uac - Unaligned Access Message Control
uac p | s [value]
uac p | s [keyword]
The following options are used with the uac command: Sets
or displays the current option setting for the parent process
Sets or displays the current option setting for the
The following keywords are used with the p option: Reset
the current option settings to the default for the parent
process Do not print the unaligned access warning message
for the parent process Do not fix the unaligned access for
the parent process Deliver a SIGBUS signal to the parent
The following keywords are used with the s option: Reset
the current option settings to the default for the system
Do not print the unaligned access warning message for the
Usually, the parent process is the shell.
Most complex instruction set computers (CISC) can access
unaligned data but only at a significantly reduced speed.
Most reduced instruction set computers (RISC) do not
attempt to handle unaligned accesses. Instead, they generate
an unaligned access trap and let the operating system
handle the fault.
The default action for the operating system when an
unaligned access fault occurs is to fix the unaligned
access fault and then display a warning message informing
the user process that an unaligned access fault has
occurred. The warning message has the following form:
"Unaligned access pid=nnn <prog_name> va=virtual_addr
pc=pc_addr". If you want to change the code to prevent the
unaligned access faults from occurring in the future, the
warning message gives you the information you need to
locate the code causing a fault. (See the Programmer's
Guide for details on the causes and effects of misaligned
In addition, the default action for the operating system
is to not deliver a SIGBUS signal to the parent process
when an unaligned access fault occurs.
These defaults are satisfactory for most users, but some
users require a different behavior and want to specify
their own Unaligned Access Control (UAC).
The uac command enables or disables the display of
"Unaligned access" messages. The command sets or displays
the UAC_NOPRINT, UAC_NOFIX, and UAC_SIGBUS options, as
defined in setsysinfo(2).
If value is not specified, the current option setting is
displayed. You can specify value as either 0 (zero) or 1
(one). If value is 0 (zero), the option is turned on and
messages are not displayed. If value is 1 (one), the
option is turned off and messages are displayed.
Keywords can be used singly, or in any combination. However,
the reset keyword overrides any other keywords it is
You must be superuser to set the system option.
Both of the following commands disable messages for the
parent process: # uac p 0
# uac p noprint Both of the following commands
enable messages for the system: # uac s 1
# uac s reset The following command disables fixing
of an unaligned access fault, disables printing a
message for an unaligned access fault to the parent
process, and enables delivery of a SIGBUS signal to
the parent process: # uac p nofix noprint sigbus
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