MPE/iX Shell and Utilities Reference Manual, Vol 2

Table A-3 shows how POSIX.1 file user classes correspond to MPE/iX ACD user specifications.
POSIX.1 File Classes ACD User Specifications
File owner $OWNER
File group $GROUP and $GROUP_MASK
File other @.@
Table A-3: Mapping Between POSIX.1 and ACD User Classes
If you are accessing files that were not created through
MPE/iX Shell and Utilities or whose security was
modified by MPE/iX commands or system intrinsics, the ACDs may be missing any or all of the four
POSIX.1 ACD user specification entries ($OWNER, $GROUP, $GROUP_MASK, and @.@). ACDs missing
any of these four ACD user specification entries are still POSIX.1-compliant since they can be regarded
as containing default values for the missing ACD entries.
MPE/iX ACD evaluation is consistent with default access values of all for missing $OWNER and
$GROUP_MASK entries and no for missing $GROUP and @.@ entries. If an
ACD contains an @.@ entry
and lacks a $GROUP entry, members of the file group class are mapped to the @.@ entry.
For example, the
ACD
entries R,W,X:$OWNER and R,W,X:JOE.FINANCE are equivalent if the user
ID located in the file label of the file is JOE.FINANCE. Likewise, the ACD entries R,W,X:$GROUP
and R,W,X:@.FINANCE are equivalent if the group
ID (GID) associated with the MPE/iX FINANCE
account is the same as the GID
associated with the file.
$GROUP_MASK
The MPE/iX ACD user specification, $GROUP_MASK, allows the less expressive 9-bit security in MPE/iX
Shell and Utilities to provide greater control over the more expressive security provided by MPE/iX
ACDs. Access modes specified by $GROUP_MASK serve to restrict access permissions specified by
$GROUP.
When the
ACD contains only the four POSIX.1 ACD user specifications (the default condition for all files
and directories created through MPE/iX Shell and Utilities), modifications made to access permissions
associated with the $GROUP entry are also made to the access permissions associated with the
$GROUP_MASK. This ensures that the two entries remain synchronized.
The behavior of $GROUP and $GROUP_MASK is different when an ACD contains user specification
entries beyond $OWNER, $GROUP, $GROUP_MASK, and @.@that are not $OWNER or @.@ are consid-
ered to be members of the POSIX.1 file group class. The $GROUP_MASK entry restricts all members of
the file group class to the access permissions associated with $GROUP_MASK.
Note: When an ACD contains entries other than the four POSIX.1 ACD user specification entries
($OWNER, $GROUP, $GROUP_MASK, and @.@), modifications made to the file group class using the
shell chmod command modify only the ACD $GROUP_MASK user specification (instead of both
$GROUP and $GROUP_MASK). This conforms to rules specified in the current draft of the POSIX.6
standard.
MPE/iX Implementation Considerations A-5