Re: [9fans] fd and /srv filesystem

[hiro <23hiro@gmail.com>]

btw it's very common on unix to share FDs in multi-threaded programs.
and all the pain resulting from un-synchronised FD access is available
as expected :)

On 10/4/23, hiro <23hiro@gmail.com> wrote:
> file descriptors describe to the kernel which of the files you
> previously open()'ed (a syscall) you want to operator on.
>
> it's not about security: if you want to operate on a file that another
> process might have opened before, you have to be careful that the
> other process isn't writing to the same location in the file at the
> same time. the kernel also keeps offsets for you.
>
> if you share FDs between multiple processes you might want some
> synchronisation like locking.
>
> On 10/4/23, Chris McGee <newton688@gmail.com> wrote:
>> Hi All,
>> 
>> I was thinking about file descriptors in the context of Plan 9. On Unix
>> an
>> fd is generally only usable by the current process, and child ones
>> through
>> a fork with some special incantation if one wants to communicate one over
>> a
>> domain socket. This is possibly for security reasons, avoiding other
>> users'
>> processes from trying to guess the fd of a critical file.
>> 
>> It's common practice in Plan 9 to post an fd (sometimes via a pipe) from
>> one process to the /srv filesystem so that others can discover it and
>> open
>> a comms channel. Does the kernel transform the fd into something when
>> posted to /srv so that it can be consumed by any other process in the
>> system?
>> 
>> Thanks,
>> Chris

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