How write() works

[Marcus Clyne <maccaday at gmail dot com>]

Hi,

I'm trying to find out how write() works, specifically what has happened 
before it returns successfully - in relation to regular files.  What I 
want to know is has that data been written to its final storage medium, 
or has it just been transferred to kernel space?

Assuming that the length returned is the same length requested, I assume 
that one of the two scenarios are true :

1) The data has been written to kernel memory, and may or may not have 
yet been written to the final storage location.  The data will be 
flushed to the final storage medium asynchronously by the kernel, but 
all reads will return the updated data.

2) The data has been written to the final storage medium (and hence the 
write() function blocked until the writing finished).

I'm assuming that it is situation 1, but I'd like to be certain.  Any 
clarification on this would be much appreciated.

Thanks,

Marcus.