready */
                FIFO_PORT = *p++;
                cnt++;
            } while (cnt < btf && FIFO_READY);
        }

        return cnt;
    }

    /*------------------------------------------------------------------------*/
    /* Sample code using f_forward function                                   */
    /*------------------------------------------------------------------------*/

    FRESULT play_file (
        char *fn        /* Pointer to the audio file name to be played */
    )
    {
        FRESULT rc;
        FIL fil;
        UINT dmy;

        /* Open the audio file in read only mode */
        rc = f_open(&fil, fn, FA_READ);
        if (rc) return rc;

        /* Repeat until the file pointer reaches end of the file */
        while (rc == FR_OK && !f_eof(&fil)) {

            /* some processes... */

            /* Fill output stream periodicaly or on-demand */
            rc = f_forward(&fil, out_stream, 1000, &dmy);
        }

        /* Close the file and return */
        f_close(&fil);
        return rc;
    }

#### See Also

`f_open, fgets, f_write, f_close, FIL`

Return