Reference Guide

ZCOM Tables and Data Structures
Differences in 32-bit and 64-bit Data Structures
Chapter 340
Differences in 32-bit and 64-bit Data
Structures
HP-UX supports 64-bit mode using the LP64 model. This means the long
and pointer fields are 64-bits in length. The ZCOM memory data
structures are designed to be 32/64-bit neutral by using conditional
padding fields. For example, a pointer field is defined as a 64-bit pointer
in 64-bit mode. But when the same field is used in the 32-bit kernel, it is
a 32-bit pointer prefixed with a 32-bit padding field (the padding is
inserted in front of the field). The data structures hence are the same
sizes in both 32 and 64-bit modes. This gives better control over data
alignment in the data structures, and allows 32-bit utilities to
manipulate both the 32and 64-bit data structures easily.
But this still does not give full transparency to 32-bit programs that
access ZCOM kernel tables on a 64-bit system. When a 32-bit program
manipulates a 64-bit long or pointer field in a data structure, the
program should consider whether the data structure comes from a 32-bit
or 64-bit kernel, even though it has the same size and layout. The
program should treat the padding fields as hidden fields (same as the
data structure alignment gaps inserted by the compiler) and should not
reference them directly.
Another side-effect is the ‘shifting’ of the long or pointer fields. Although
the overall layout is the same, the location offset of a long or pointer field
within a data structure is different in 32-bit and 64-bit modes. When
used in 32-bit mode, due to the presence of the 32-bit padding, the
position of a long or pointer field will be shifted 4-bytes towards the end,
compared to the same field when used in 64-bit mode.
Some of the data fields are of fixed 64-bit (e.g. hssoff in zheader_type) in
both 32/64 modes. If a 32-bit program is to manipulate these 64-bit fields,
it is recommended that the program should be compiled with the option
-Ae (extended ANSI) or -Ac (K&R C mode), so as to enable the ‘long long’
type for 64-bit integer.
The following sections describe the ZCOM data structures in 64-bit
mode. Hence the conditional padding fields are suppressed. If they are
used in a 32-bit environment, the long and pointer fields will become
32-bit, and the 32-bit padding will be present.