I am adding some code to an existing FreeBSD device driver and I am trying to pass a char* from user space to the driver. I've implemented a custom ioctl() command using the _IOW macro like so: #define TIBLOOMFILTER _IOW(0,253,char*) My call looks something like this: int file_desc = open("/dev/ti0", O_RDWR); ioctl(file_desc, TIBLOOMFILTER, (*filter).getBitArray()); close(file_desc); When I call ioctl() I get: Inappropriate ioctl for device as an error message. Any guess as to what may be doing wrong? I've defined the same macro in my device driver, and added it to the case statement.