Cannot mmap character device

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

Hello

the mmap(2) page explicitely notes that we may map character devices.
Anyone can tell me why this fails?

# cat test.c                                                                 
#include <err.h>
#include <fcntl.h>
#include <sys/mman.h>

int
main(void)
{
        int fd;
        void *map;

        if ((fd = open("/dev/rxbd1a", O_RDONLY, 0)) == -1)
                err(1, "open /dev/rxbd1a failed");

        map = mmap(NULL, 4096, PROT_READ, MAP_FILE, fd, 8192);
        if (map == MAP_FAILED)
                err(1, "mmap /dev/rxbd1a failed");

        return 0;
}
# cc -Wall -ansi -o test test.c
# ./test
test: mmap /dev/rxbd1a failed: Invalid argument
# ls -l /dev/*xbd1a 
crw-r-----  1 root  backups        142, 8 Nov 17  2006 /dev/rxbd1a
brw-r-----  1 root  backups        142, 8 Nov 17  2006 /dev/xbd1a
# mount|grep xbd1a
/dev/xbd1a on /htdocs2 type ffs (log, local)

-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost