Historically the linking of PL/I programs was slightly complicated because
there were two versions of certain library routines, one for COBOL-like
single-task applications and another for multitasking ones. These routines,
located in two different libraries, had the same names.
The scheme used to parameterize this situation was a simple one. It exploited
the fact that
//LIB1 DD DSN=A.B.C,DISP=SHR
// DD DSN=A.B.C,DISP=SHR
is licit. One may, that is, concatenate a library with itself; and the only
penalty incurred by doing so is a small amount of sometimes gratuitous overhead.
That said, why do this? The answer is that if the first DSN= value is
parameterized and given a default value equal to the value of the second, the
default value of the first can be overridden if a second library is needed and
ignored if not.
This scheme also generalizes readily to situations in which more than one
placeholder is needed.
About the differences between DSN=NULLFILE and DUMMY I have nothing new to say.
The mob was wrong on that earlier occasion, and it is wrong again now. It may
be, however, that this placeholder example will suggest one of the crucial
differences between DUMMY and DSN=NULLFILE to an 'alert reader'. John
Gilmore Ashland, MA 01721-1817 USA
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