On 14Apr2012 09:45, John Wendel <jwendel10@xxxxxxxxxxx> wrote:
| > I don't see how echoing into a pipe would change the problem.
| > Theoretically, if several processes (e.g. echo) are running in the
| > background, e.g. on a round robin basis, then potentially I could see
| > random sequences of As, Bs and Cs. It doesn't seem to be the case in
| > practice though. So which is it?
Writes certainly _used_ to be atomic unless they overflowed a buffer.
For example,
echo 1 &
echo 2 &
will always write "1\n" and "2\n" (in whichever order runs first) and
never interleave them. 500 "A"s shouldn't overflow a buffer either.
| > This has to do with the operating system internals, it's not a trivial
| > question.
| >
| Actually it is semi-non-trivial. :-)
|
| Unix/Linux makes the following guarantee ...
Ireelevant promise:
| Multiple processes that open the same file for writing each maintain
| their own file positions, so they may overwrite the output of another
| process, unless the processes all open the file with the "O_APPEND"
| option.
This only matters if the processes _independently_ opened the file.
So this:
echo AAA >foo &
echo BBB >foo &
pretty much _will_ overwrite each other. But the OP effectively has this:
( echo AAA &
echo BBB &
) >foo
Only _one_ open file handle in play. The writes may happen in either
order but they will _not_ overwrite each other because there is only one
file handle, and thus only one file position pointer.
| Of course, if you didn't write the code that is doing the output, you'll
| have to examine the source to see if it uses the "O_APPEND" open option.
| This may be non-trivial.
Not in the shell. This:
echo AAA >>foo
uses O_APPEND.
But it is not the issue at hand.
Cheers,
--
Cameron Simpson <cs@xxxxxxxxxx> DoD#743
http://www.cskk.ezoshosting.com/cs/
Reality is that which refuses to go away when I stop believing in it.
- Phillip K. Dick
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