And in many cases, the data is used for ioctl to communicate with the caller.

  > flags will have FUSE_IOCTL_COMPAT set for 32bit ioctls in 64bit environment. The size and direction of data is determined by IOC*() decoding of cmd. For _IOC_NONE, data will be NULL, for _IOC_WRITE data is out area, for _IOC_READ in area and if both are set in/out area. In all non-NULL cases, the area is of _IOC_SIZE(cmd) bytes.

From the fuse API document: http://fuse.sourceforge.net/doxygen/structfuse__operations.html#a37f0612d67a6b76bf10fe6a71b0e3b5b