prctl


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  sig-
       nificance depending on the first one.  The first argument can be:

       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 capa-
              bility  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_DUMPABLE (since Linux 2.3.20)
              Set  the  state  of  the flag determining whether core dumps are
              produced for this  process  upon  delivery  of  a  signal  whose
              default behavior is to produce a core dump.  (Normally this flag
              is set for a process by default, but it is cleared when  a  set-
              user-ID  or set-group-ID program is executed and also by various
              system calls that manipulate process UIDs and GIDs).  In kernels
              up  to  and  including 2.6.12, arg2 must be either 0 (process is
              not dumpable) or  1  (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 read-
              able  by  root only; for security reasons, this feature has been
              removed.       (See      also      the      description       of
              /proc/sys/fs/suid_dumpable 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)
              PR_FPEMU_SIGFPE to not emulate fp  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_NONRE-
              COV  for  async  non-recoverable exception mode, PR_FP_EXC_ASYNC
              for async recoverable exception mode, PR_FP_EXC_PRECISE for pre-
              cise 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 effective and permitted capa-
              bility sets are 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 non-zero when at least one of  them
              previously  had the value 0.  (By default, these credential sets
              are cleared).  arg2 must be either 0 (capabilities are  cleared)
              or  1 (capabilities are kept).  This 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_SET_NAME (since Linux 2.6.9)
              Set the process name for the calling process, using the value in
              the location pointed to by (char *) arg2.  The name can be up to
              16  bytes  long,  and  should  be null terminated if it contains
              fewer bytes.

       PR_GET_NAME (since Linux 2.6.11)
              Return the process name for the calling process, 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  if
              it is shorter than that.

       PR_SET_PDEATHSIG (since Linux 2.1.57)
              Set  the  parent  process 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
              sigreturn(2).  Other system calls result in the  delivery  of  a
              SIGKILL  signal.   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 only available if the  kernel  is  configured  with
              CONFIG_SECCOMP enabled.

       PR_GET_SECCOMP (since Linux 2.6.23)
              Return  the  secure  computing  mode of the calling thread.  Not
              very useful for the current implementation (mode equals 1),  but
              may  be useful for other possible future modes: if the caller is
              not in secure computing mode, this operation returns 0;  if  the
              caller  is  in secure computing mode, then the prctl() call will
              cause a SIGKILL signal to be sent to the process.   This  opera-
              tion  is  only  available  if the kernel is configured with CON-
              FIG_SECCOMP enabled.

       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_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_TIM-
              ING_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
       EFAULT arg2 is an invalid address.

       EINVAL The value of option is not recognized.

       EINVAL arg2 is not valid value for this option.

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

       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     callers'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.

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 maxi-
       mum stack size, etc.

SEE ALSO
       signal(2), core(5)

COLOPHON
       This page is part of release 3.23 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-07-16                          PRCTL(2)
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