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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 argu-
ments 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 capability bound-
ing 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 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 fp operations
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 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 capabil-
ity 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, regard-
less of the setting of the "keep capabilities" flag.) The "keep capabilities" value will be reset to 0 on subse-
quent 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 com-
puting 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 CON-
FIG_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 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.

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 task
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 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.

RETURN VALUE
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.

ERRORS
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 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.