Le 27/07/2017 à 16:18, Enrico Weigelt, metux IT consult a écrit :
On 27.07.2017 08:22, Didier Kryn wrote:
At first glance at least, it means that file offsets are managed in the
kernel or VFS
Of course they are. That's required for any sane multiprocessing
implementation. And some files/devices don't even have the notion
of a current position (IOW: not seekable at all).
> but they can be bypassed by pwrite(). AFAIR pwrite()
doesn't change the "current" file offset; it simply ignores and bypasses
it, which isn't exactly what your example does.
Yes, that's an separate syscall for direct access. Simple streams
(eg. tty's, pipes, stream sockets, etc) usually don't support it.
Using unistd's read() and write() in C means you are dealing with low
level issues; otherwise why would you bother with the complexity it
introduces - not only you need to deal with buffering but also with
retries when interrupted by signals.
Usually you wanna care about signals - they actually mean something.
The ansi stream functions might or might not make it easier - depending
on your actual usecase. If you care about performance, you likely
don't wanna use them.
In an ideal word, software would have
- maximum performance
- minimum resource usage
- minimum of dangerous bugs
- easy maintainability
- fast development
- what else?
It's impossible to reach all these targets in the same time;
therefore the first design decision is which compromise between these is
the target. This implies choices in the organization of the program, the
language(s) to use, and possibly the programmer(s). I'm sure you know
this very well :-)
Didier
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