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PRCTL(2)                   Linux Programmer's Manual                  PRCTL(2)



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_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  capa-
              bility 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_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
              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).)

       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  fol-
              lowing: 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 fp opera-
              tions accesses, or 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_NONRECOV for async non-recoverable  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 effective
              and permitted capability 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 when its  parent  dies.
              This value is cleared for the child of a fork(2).

       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_SECCOMP (since Linux 2.6.23)
              Set the secure computing mode for the calling thread.  In the current implementation, arg2  must  be  1.
              After the secure computing mode has been set to 1, the only system calls that the thread is permitted to
              make are read(2), write(2), _exit(2), and 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 exe-
              cute 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  com-
              puting  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 operation is only available if  the  kernel
              is configured with CONFIG_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_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_CAPBSET_READ, PR_GET_TIMING, PR_GET_SECUREBITS, and (if it
       returns) PR_GET_SECCOMP return the non-negative values described above.  All other option values  return  0  on
       success.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       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 maximum stack size,
       etc.

SEE ALSO
       signal(2), core(5)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-07-16                          PRCTL(2)