> By the way, what ~is~ SVC 99?
First, note that three terms are used pretty much interchangeably: SVC 99,
DYNALLOC, and "Dynamic Allocation." I will use the term SVC 99 here. DYNALLOC
is an assembler macro that does not do much of anything except issue SVC 99.
(You know what a supervisor call is? If not, beyond the scope of this e-mail.
It's a low-level interface into z/OS services.)
Big picture, envision what a DD statement does. It allocates a dataset to a DD
name. You need DD statements because to open a dataset in z/OS you need a DD
name: when you open a DCB it references a DD name, not a dataset name. Same
thing in Rexx. Now picture a subroutine or API that you could call from a
running program that would do what a DD statement does: allocate a dataset to a
DD name, but in the middle of your program, not before your program starts.
(Yes, you can also do other things with SVC 99, but that's a good start.) In
the situation you wrote about earlier, you could get rid of your DD statement
for SORTOUT DISP=OLD and instead use SVC 99 to accomplish the same thing --
only 4.5 hours later in the process.
There are three major ways (yes, there are more than three) to access SVC 99
services:
- Some higher level languages do it under the covers. In C I can say
fopen("//MY.DATA.SET", "r") and under the covers LE or C allocates (SVC 99)
MY.DATA.SET to some generated DD name and then opens a DCB using that DD name.
COBOL can do something somewhat similar. Perhaps other languages also.
- The traditional way of using SVC 99 is here:
https://www.ibm.com/support/knowledgecenter/SSLTBW_2.1.0/com.ibm.zos.v2r1.ieaa800/sjdynal.htm.
Don't let the title fool you: most SVC 99 services do not require any special
authorization. It's complicated: lots of control blocks pointing to each other.
But if you need access to every SVC 99 feature and facility, this is the way to
go. Not for the feint-hearted. Not recommended as your first HLASM project.
- The new, easy way is:
https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.2.0/com.ibm.zos.v2r2.bpxb600/wdyn.htm.
Pretty well explained. You code CALL BPXWDYN "ALLOC FI(SORTOUT)
DA(target.dataset) OLD" and voila! (Yes, you probably need some return code
checking also.)
Charles
-----Original Message-----
From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf
Of Bob Bridges
Sent: Tuesday, March 31, 2020 4:33 PM
To: [email protected]
Subject: Re: PARM= vs PARMDD= and symbol substitution
I'm trying to picture what this would involve. I don't know from SVC 99, but
it seems to me nothing very bad would happen. Suppose my TSO session is
running two threads at the same time. (That never happens, although I get the
impression I could make it possible by some exotic coding. We'll pretend I
know how, for the moment.) One thread RR1 calls TEMPDD, which claims that
DDN12345 is available. Then RR2 grabs DDN12345. When (a millisecond or so
later) RR1 tries to use DDN12345, the system says "no, sorry, no can do" and
abends. I, of course, am puzzled (how could such a thing happen? Practically
speaking it can't, although hypothetically I guess it can), but that's the
worst that'll happen; in reality I'll try RR1 again, it'll work, and I'll shrug
and move on. It's not like a deadly embrace where two routines just freeze
up...is it?
By the way, what ~is~ SVC 99? Some kind of assembler call, I suppose? I've
written in assemblers, but haven't yet learned HLASM. One of my many
ambitions; still trying to get around to it.
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