mmap port from 9 not working

[Laurie Jennings]

I have some custom stuff I'm porting from Freebsd 9.x using mmap. I get a pointer from the kernel via an ioctl and I map it into a shared buffer.
char *kptr;   // mem ptr from kernel
fd=open("/dev/kmem",O_RDWR);memp=mmap(0,size,PROT_READ|PROT_WRITE,MAP_SHARED,fd,(off_t) ptr);

This worked perfectly in 9; memp I had a shared block of memory between the kernel and user space.
In 11.3 this returns an errno 22, which is pretty murky. I did notice that off_t doesnt yield an actual offset; I've tried putting in the correct value manuallybut it just fails and fails.I've tried read only also. 
Please Help!
Laurie