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_CAP_AMBIENT (since Linux 4.3)
    Reads or changes the ambient capability set of the calling thread, 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() 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 within its user namespace, 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.

    A subreaper fulfills the role of init(1) for its descendant processes. When a process becomes orphaned (i.e., its immediate parent terminates) then that process will be reparented to the nearest still living ancestor subreaper. Subsequently, calls to getppid() in the orphaned process will now return the PID of the subreaper process, and when the orphan terminates, it is the subreaper process that will receive a SIGCHLD signal and will be able to wait(2) on the process to discover its termination status.

    The setting of this bit is not inherited by children created by fork(2) and clone(2). The setting is preserved across execve(2).

    Establishing a subreaper process is useful in session management frameworks where a hierarchical group of processes is managed by a subreaper process that needs to be informed when one of the processes—for example, a double-forked daemon—terminates (perhaps so that it can restart that process). Some init(1) frameworks (e.g., systemd(1)) employ a subreaper process for similar reasons.

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), in the following circumstances:

    *   The process's effective user or group ID is changed.

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

    *   The process executes (execve(2)) a set-user-ID or set-group-ID program, resulting in a change of either the effective user ID or the effective group ID.

    *   The process executes (execve(2)) a program that has file capabilities (see capabilities(7)), but only if the permitted capabilities gained exceed those already permitted for the process.

    Processes that are not dumpable can not be attached via ptrace(2) PTRACE_ATTACH; see ptrace(2) for further details.

    If a process is not dumpable, the ownership of files in the process's /proc/[pid] directory is affected as described 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_FP_MODE (since Linux 4.0, only on MIPS)
    On the MIPS architecture, user-space code can be built using an ABI which permits linking with code that has more restrictive floating-point (FP) requirements. For example, user-space code may be built to target the O32 FPXX ABI and linked with code built for either one of the more restrictive FP32 or FP64 ABIs. When more restrictive code is linked in, the overall requirement for the process is to use the more restrictive floating-point mode.

    Because the kernel has no means of knowing in advance which mode the process should be executed in, and because these restrictions can change over the lifetime of the process, the PR_SET_FP_MODE operation is provided to allow control of the floating-point mode from user space.

    The (unsigned int) arg2 argument is a bit mask describing the floating-point mode used:

    PR_FP_MODE_FR
        When this bit is unset (so called FR=0 or FR0 mode), the 32 floating-point registers are 32 bits wide, and 64-bit registers are represented as a pair of registers (even- and odd- numbered, with the even-numbered register containing the lower 32 bits, and the odd-numbered register containing the higher 32 bits).

        When this bit is set (on supported hardware), the 32 floating-point registers are 64 bits wide (so called FR=1 or FR1 mode). Note that modern MIPS implementations (MIPS R6 and newer) support FR=1 mode only.

        Applications that use the O32 FP32 ABI can operate only when this bit is unset (FR=0; or they can be used with FRE enabled, see below). Applications that use the O32 FP64 ABI (and the O32 FP64A ABI, which exists to provide the ability to operate with existing FP32 code; see below) can operate only when this bit is set (FR=1). Applications that use the O32 FPXX ABI can operate with either FR=0 or FR=1.

    PR_FP_MODE_FRE
        Enable emulation of 32-bit floating-point mode. When this mode is enabled, it emulates 32-bit floating-point operations by raising a reserved-instruction exception on every instruction that uses 32-bit formats and the kernel then handles the instruction in software. (The problem lies in the discrepancy of handling odd-numbered registers which are the high 32 bits of 64-bit registers with even numbers in FR=0 mode and the lower 32-bit parts of odd-numbered 64-bit registers in FR=1 mode.) Enabling this bit is necessary when code with the O32 FP32 ABI should operate with code with compatible the O32 FPXX or O32 FP64A ABIs (which require FR=1 FPU mode) or when it is executed on newer hardware (MIPS R6 onwards) which lacks FR=0 mode support when a binary with the FP32 ABI is used.

        Note that this mode makes sense only when the FPU is in 64-bit mode (FR=1).

        Note that the use of emulation inherently has a significant performance hit and should be avoided if possible.

    In the N32/N64 ABI, 64-bit floating-point mode is always used, so FPU emulation is not required and the FPU always operates in FR=1 mode.

    This option is mainly intended for use by the dynamic linker (ld.so(8)).

    The arguments arg3, arg4, and arg5 are ignored.

PR_GET_FP_MODE (since Linux 4.0, only on MIPS)
    Get the current floating-point mode (see the description of PR_SET_FP_MODE for details).

    On success, the call returns a bit mask which represents the current floating-point mode.

    The arguments arg2, arg3, arg4, and arg5 are ignored.

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 calling thread's "keep capabilities" flag. The effect if this flag is described in capabilities(7). arg2 must be either 0 (clear the flag) or 1 (set the 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 thread's "keep capabilities" flag. See capabilities(7) for a description of this flag.

PR_MCE_KILL (since Linux 2.6.32)
    Set the machine check memory corruption kill policy for the calling 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.

    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. The arg4 and arg5 arguments must be zero if unused.

    Since Linux 3.10, this feature is available all the time. Before Linux 3.10, this feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE option enabled.

    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 shareable (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 shareable.

    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 shareable.

    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 shareable.

    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.

    The following options are available since Linux 3.18.

    PR_SET_MM_MAP
        Provides one-shot access to all the addresses by passing in a struct prctl_mm_map (as defined in <linux/prctl.h>). The arg4 argument should provide the size of the struct.

        This feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE option enabled.

    PR_SET_MM_MAP_SIZE
        Returns the size of the struct prctl_mm_map the kernel expects. This allows user space to find a compatible struct. The arg4 argument should be a pointer to an unsigned int.

        This feature is available only if the kernel is built with the CONFIG_CHECKPOINT_RESTORE option enabled.

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 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 thread'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).

    Since Linux 4.10, the value of a thread's no_new_privs bit can be viewed via the NoNewPrivs field in the /proc/[pid]/status file.

    For more information, see the kernel source file Documentation/userspace-api/no_new_privs.rst (or Documentation/prctl/no_new_privs.txt before Linux 4.13). See also seccomp(2).

PR_GET_NO_NEW_PRIVS (since Linux 3.5)
    Return (as the function result) the value of the no_new_privs bit for the calling thread. 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/admin-guide/LSM/Yama.rst (or Documentation/security/Yama.txt before Linux 4.13).

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/userspace-api/seccomp_filter.rst (or Documentation/prctl/seccomp_filter.txt before Linux 4.13).

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 (retaining the 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. Note that since the prctl() system call does not have a compat implementation for the AMD64 x32 and MIPS n32 ABIs, and the kernel writes out a pointer using the kernel's pointer size, this operation expects a user-space buffer of 8 (not 4) bytes on these ABIs.

PR_SET_TIMERSLACK (since Linux 2.6.28)
    Each thread has two associated timer slack values: a "default" value, and a "current" value. This operation sets the "current" timer slack value for the calling thread. If the nanosecond value supplied in arg2 is greater than zero, then the "current" value is set to this value. If arg2 is less than or equal to zero, the "current" timer slack is reset to the thread's "default" timer slack value.

    The "current" 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)).

    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).

    Since Linux 4.6, the "current" timer slack value of any process can be examined and changed via the file /proc/[pid]/timerslack_ns. See proc(5).

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; sh, since Linux 2.6.34; tile, since Linux 3.12) 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. Alpha also supports an additional flag with the value of 4 and no corresponding named constant, which instructs kernel to not fix up unaligned accesses (it is analogous to providing the UAC_NOFIX flag in SSI_NVPAIRS operation of the setsysinfo() system call on Tru64).

PR_GET_UNALIGN
    (see PR_SET_UNALIGN for information on versions and architectures) Return unaligned access control bits, in the location pointed to by (unsigned 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_SECCOMP and arg2 is SECCOMP_MODE_FILTER, but the process does not have the CAP_SYS_ADMIN capability or has not set the no_new_privs attribute (see the discussion of PR_SET_NO_NEW_PRIVS above).

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.

EOPNOTSUPP
    option is PR_SET_FP_MODE and arg2 has an invalid or unsupported value.

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 SECBIT_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 4.15 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 https://www.kernel.org/doc/man-pages/.