off topic: combined output of concurrent processes

[John Wendel]

On 04/14/2012 08:20 AM, Amadeus W.M. wrote:
>> If you really would like to get output in sequence, write to a pipe, and
>> have a reader process drain the pipe to a logfile.  It's pretty easy;
>> look at "mknod" with the 'p' option, or "mkfifo".  I'd still suggest
>> tagging each output line with an identifier and sequence number.
>>
> For the sake of the argument, assume I echo 500 As, 500 Bs and 500 Cs.
>
> I don't care which process the output is coming from. It doesn't matter
> which order the As, Bs and Cs are output. All I care about is that I
> don't get 349As followed by 245Bs, etc. I want to see blocks of 500 each.
>
> 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?
>
> 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 ...

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. With the "O_APPEND" option, the system guarantees that the 
entire data from a single write by a process will be written to the end 
of the file as a indivisible block and will not be mixed with the output 
from another process. Without "O_APPEND", data from multiple processes 
may be intermixed in any order, or may seem to "disappear" (is 
overwritten by other data).

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.

Regards,

John