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NAME

       prctl - operations on a process

SYNOPSIS

       #include <sys/prctl.h>

       int prctl(int option, unsigned long arg2, unsigned long arg3,
                 unsigned long arg4, unsigned long arg5);

DESCRIPTION

       prctl()  is  called  with  a first argument describing what to do (with
       values defined  in  <linux/prctl.h>),  and  further  arguments  with  a
       significance depending on the first one.  The first argument can be:

       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(2) 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, 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  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),  if  any  of  the  following
              attributes  of  the process are changed by the operations listed
              below:

              *  The effective user or group ID is changed.

              *  The  filesystem  user   or   group   ID   is   changed   (see
                 credentials(7)).

              *  The    process's   set   of   permitted   capabilities   (see
                 capabilities(7))  is  changed  such  that  its  new  set   of
                 capabilities   is  not  a  subset  of  its  previous  set  of
                 capabilities.

              The operations that may trigger changes  to  the  dumpable  flag
              include:

              *  execution   (execve(2))  of  a  set-user-ID  or  set-group-ID
                 program,  or   a   program   that   has   capabilities   (see
                 capabilities(7));

              *  capset(2); and

              *  system  calls  that  change  process  credentials  (setuid(2)
                 setgid(2), setresuid(2), setresgid(2), setgroups(2),  and  so
                 on).

              Processes  that  are  not  dumpable  can  not  be  attached  via
              ptrace(2) PTRACE_ATTACH.

       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_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 threads's  permitted  capability  set  is
              cleared  when  a  change  is made to the threads's user IDs such
              that the threads'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
              threads'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.

       PR_SET_TIMERSLACK (since Linux 2.6.28)
              Set  the  current  timer  slack  for  the  calling thread to the
              nanosecond value supplied in arg2.  If  arg2  is  less  than  or
              equal  to  zero,  reset  the current timer slack to the thread's
              default timer slack value.  The  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)).

              Each  thread  has two associated timer slack values: a "default"
              value, and a "current" value.  The current value is the one that
              governs  grouping  of  timer  expirations.  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).

       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) 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.

       PR_GET_UNALIGN
              (see   PR_SET_UNALIGN   for   information   on   versions    and
              architectures)  Return  unaligned  access  control  bits, in the
              location pointed to by (int *) arg2.

RETURN VALUE

       On  success,  PR_GET_DUMPABLE,  PR_GET_KEEPCAPS,   PR_GET_NO_NEW_PRIVS,
       PR_GET_THP_DISABLE,  PR_CAPBSET_READ, PR_GET_TIMING, PR_GET_TIMERSLACK,
       PR_GET_SECUREBITS,                                     PR_MCE_KILL_GET,
       PR_CAP_AMBIENT+PR_CAP_AMBIENT_IS_SET,     and     (if    it    returns)
       PR_GET_SECCOMP return the  nonnegative  values  described  above.   All
       other option values return 0 on success.  On error, -1 is returned, and
       errno is set appropriately.

ERRORS

       EACCES option is PR_SET_MM, and arg3 is PR_SET_MM_EXE_FILE, the file is
              not executable.

       EBADF  option  is  PR_SET_MM,  arg3 is PR_SET_MM_EXE_FILE, and the file
              descriptor passed in arg4 is not valid.

       EBUSY  option is PR_SET_MM, arg3 is PR_SET_MM_EXE_FILE,  and  this  the
              second  attempt to change the /proc/pid/exe symbolic link, which
              is prohibited.

       EFAULT arg2 is an invalid address.

       EFAULT option  is  PR_SET_SECCOMP,  arg2  is  SECCOMP_MODE_FILTER,  the
              system  was  built  with  CONFIG_SECCOMP_FILTER,  and arg3 is an
              invalid address.

       EINVAL The value of option is not recognized.

       EINVAL option is  PR_MCE_KILL  or  PR_MCE_KILL_GET  or  PR_SET_MM,  and
              unused prctl() arguments were not specified as zero.

       EINVAL arg2 is not valid value for this option.

       EINVAL option  is  PR_SET_SECCOMP or PR_GET_SECCOMP, and the kernel was
              not configured with CONFIG_SECCOMP.

       EINVAL option is PR_SET_SECCOMP, arg2 is SECCOMP_MODE_FILTER,  and  the
              kernel was not configured with CONFIG_SECCOMP_FILTER.

       EINVAL option is PR_SET_MM, and one of the following is true

              *  arg4 or arg5 is nonzero;

              *  arg3  is greater than TASK_SIZE (the limit on the size of the
                 user address space for this architecture);

              *  arg2     is     PR_SET_MM_START_CODE,     PR_SET_MM_END_CODE,
                 PR_SET_MM_START_DATA,          PR_SET_MM_END_DATA,         or
                 PR_SET_MM_START_STACK,   and   the   permissions    of    the
                 corresponding memory area are not as required;

              *  arg2  is  PR_SET_MM_START_BRK  or  PR_SET_MM_BRK, and arg3 is
                 less than or  equal  to  the  end  of  the  data  segment  or
                 specifies  a  value that would cause the RLIMIT_DATA resource
                 limit to be exceeded.

       EINVAL option is PR_SET_PTRACER and arg2 is not 0,  PR_SET_PTRACER_ANY,
              or the PID of an existing process.

       EINVAL option  is  PR_SET_PDEATHSIG  and  arg2  is  not  a valid signal
              number.

       EINVAL option is PR_SET_DUMPABLE and arg2 is neither  SUID_DUMP_DISABLE
              nor SUID_DUMP_USER.

       EINVAL option is PR_SET_TIMING and arg2 is not PR_TIMING_STATISTICAL.

       EINVAL option  is  PR_SET_NO_NEW_PRIVS  and  arg2  is not equal to 1 or
              arg3, arg4, or arg5 is nonzero.

       EINVAL option is PR_GET_NO_NEW_PRIVS and arg2, arg3, arg4, or  arg5  is
              nonzero.

       EINVAL option is PR_SET_THP_DISABLE and arg3, arg4, or arg5 is nonzero.

       EINVAL option  is  PR_GET_THP_DISABLE  and arg2, arg3, arg4, or arg5 is
              nonzero.

       EINVAL option is PR_CAP_AMBIENT and an unused argument (arg4, arg5, or,
              in  the  case  of PR_CAP_AMBIENT_CLEAR_ALL, arg3) is nonzero; or
              arg2 has an invalid  value;  or  arg2  is  PR_CAP_AMBIENT_LOWER,
              PR_CAP_AMBIENT_RAISE, or PR_CAP_AMBIENT_IS_SET and arg3 does not
              specify a valid capability.

       ENXIO  option was PR_MPX_ENABLE_MANAGEMENT or PR_MPX_DISABLE_MANAGEMENT
              and  the  kernel  or  the  CPU  does not support MPX management.
              Check that the kernel and processor have MPX support.

       EPERM  option is PR_SET_SECUREBITS, and the caller does  not  have  the
              CAP_SETPCAP  capability,  or  tried to unset a "locked" flag, or
              tried to set a flag whose corresponding locked flag was set (see
              capabilities(7)).

       EPERM  option      is     PR_SET_KEEPCAPS,     and     the     caller's
              SECURE_KEEP_CAPS_LOCKED flag is set (see capabilities(7)).

       EPERM  option is PR_CAPBSET_DROP, and the  caller  does  not  have  the
              CAP_SETPCAP capability.

       EPERM  option   is   PR_SET_MM,  and  the  caller  does  not  have  the
              CAP_SYS_RESOURCE capability.

       EPERM  option is PR_CAP_AMBIENT and arg2 is  PR_CAP_AMBIENT_RAISE,  but
              either  the  capability  specified in arg3 is not present in the
              process's permitted and  inheritable  capability  sets,  or  the
              PR_CAP_AMBIENT_LOWER securebit has been set.

VERSIONS

       The prctl() system call was introduced in Linux 2.1.57.

CONFORMING TO

       This  call  is  Linux-specific.   IRIX  has a prctl() system call (also
       introduced in Linux 2.1.44 as irix_prctl  on  the  MIPS  architecture),
       with prototype

       ptrdiff_t prctl(int option, int arg2, int arg3);

       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.

SEE ALSO

       signal(2), core(5)

COLOPHON

       This page is part of release 4.04 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       http://www.kernel.org/doc/man-pages/.