> -----Original Message-----
> From: IBM Mainframe Discussion List On Behalf Of John McKown
>
> Just to be my usual self. In general in CS, "reentrant" does not mean
> "non-self modifying". Many programs which do not modify themselves are
> still not reentrant due to uncoordinated updating of shared resources.
> Likewise, if done properly a self modifying program can still be
> reentrant.
>
> ref: http://en.wikipedia.org/wiki/Reentrant_%28subroutine%29
> <quote>
> In computing, a computer program or subroutine is called reentrant if
it
> can be interrupted in the middle of its execution and then safely
called
> again before its previous invocations complete executing.
> </quote>
>
> simple non-reentrant code example:
>
> L R1,SOMEVALUE
> AHI R1,+1
> ST R1,SOMEVALUE
>
> Note that it is possible for the program to be interrupted after the L
> and before the ST and some other function (or another task running
this
> function) to update SOMEVALUE. So SOMEVALUE can become corrupted
(update
> lost).
>
> IIRC, non-self modifying code is called "pure code" in UNIX.
OK, so what's the difference between "reentrant" (or "re-enterable") and
"threadsafe"? CICS (e.g.) asserts there is a difference: DFHSIT parm
RENTPGM=[PROTECT | NOPROTECT] and PROGRAM attribute
CONCURRENCY=[THREADSAFE | QUASIRENT], each independent of the other.
RENTPGM=PROTECT causes CICS to allocate its RDSAs in key-0 storage,
while CONCURRENCY=THREADSAFE causes CICS to leave programs running on an
"open" TCB between resource manager (usually DB2) calls, or until the
program issues a "non-threadsafe" EXEC CICS command.
-jc-
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
