Hi,
I forgot to include some description how it should work. The guarded
file is not %lockdir/lock, but any other file - because read-only
accesses to the file don't need to be guarded, and would fail if the
guarded file is currently renamed. You should make a directory
%lockdir/ with an empty file %lock
then you should use:
obtain-semaphore
something: read %my-guarded-file.txt
write %my-guarded-file.txt "something new"
release-semaphore
If you want to only read from your file, you can use just
something: read %my-guarded-file.txt
print something
If I remember correctly, you must only guard one situation,
"read-modify-write", because two simultaneous reads do not interfere,
and two simultaneous writes are a logic error of your program.
obtain-semaphore function uses loop and random wait interval to have a
better chance of succeeding if several processes run it simultaneously.
release-semaphore function should be simpler, loop is not necessary:
release-semaphore: func [] [
if not error? try [ rename %lockdir/lock.locked %lock ] [
return
]
print "Could not release semaphore"
quit
]
--
Michal Kracik
[EMAIL PROTECTED] wrote:
>
> Thank you Mr. Kracik!
>
> If only I undersatood completely whats going on in the script. Please
> correct me if I´m wrong:
> 1.Files in use are flagged with "semaphore"
> 2. The files in use are renamed as "%lock.locked"
>
> The script loops, continues to "Try" to rename, if it encounters an error
> then it "waits". Wonßt I get an error anyway if the file is directly
> renamed when I CGI script user requests a file?
>
> Excuse my newbie question
>
> Best regards
>
> Sharriff Aina
> med.iq information & quality in healthcare AG
> Gutenbergstr. 42
>
>
> [EMAIL PROTECTED]
> km.cz An: [EMAIL PROTECTED]
> Kopie:
> 11.10.00 Thema: [REBOL] How does one persist data?
>Re:
> 21:56
> Bitte
> antworten an
> list
>
>
>
> Hi Sharriff,
>
> be careful when saving to a file and reading it later, as many users
> may run your CGI programs simultaneously. You should use some form of
> inter-process synchronization to serialize access to your file. AFAIK
> from REBOL you can reliably synchronize by opening TCP sockets or
> renaming a file. I use these two functions:
>
> obtain-semaphore: func [] [
> loop 100 [
> if not error? try [ rename %lockdir/lock %lock.locked ] [
> return
> ]
> wait ( random 5 ) / 10
> ]
> print "Could not obtain semaphore"
> quit
> ]
>
> release-semaphore: func [] [
> loop 100 [
> if not error? try [ rename %lockdir/lock.locked %lock ] [
> return
> ]
> wait ( random 5 ) / 10
> ]
> print "Could not release semaphore"
> quit
> ]
>
> --
> Michal Kracik
>
> [EMAIL PROTECTED] wrote:
> >
> > Does anyone have a soultion of persisting data across HTML pages? I want
> > data submitted through a CGI from a HTML form to be usable in other
> pages.
> >
> > Many thanks for the anticipated help guys
> >
> > Sharriff Aina
> > med.iq information & quality in healthcare AG
