uac(1)uac(1)NAMEuac - Unaligned Access Message Control
SYNOPSISuac p | s [value]
uac p | s [keyword]
OPTIONS
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 system
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 process
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 system
Usually, the parent process is the shell.
DESCRIPTION
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 data.)
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 used with.
RESTRICTIONS
You must be superuser to set the system option.
EXAMPLES
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 deliv‐
ery of a SIGBUS signal to the parent process: # uac p nofix
noprint sigbus
SEE ALSOgetsysinfo(2), setsysinfo(2)
Programmer's Guide
uac(1)