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MVS Low-Level I/O Functions |
This chapter discusses functions that can be used for
low level access to MVS sequential data sets. Three sets of functions are
provided, two of which perform low-level I/O, and the third of which performs
file allocation. There are two I/O interfaces: a full-function interface
to BSAM and an interface similar to QSAM. The QSAM-like interface is record-oriented
rather than block-oriented, and so is easier to use in many situations. Both
of these interfaces access files by DDname, not by data set name. An additional
function,
osdynalloc
, is
provided to interface to the MVS dynamic allocation facility. This function
can be used to allocate a data set to a DDname, or to obtain information about
existing allocations. These functions are characterized as low level because
they use the MVS access method and supervisor services directly.
The direct BSAM interface functions are as follows:
The functions
osfind
,
ostclose
,
and
osstow
from the BSAM
interface can be used with files opened using the record interface. You can
also initialize a DCB using
osdcb
and then process it entirely with the direct BSAM interface. The IBM
publication MVS/DFP Using Data Sets (SC26-4749) contains additional
information about BSAM.
There is only one dynamic allocation interface function,
osdynalloc
. This function offers
a number of different subfunctions, including allocation, deallocation, concatenation,
and retrieval of attributes of data sets. The IBM publication MVS/ESA
Application Development Guide: Authorized Assembler Language Programs
(GC28-1645) provides additional information about dynamic allocation.
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