- PR_CAP_AMBIENT (since Linux 4.3)
-
Reads or changes the ambient capability set, according to the value of
arg2,
which must be one of the following:
-
- PR_CAP_AMBIENT_RAISE
-
The capability specified in
arg3
is added to the ambient set.
The specified capability must already be present in
both the permitted and the inheritable sets of the process.
This operation is not permitted if the
SECBIT_NO_CAP_AMBIENT_RAISE
securebit is set.
- PR_CAP_AMBIENT_LOWER
-
The capability specified in
arg3
is removed from the ambient set.
- PR_CAP_AMBIENT_IS_SET
-
The
prctl()
call returns 1 if the capability in
arg3
is in the ambient set and 0 if it is not.
- PR_CAP_AMBIENT_CLEAR_ALL
-
All capabilities will be removed from the ambient set.
This operation requires setting
arg3
to zero.
-
In all of the above operations,
arg4
and
arg5
must be specified as 0.
- PR_CAPBSET_READ (since Linux 2.6.25)
-
Return (as the function result) 1 if the capability specified in
arg2
is in the calling thread's capability bounding set,
or 0 if it is not.
(The capability constants are defined in
<linux/capability.h>.)
The capability bounding set dictates
whether the process can receive the capability through a
file's permitted capability set on a subsequent call to
execve(2).
If the capability specified in
arg2
is not valid, then the call fails with the error
EINVAL.
- PR_CAPBSET_DROP (since Linux 2.6.25)
-
If the calling thread has the
CAP_SETPCAP
capability within its user namespace, then drop the capability specified by
arg2
from the calling thread's capability bounding set.
Any children of the calling thread will inherit the newly
reduced bounding set.
The call fails with the error:
EPERM
if the calling thread does not have the
CAP_SETPCAP;
EINVAL
if
arg2
does not represent a valid capability; or
EINVAL
if file capabilities are not enabled in the kernel,
in which case bounding sets are not supported.
- PR_SET_CHILD_SUBREAPER (since Linux 3.4)
-
If
arg2
is nonzero,
set the "child subreaper" attribute of the calling process;
if
arg2
is zero, unset the attribute.
When a process is marked as a child subreaper,
all of the children that it creates, and their descendants,
will be marked as having a subreaper.
In effect, a subreaper fulfills the role of
init(1)
for its descendant processes.
Upon termination of a process
that is orphaned (i.e., its immediate parent has already terminated)
and marked as having a subreaper,
the nearest still living ancestor subreaper
will receive a
SIGCHLD
signal and will be able to
wait(2)
on the process to discover its termination status.
- PR_GET_CHILD_SUBREAPER (since Linux 3.4)
-
Return the "child subreaper" setting of the caller,
in the location pointed to by
(int *) arg2.
- PR_SET_DUMPABLE (since Linux 2.3.20)
-
Set the state of the "dumpable" flag,
which determines whether core dumps are produced for the calling process
upon delivery of a signal whose default behavior is to produce a core dump.
In kernels up to and including 2.6.12,
arg2
must be either 0
(SUID_DUMP_DISABLE,
process is not dumpable) or 1
(SUID_DUMP_USER,
process is dumpable).
Between kernels 2.6.13 and 2.6.17,
the value 2 was also permitted,
which caused any binary which normally would not be dumped
to be dumped readable by root only;
for security reasons, this feature has been removed.
(See also the description of
/proc/sys/fs/:suid_dumpable
in
proc(5).)
Normally, this flag is set to 1.
However, it is reset to the current value contained in the file
/proc/sys/fs/:suid_dumpable
(which by default has the value 0),
in the following circumstances:
-
- *
-
The process's effective user or group ID is changed.
- *
-
The process's filesystem user or group ID is changed (see
credentials(7)).
- *
-
The process executes
(execve(2))
a set-user-ID or set-group-ID program,
or a program that has capabilities (see
capabilities(7)).
-
Processes that are not dumpable can not be attached via
ptrace(2)
PTRACE_ATTACH;
see
ptrace(2)
for further details.
If a process is not dumpable,
the ownership of files in the process's
/proc/[pid]
directory is affected as described in
proc(5).
- PR_GET_DUMPABLE (since Linux 2.3.20)
-
Return (as the function result) the current state of the calling
process's dumpable flag.
- PR_SET_ENDIAN (since Linux 2.6.18, PowerPC only)
-
Set the endian-ness of the calling process to the value given
in arg2, which should be one of the following:
PR_ENDIAN_BIG,
PR_ENDIAN_LITTLE,
or
PR_ENDIAN_PPC_LITTLE
(PowerPC pseudo little endian).
- PR_GET_ENDIAN (since Linux 2.6.18, PowerPC only)
-
Return the endian-ness of the calling process,
in the location pointed to by
(int *) arg2.
- PR_SET_FP_MODE (since Linux 4.0, only on MIPS)
-
On the MIPS architecture,
user-space code can be built using an ABI which permits linking
with code that has more restrictive floating-point (FP) requirements.
For example, user-space code may be built to target the O32 FPXX ABI
and linked with code built for either one of the more restrictive
FP32 or FP64 ABIs.
When more restrictive code is linked in,
the overall requirement for the process is to use the more
restrictive floating-point mode.
Because the kernel has no means of knowing in advance
which mode the process should be executed in,
and because these restrictions can
change over the lifetime of the process, the
PR_SET_FP_MODE
operation is provided to allow control of the floating-point mode
from user space.
The
(unsigned int) arg2
argument is a bit mask describing the floating-point mode used:
-
- PR_FP_MODE_FR
-
When this bit is
unset
(so called
FR=0 or FR0
mode), the 32 floating-point registers are 32 bits wide,
and 64-bit registers are represented as a pair of registers
(even- and odd- numbered,
with the even-numbered register containing the lower 32 bits,
and the odd-numbered register containing the higher 32 bits).
When this bit is
set
(on supported hardware),
the 32 floating-point registers are 64 bits wide (so called
FR=1 or FR1
mode).
Note that modern MIPS implementations (MIPS R6 and newer) support
FR=1
mode only.
Applications that use the O32 FP32 ABI can operate only when this bit is
unset
(FR=0;
or they can be used with FRE enabled, see below).
Applications that use the O32 FP64 ABI
(and the O32 FP64A ABI, which exists to
provide the ability to operate with existing FP32 code; see below)
can operate only when this bit is
set
(FR=1).
Applications that use the O32 FPXX ABI can operate with either
FR=0
or
FR=1.
- PR_FP_MODE_FRE
-
Enable emulation of 32-bit floating-point mode.
When this mode is enabled,
it emulates 32-bit floating-point operations
by raising a reserved-instruction exception
on every instruction that uses 32-bit formats and
the kernel then handles the instruction in software.
(The problem lies in the discrepancy of handling odd-numbered registers
which are the high 32 bits of 64-bit registers with even numbers in
FR=0
mode and the lower 32-bit parts of odd-numbered 64-bit registers in
FR=1
mode.)
Enabling this bit is necessary when code with the O32 FP32 ABI should operate
with code with compatible the O32 FPXX or O32 FP64A ABIs (which require
FR=1
FPU mode) or when it is executed on newer hardware (MIPS R6 onwards)
which lacks
FR=0
mode support when a binary with the FP32 ABI is used.
-
Note that this mode makes sense only when the FPU is in 64-bit mode
(FR=1).
-
Note that the use of emulation inherently has a significant performance hit
and should be avoided if possible.
-
In the N32/N64 ABI, 64-bit floating-point mode is always used,
so FPU emulation is not required and the FPU always operates in
FR=1
mode.
-
This option is mainly intended for use by the dynamic linker
(ld.so(8)).
-
The arguments
arg3,
arg4,
and
arg5
are ignored.
- PR_GET_FP_MODE (since Linux 4.0, only on MIPS)
-
Get the current floating-point mode (see the description of
PR_SET_FP_MODE
for details).
On success,
the call returns a bit mask which represents the current floating-point mode.
The arguments
arg2,
arg3,
arg4,
and
arg5
are ignored.
- PR_SET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
-
Set floating-point emulation control bits to arg2.
Pass
PR_FPEMU_NOPRINT
to silently emulate floating-point operation accesses, or
PR_FPEMU_SIGFPE
to not emulate floating-point operations and send
SIGFPE
instead.
- PR_GET_FPEMU (since Linux 2.4.18, 2.5.9, only on ia64)
-
Return floating-point emulation control bits,
in the location pointed to by
(int *) arg2.
- PR_SET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
-
Set floating-point exception mode to arg2.
Pass PR_FP_EXC_SW_ENABLE to use FPEXC for FP exception enables,
PR_FP_EXC_DIV for floating-point divide by zero,
PR_FP_EXC_OVF for floating-point overflow,
PR_FP_EXC_UND for floating-point underflow,
PR_FP_EXC_RES for floating-point inexact result,
PR_FP_EXC_INV for floating-point invalid operation,
PR_FP_EXC_DISABLED for FP exceptions disabled,
PR_FP_EXC_NONRECOV for async nonrecoverable exception mode,
PR_FP_EXC_ASYNC for async recoverable exception mode,
PR_FP_EXC_PRECISE for precise exception mode.
- PR_GET_FPEXC (since Linux 2.4.21, 2.5.32, only on PowerPC)
-
Return floating-point exception mode,
in the location pointed to by
(int *) arg2.
- PR_SET_KEEPCAPS (since Linux 2.2.18)
-
Set the state of the thread's "keep capabilities" flag,
which determines whether the thread's permitted
capability set is cleared when a change is made to the thread's user IDs
such that the thread's real UID, effective UID, and saved set-user-ID
all become nonzero when at least one of them previously had the value 0.
By default, the permitted capability set is cleared when such a change is made;
setting the "keep capabilities" flag prevents it from being cleared.
arg2
must be either 0 (permitted capabilities are cleared)
or 1 (permitted capabilities are kept).
(A thread's
effective
capability set is always cleared when such a credential change is made,
regardless of the setting of the "keep capabilities" flag.)
The "keep capabilities" value will be reset to 0 on subsequent calls to
execve(2).
- PR_GET_KEEPCAPS (since Linux 2.2.18)
-
Return (as the function result) the current state of the calling thread's
"keep capabilities" flag.
- PR_MCE_KILL (since Linux 2.6.32)
-
Set the machine check memory corruption kill policy for the current thread.
If
arg2
is
PR_MCE_KILL_CLEAR,
clear the thread memory corruption kill policy and use the system-wide default.
(The system-wide default is defined by
/proc/sys/vm/memory_failure_early_kill;
see
proc(5).)
If
arg2
is
PR_MCE_KILL_SET,
use a thread-specific memory corruption kill policy.
In this case,
arg3
defines whether the policy is
early kill
(PR_MCE_KILL_EARLY),
late kill
(PR_MCE_KILL_LATE),
or the system-wide default
(PR_MCE_KILL_DEFAULT).
Early kill means that the thread receives a
SIGBUS
signal as soon as hardware memory corruption is detected inside
its address space.
In late kill mode, the process is killed only when it accesses a corrupted page.
See
sigaction(2)
for more information on the
SIGBUS
signal.
The policy is inherited by children.
The remaining unused
prctl()
arguments must be zero for future compatibility.
- PR_MCE_KILL_GET (since Linux 2.6.32)
-
Return the current per-process machine check kill policy.
All unused
prctl()
arguments must be zero.
- PR_SET_MM (since Linux 3.3)
-
Modify certain kernel memory map descriptor fields
of the calling process.
Usually these fields are set by the kernel and dynamic loader (see
ld.so(8)
for more information) and a regular application should not use this feature.
However, there are cases, such as self-modifying programs,
where a program might find it useful to change its own memory map.
This feature is available only if the kernel is built with the
CONFIG_CHECKPOINT_RESTORE
option enabled.
The calling process must have the
CAP_SYS_RESOURCE
capability.
The value in
arg2
is one of the options below, while
arg3
provides a new value for the option.
-
- PR_SET_MM_START_CODE
-
Set the address above which the program text can run.
The corresponding memory area must be readable and executable,
but not writable or sharable (see
mprotect(2)
and
mmap(2)
for more information).
- PR_SET_MM_END_CODE
-
Set the address below which the program text can run.
The corresponding memory area must be readable and executable,
but not writable or sharable.
- PR_SET_MM_START_DATA
-
Set the address above which initialized and
uninitialized (bss) data are placed.
The corresponding memory area must be readable and writable,
but not executable or sharable.
- PR_SET_MM_END_DATA
-
Set the address below which initialized and
uninitialized (bss) data are placed.
The corresponding memory area must be readable and writable,
but not executable or sharable.
- PR_SET_MM_START_STACK
-
Set the start address of the stack.
The corresponding memory area must be readable and writable.
- PR_SET_MM_START_BRK
-
Set the address above which the program heap can be expanded with
brk(2)
call.
The address must be greater than the ending address of
the current program data segment.
In addition, the combined size of the resulting heap and
the size of the data segment can't exceed the
RLIMIT_DATA
resource limit (see
setrlimit(2)).
- PR_SET_MM_BRK
-
Set the current
brk(2)
value.
The requirements for the address are the same as for the
PR_SET_MM_START_BRK
option.
The following options are available since Linux 3.5.
- PR_SET_MM_ARG_START
-
Set the address above which the program command line is placed.
- PR_SET_MM_ARG_END
-
Set the address below which the program command line is placed.
- PR_SET_MM_ENV_START
-
Set the address above which the program environment is placed.
- PR_SET_MM_ENV_END
-
Set the address below which the program environment is placed.
-
The address passed with
PR_SET_MM_ARG_START,
PR_SET_MM_ARG_END,
PR_SET_MM_ENV_START,
and
PR_SET_MM_ENV_END
should belong to a process stack area.
Thus, the corresponding memory area must be readable, writable, and
(depending on the kernel configuration) have the
MAP_GROWSDOWN
attribute set (see
mmap(2)).
- PR_SET_MM_AUXV
-
Set a new auxiliary vector.
The
arg3
argument should provide the address of the vector.
The
arg4
is the size of the vector.
- PR_SET_MM_EXE_FILE
-
Supersede the
/proc/pid/exe
symbolic link with a new one pointing to a new executable file
identified by the file descriptor provided in
arg3
argument.
The file descriptor should be obtained with a regular
open(2)
call.
-
To change the symbolic link, one needs to unmap all existing
executable memory areas, including those created by the kernel itself
(for example the kernel usually creates at least one executable
memory area for the ELF
.text
section).
-
The second limitation is that such transitions can be done only once
in a process life time.
Any further attempts will be rejected.
This should help system administrators monitor unusual
symbolic-link transitions over all processes running on a system.
- PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT (since Linux 3.19)
-
Enable or disable kernel management of Memory Protection eXtensions (MPX)
bounds tables.
The
arg2,
arg3,
arg4,
and
arg5
arguments must be zero.
MPX is a hardware-assisted mechanism for performing bounds checking on
pointers.
It consists of a set of registers storing bounds information
and a set of special instruction prefixes that tell the CPU on which
instructions it should do bounds enforcement.
There is a limited number of these registers and
when there are more pointers than registers,
their contents must be "spilled" into a set of tables.
These tables are called "bounds tables" and the MPX
prctl()
operations control
whether the kernel manages their allocation and freeing.
When management is enabled, the kernel will take over allocation
and freeing of the bounds tables.
It does this by trapping the #BR exceptions that result
at first use of missing bounds tables and
instead of delivering the exception to user space,
it allocates the table and populates the bounds directory
with the location of the new table.
For freeing, the kernel checks to see if bounds tables are
present for memory which is not allocated, and frees them if so.
Before enabling MPX management using
PR_MPX_ENABLE_MANAGEMENT,
the application must first have allocated a user-space buffer for
the bounds directory and placed the location of that directory in the
bndcfgu
register.
These calls will fail if the CPU or kernel does not support MPX.
Kernel support for MPX is enabled via the
CONFIG_X86_INTEL_MPX
configuration option.
You can check whether the CPU supports MPX by looking for the 'mpx'
CPUID bit, like with the following command:
cat /proc/cpuinfo | grep ' mpx '
A thread may not switch in or out of long (64-bit) mode while MPX is
enabled.
All threads in a process are affected by these calls.
The child of a
fork(2)
inherits the state of MPX management.
During
execve(2),
MPX management is reset to a state as if
PR_MPX_DISABLE_MANAGEMENT
had been called.
For further information on Intel MPX, see the kernel source file
Documentation/x86/intel_mpx.txt.
- PR_SET_NAME (since Linux 2.6.9)
-
Set the name of the calling thread,
using the value in the location pointed to by
(char *) arg2.
The name can be up to 16 bytes long,
including the terminating null byte.
(If the length of the string, including the terminating null byte,
exceeds 16 bytes, the string is silently truncated.)
This is the same attribute that can be set via
pthread_setname_np(3)
and retrieved using
pthread_getname_np(3).
The attribute is likewise accessible via
/proc/self/task/[tid]/comm,
where
tid
is the name of the calling thread.
- PR_GET_NAME (since Linux 2.6.11)
-
Return the name of the calling thread,
in the buffer pointed to by
(char *) arg2.
The buffer should allow space for up to 16 bytes;
the returned string will be null-terminated.
- PR_SET_NO_NEW_PRIVS (since Linux 3.5)
-
Set the calling process's
no_new_privs
bit to the value in
arg2.
With
no_new_privs
set to 1,
execve(2)
promises not to grant privileges to do anything
that could not have been done without the
execve(2)
call (for example,
rendering the set-user-ID and set-group-ID mode bits,
and file capabilities non-functional).
Once set, this bit cannot be unset.
The setting of this bit is inherited by children created by
fork(2)
and
clone(2),
and preserved across
execve(2).
For more information, see the kernel source file
Documentation/prctl/no_new_privs.txt.
- PR_GET_NO_NEW_PRIVS (since Linux 3.5)
-
Return (as the function result) the value of the
no_new_privs
bit for the current process.
A value of 0 indicates the regular
execve(2)
behavior.
A value of 1 indicates
execve(2)
will operate in the privilege-restricting mode described above.
- PR_SET_PDEATHSIG (since Linux 2.1.57)
-
Set the parent death signal
of the calling process to arg2 (either a signal value
in the range 1..maxsig, or 0 to clear).
This is the signal that the calling process will get when its
parent dies.
This value is cleared for the child of a
fork(2)
and (since Linux 2.4.36 / 2.6.23)
when executing a set-user-ID or set-group-ID binary,
or a binary that has associated capabilities (see
capabilities(7)).
This value is preserved across
execve(2).
Warning:
the "parent" in this case is considered to be the
thread
that created this process.
In other words, the signal will be sent when that thread terminates
(via, for example,
pthread_exit(3)),
rather than after all of the threads in the parent process terminate.
- PR_GET_PDEATHSIG (since Linux 2.3.15)
-
Return the current value of the parent process death signal,
in the location pointed to by
(int *) arg2.
- PR_SET_PTRACER (since Linux 3.4)
-
This is meaningful only when the Yama LSM is enabled and in mode 1
("restricted ptrace", visible via
/proc/sys/kernel/yama/ptrace_scope).
When a "ptracer process ID" is passed in arg2,
the caller is declaring that the ptracer process can
ptrace(2)
the calling process as if it were a direct process ancestor.
Each
PR_SET_PTRACER
operation replaces the previous "ptracer process ID".
Employing
PR_SET_PTRACER
with
arg2
set to 0 clears the caller's "ptracer process ID".
If
arg2
is
PR_SET_PTRACER_ANY,
the ptrace restrictions introduced by Yama are effectively disabled for the
calling process.
For further information, see the kernel source file
Documentation/security/Yama.txt.
- PR_SET_SECCOMP (since Linux 2.6.23)
-
Set the secure computing (seccomp) mode for the calling thread, to limit
the available system calls.
The more recent
seccomp(2)
system call provides a superset of the functionality of
PR_SET_SECCOMP.
The seccomp mode is selected via
arg2.
(The seccomp constants are defined in
<linux/seccomp.h>.)
With
arg2
set to
SECCOMP_MODE_STRICT,
the only system calls that the thread is permitted to make are
read(2),
write(2),
_exit(2)
(but not
exit_group(2)),
and
sigreturn(2).
Other system calls result in the delivery of a
SIGKILL
signal.
Strict secure computing mode is useful for number-crunching applications
that may need to execute untrusted byte code,
perhaps obtained by reading from a pipe or socket.
This operation is available only
if the kernel is configured with
CONFIG_SECCOMP
enabled.
With
arg2
set to
SECCOMP_MODE_FILTER (since Linux 3.5),
the system calls allowed are defined by a pointer
to a Berkeley Packet Filter passed in
arg3.
This argument is a pointer to
struct sock_fprog;
it can be designed to filter
arbitrary system calls and system call arguments.
This mode is available only if the kernel is configured with
CONFIG_SECCOMP_FILTER
enabled.
If
SECCOMP_MODE_FILTER
filters permit
fork(2),
then the seccomp mode is inherited by children created by
fork(2);
if
execve(2)
is permitted, then the seccomp mode is preserved across
execve(2).
If the filters permit
prctl()
calls, then additional filters can be added;
they are run in order until the first non-allow result is seen.
For further information, see the kernel source file
Documentation/prctl/seccomp_filter.txt.
- PR_GET_SECCOMP (since Linux 2.6.23)
-
Return (as the function result)
the secure computing mode of the calling thread.
If the caller is not in secure computing mode, this operation returns 0;
if the caller is in strict secure computing mode, then the
prctl()
call will cause a
SIGKILL
signal to be sent to the process.
If the caller is in filter mode, and this system call is allowed by the
seccomp filters, it returns 2; otherwise, the process is killed with a
SIGKILL
signal.
This operation is available only
if the kernel is configured with
CONFIG_SECCOMP
enabled.
Since Linux 3.8, the
Seccomp
field of the
/proc/[pid]/status
file provides a method of obtaining the same information,
without the risk that the process is killed; see
proc(5).
- PR_SET_SECUREBITS (since Linux 2.6.26)
-
Set the "securebits" flags of the calling thread to the value supplied in
arg2.
See
capabilities(7).
- PR_GET_SECUREBITS (since Linux 2.6.26)
-
Return (as the function result)
the "securebits" flags of the calling thread.
See
capabilities(7).
- PR_SET_THP_DISABLE (since Linux 3.15)
-
Set the state of the "THP disable" flag for the calling thread.
If
arg2
has a nonzero value, the flag is set, otherwise it is cleared.
Setting this flag provides a method
for disabling transparent huge pages
for jobs where the code cannot be modified, and using a malloc hook with
madvise(2)
is not an option (i.e., statically allocated data).
The setting of the "THP disable" flag is inherited by a child created via
fork(2)
and is preserved across
execve(2).
- PR_TASK_PERF_EVENTS_DISABLE (since Linux 2.6.31)
-
Disable all performance counters attached to the calling process,
regardless of whether the counters were created by
this process or another process.
Performance counters created by the calling process for other
processes are unaffected.
For more information on performance counters, see the Linux kernel source file
tools/perf/design.txt.
-
Originally called
PR_TASK_PERF_COUNTERS_DISABLE;
renamed (with same numerical value)
in Linux 2.6.32.
- PR_TASK_PERF_EVENTS_ENABLE (since Linux 2.6.31)
-
The converse of
PR_TASK_PERF_EVENTS_DISABLE;
enable performance counters attached to the calling process.
-
Originally called
PR_TASK_PERF_COUNTERS_ENABLE;
renamed
in Linux 2.6.32.
- PR_GET_THP_DISABLE (since Linux 3.15)
-
Return (via the function result) the current setting of the "THP disable"
flag for the calling thread:
either 1, if the flag is set, or 0, if it is not.
- PR_GET_TID_ADDRESS (since Linux 3.5)
-
Retrieve the
clear_child_tid
address set by
set_tid_address(2)
and the
clone(2)
CLONE_CHILD_CLEARTID
flag, in the location pointed to by
(int **) arg2.
This feature is available only if the kernel is built with the
CONFIG_CHECKPOINT_RESTORE
option enabled.
Note that since the
prctl()
system call does not have a compat implementation for
the AMD64 x32 and MIPS n32 ABIs,
and the kernel writes out a pointer using the kernel's pointer size,
this operation expects a user-space buffer of 8 (not 4) bytes on these ABIs.
- PR_SET_TIMERSLACK (since Linux 2.6.28)
-
Each thread has two associated timer slack values:
a "default" value, and a "current" value.
This operation sets the "current" timer slack value for the calling thread.
If the nanosecond value supplied in
arg2
is greater than zero, then the "current" value is set to this value.
If
arg2
is less than or equal to zero,
the "current" timer slack is reset to the
thread's "default" timer slack value.
The "current" timer slack is used by the kernel to group timer expirations
for the calling thread that are close to one another;
as a consequence, timer expirations for the thread may be
up to the specified number of nanoseconds late (but will never expire early).
Grouping timer expirations can help reduce system power consumption
by minimizing CPU wake-ups.
The timer expirations affected by timer slack are those set by
select(2),
pselect(2),
poll(2),
ppoll(2),
epoll_wait(2),
epoll_pwait(2),
clock_nanosleep(2),
nanosleep(2),
and
futex(2)
(and thus the library functions implemented via futexes, including
pthread_cond_timedwait(3),
pthread_mutex_timedlock(3),
pthread_rwlock_timedrdlock(3),
pthread_rwlock_timedwrlock(3),
and
sem_timedwait(3)).
Timer slack is not applied to threads that are scheduled under
a real-time scheduling policy (see
sched_setscheduler(2)).
When a new thread is created,
the two timer slack values are made the same as the "current" value
of the creating thread.
Thereafter, a thread can adjust its "current" timer slack value via
PR_SET_TIMERSLACK.
The "default" value can't be changed.
The timer slack values of
init
(PID 1), the ancestor of all processes,
are 50,000 nanoseconds (50 microseconds).
The timer slack values are preserved across
execve(2).
Since Linux 4.6, the "current" timer slack value of any process
can be examined and changed via the file
/proc/[pid]/timerslack_ns.
See
proc(5).
- PR_GET_TIMERSLACK (since Linux 2.6.28)
-
Return (as the function result)
the "current" timer slack value of the calling thread.
- PR_SET_TIMING (since Linux 2.6.0-test4)
-
Set whether to use (normal, traditional) statistical process timing or
accurate timestamp-based process timing, by passing
PR_TIMING_STATISTICAL
or
PR_TIMING_TIMESTAMP
to arg2.
PR_TIMING_TIMESTAMP
is not currently implemented
(attempting to set this mode will yield the error
EINVAL).
- PR_GET_TIMING (since Linux 2.6.0-test4)
-
Return (as the function result) which process timing method is currently
in use.
- PR_SET_TSC (since Linux 2.6.26, x86 only)
-
Set the state of the flag determining whether the timestamp counter
can be read by the process.
Pass
PR_TSC_ENABLE
to
arg2
to allow it to be read, or
PR_TSC_SIGSEGV
to generate a
SIGSEGV
when the process tries to read the timestamp counter.
- PR_GET_TSC (since Linux 2.6.26, x86 only)
-
Return the state of the flag determining whether the timestamp counter
can be read,
in the location pointed to by
(int *) arg2.
- PR_SET_UNALIGN
-
(Only on: ia64, since Linux 2.3.48; parisc, since Linux 2.6.15;
PowerPC, since Linux 2.6.18; Alpha, since Linux 2.6.22;
sh, since Linux 2.6.34; tile, since Linux 3.12)
Set unaligned access control bits to arg2.
Pass
PR_UNALIGN_NOPRINT to silently fix up unaligned user accesses,
or PR_UNALIGN_SIGBUS to generate
SIGBUS
on unaligned user access.
Alpha also supports an additional flag with the value
of 4 and no corresponding named constant,
which instructs kernel to not fix up
unaligned accesses (it is analogous to providing the
UAC_NOFIX
flag in
SSI_NVPAIRS
operation of the
setsysinfo()
system call on Tru64).
- PR_GET_UNALIGN
-
(see
PR_SET_UNALIGN
for information on versions and architectures)
Return unaligned access control bits, in the location pointed to by
(unsigned int *) arg2.
and options to get the maximum number of processes per user,
get the maximum number of processors the calling process can use,
find out whether a specified process is currently blocked,
get or set the maximum stack size, and so on.