Probably because you're using the wrong tool.
And the "correct" tool in your opinion is?
What do you mean by that? The OPENJ does not[1] change the TIOT, so you're
still pointing to the same UCB's as before. Since MVS no longer supports
mountable DASD, that means that you can only succeed in changing a volser
that you already have allocated. That means that you don't need an RDJFCB
at all, just a correct calculation of the extent number.
I think that you misunderstand. I'm not trying to change the volser on
which the dataset resides. I'm trying to open a particular volume
sequence number in a multivolume set. You do that by specifying the
JFCBVLSQ field specifying 1 if you want the first volume, 2 for the
second etc.
It seems to me like I always get the first volume.
That's expected if you always set M to 0.
The M field in a MCCHHHHR is the extent number. Once I've got a DCB
pointing at the correct volume I construct a MCCHHRR corresponding to
the TTTR that I want within that volume using the normal methods. I
don't understand how the 'M' has a bearing on which volume I'm talking to.
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