[TUHS] FreeBSD disk devices (Re: Choice of Unix for 11/03 and 11/23+ Systems

[Bakul Shah]

On Oct 12, 2020, at 9:57 AM, Arthur Krewat <krewat at kilonet.net> wrote:
> 
> On 10/11/2020 9:56 PM, Warner Losh wrote:
>> unaligned I/O on the block device
> Sorry, I have to laugh... isn't that an oxymoron? ;)

Actually there are no *block* devices in FreeBSD.
And while raw device files such as /dev/r<blkdev>
no longer exist, it is the block devices (with
their buffering etc.) that are gone. Only raw devices
exist now.

And FreeBSD raw disk device drivers don't allow
unaligned I/O access. Nor should they paper over
what is not allowed by the underlying device. Try

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

unsigned char buf[1024];

int main(int c, char**v) {
        char* a = c > 1? v[1] : "1";
        off_t off = (off_t)strtoull(a, 0, 0);
        ssize_t r = pread(0, buf, sizeof buf, off);
        if (r < 0) { perror("read"); return 1; }
        for (int i = 0; i < 4; i++) printf(" %02x", buf[i]);
        printf("\n");
        return 0;
}