int prctl(int option, unsigned long arg2, unsigned long arg3,
unsigned long arg4, unsigned long arg5);
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).) 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_FPEMU_NOPRINT to silently emulate fp operations accesses, or
PR_FPEMU_SIGFPE to not emulate fp operations and send SIGFPE
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 nonrecoverable exception mode, PR_FP_EXC_ASYNC for
async recoverable exception mode, PR_FP_EXC_PRECISE for precise
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 capabili-
ties" flag prevents it from being cleared. arg2 must be either
0 (permitted capabilities are cleared) or 1 (permitted capabili-
ties 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 capabili-
ties" 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
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_PTRACER (since Ubuntu 10.10)
Sets the top of the process tree that is allowed to use PTRACE
on the calling process, assuming other requirements are met
(matching uid, wasn't setuid, etc). The allowed process id is
specified in arg2 (or 0 to clear). For more details, see
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 com-
puting 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 execute untrusted byte
code, perhaps obtained by reading from a pipe or socket. This
operation is only available if the kernel is configured with
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-
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.
(see PR_SET_UNALIGN for information on versions and architec-
tures) Return unaligned access control bits, in the location
pointed to by (int *) arg2.
PR_MCE_KILL (since Linux 2.6.32)
Set the machine check memory corruption kill policy for the cur-
rent 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_fail-
ure_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 sys-
tem-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 only killed when it accesses a corrupted
page. See sigaction(2) for more information on the SIGBUS sig-
nal. 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.
On success, PR_GET_DUMPABLE, PR_GET_KEEPCAPS, PR_CAPBSET_READ,
PR_GET_TIMING, PR_GET_SECUREBITS, PR_MCE_KILL_GET, 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.
EFAULT arg2 is an invalid address.
EINVAL The value of option is not recognized.
EINVAL option is PR_MCE_KILL or PR_MCE_KILL_GET, 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_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
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
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.
This page is part of release 3.35 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.
Linux 2011-09-17 PRCTL(2)
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