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
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