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prctl(1)

Process operations

prctl

A utility to perform process operations

NAME

prctl - Process operations

SYNOPSIS

prctl [-v] [-h|--help] [--version] <-q|<options....>> [ command ]
where <options> are:
--unaligned=[silent|signal|always-signal|default]
--fpemu=[silent|signal|default]
--mcekill=[early|late|default]

DESCRIPTION

prctl allows you to query or control certain process behavior. Supported options are:
Unaligned memory access: When a process performs an unaligned memory access, by default the kernel would emulate the unaligned access correctly and log the unaligned access in syslog. This behavior can be changed so the kernel could either emulate the unaligned access correctly without logging an error ("silent") or send SIGBUS to the process ("signal" and "always-signal"). "always-signal" is available on ia64 only.
Floating point assistance faults: when a process encounters a floating point assist fault, kernel would invoke floating point emulator and log the floating point assist fault. This behavior can be changed so the kernel could either emulate floating point operation without logging an error ("silent") or send SIGFPE to the offending process ("signal").
Machine check memory corruption kill policy: If a hardware memeory corruption is detected inside a thread’s address space, mmemory corruption kill policy determines whether the thread received SIGBUS as soon as corruption is detected ("early"), when it accesses corrupted memory ("late"), or use system wide default.
prctl can optionally be followed by a command. If a command is specified, prctl will exec the command with the processor behavior set to as specified by the options. If a command is not specified, prctl will fork a new shell unless the command only queried the current settings. The shell started by prctl will be as defined by the environment variable SHELL. If environment variable SHELL is not defined, shell in the password entry for the user will be started. If a shell is not found in the password entry, bash will be started.

OPTIONS

-v Verbose mode. In verbose mode, any new settings are confirmed with a message on stdout.
--help Print usage information and exit.
--version
Print software version and exit.
-q Query the current settings for the process options controllable by prctl.
--unaligned=[silent|signal|always-signal|default]]
Set unaligned memory access behavior to not log the access (silent), send SIGBUS to the process (signal), or do the default (default). On IA-64 architecture an additional value always-signal is supported which causes a SIGBUS to be generated every time an unaligned access occurs. Refer to the section titled "Memory Datum Alignment and Atomicity" in "Intel IA-64 Architecture Software Developer’s Manual: Volume 2" for details on when an unaligned access would not generate signal normally. If a value is not specified after "=", current setting is returned.
--fpemu=[silent|signal|default]]
Set floating point assist fault behavior to not log the faulti (silent), send SIGFPE to the process (signal), or do the default (default). If a value is not specified after "=", current setting is returned.
If an option is specified multiple times, the last one takes effect.

EXAMPLES

prctl --unaligned=signal
starts up a shell (as defined by the environment variable SHELL) and sets up any process running under this shell to be sent SIGBUS upon an unaligned memory access.
prctl --unaligned=signal gdb tst
starts up a gdb session for the program "tst" with the process set to receive SIGBUS upon unligned memory access.
prctl --unaligned=
displays the current setting for unaligned memory access handling.
prctl --unaligned= gdb tst
displays the current setting for unaligned memory access handling and starts up a gdb session for the program "tst".

NOTES

prctl works on 2.4.0 and above kernels only.
--fpemu= option is specific to IPF (Itanium Processor Family, previously known as IA-64) and is supported on kernels 2.4.11 and above. Other architectures and kernels may return "Invalid argument" error.

SEE ALSO

AUTHOR

Khalid Aziz <>

LICENSE

This software is made available under the GNU General Public License (GPL) Version 2. This software comes with NO WARRANTY.
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