Re: Lockless file reading

[Robin Rosenberg]

Aren't Linux files also streams. Writing to a stream in sequence
should update the stream in sequence, or it wouldn't be a stream,
would it?

-- robin

onsdagen den 27 augusti 2003 14.37 skrev Timo Sirainen:
> (Maybe a bit off topic, but I couldn't get answers elsewhere and it is
> about kernel behaviour)
>
> The question is what can happen if I read() a file that's being
> simultaneously updated by a write() in another process? If I'm writing
> 123 over XXX, is it possible that read() returns 1X3 in some conditions?
> Is the behaviour filesystem specific? Any idea about other operating
> systems?
>
> I'm thinking about implementing lockless file reads that work like:
>
> void write_data(int fd, off_t offset, void *data, size_t size) {
>   lock_file(fd);
>   pwrite(fd, data, size, offset); // or writev() or copy+single pwrite()
>   pwrite(fd, data, size, offset + size);
>   unlock_file(fd);
> }
>
> void read_data(int fd, off_t offset, void *data, size_t size) {
>   unsigned char buf{size*2];
>   for (;;) {
>     pread(fd, buf, size*2, offset); // or shared mmap()ed access
>     if (memcmp(buf, buf+size, size) == 0) break;
>     usleep(10);
>   }
>   memcpy(data, buf, size);
> }
>
> The only case when I see that it would break is if we write "1212" but
> pread() sees "1X1X" or "X2X2" there.
>
>
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