# proc_sys_kernel - control a range of kernel parameters - man(5) - [phpMan] [_proc_sys_kernel_(5)](https://www.chedong.com/phpMan.php/man/procsyskernel/5/markdown) File Formats Manual [_proc_sys_kernel_(5)](https://www.chedong.com/phpMan.php/man/procsyskernel/5/markdown) ## NAME /proc/sys/kernel/ - control a range of kernel parameters ## DESCRIPTION _/proc/sys/kernel/_ This directory contains files controlling a range of kernel parameters, as described below. _/proc/sys/kernel/acct_ This file contains three numbers: _highwater_, _lowwater_, and _frequency_. If BSD-style process accounting is enabled, these values control its behavior. If free space on filesystem where the log lives goes below _lowwater_ percent, accounting suspends. If free space gets above _highwater_ percent, accounting resumes. _frequency_ determines how often the kernel checks the amount of free space (value is in seconds). Default val‐ ues are 4, 2, and 30. That is, suspend accounting if 2% or less space is free; resume it if 4% or more space is free; consider information about amount of free space valid for 30 seconds. _/proc/sys/kernel/auto_msgmni_ (Linux 2.6.27 to Linux 3.18) From Linux 2.6.27 to Linux 3.18, this file was used to control recomputing of the value in _/proc/sys/kernel/msgmni_ upon the addition or removal of memory or upon IPC namespace creation/removal. Echoing "1" into this file enabled _msgmni_ automatic re‐ computing (and triggered a recomputation of _msgmni_ based on the current amount of available memory and number of IPC namespaces). Echoing "0" disabled automatic recom‐ puting. (Automatic recomputing was also disabled if a value was explicitly assigned to _/proc/sys/kernel/msgmni_.) The default value in _auto_msgmni_ was 1. Since Linux 3.19, the content of this file has no effect (because _msgmni_ defaults to near the maximum value possible), and reads from this file always return the value "0". _/proc/sys/kernel/cap_last_cap_ (since Linux 3.2) See [**capabilities**(7)](https://www.chedong.com/phpMan.php/man/capabilities/7/markdown). _/proc/sys/kernel/cap-bound_ (from Linux 2.2 to Linux 2.6.24) This file holds the value of the kernel _capability_ _bounding_ _set_ (expressed as a signed decimal number). This set is ANDed against the capabilities permitted to a process during [**execve**(2)](https://www.chedong.com/phpMan.php/man/execve/2/markdown). Starting with Linux 2.6.25, the system-wide capability bounding set disappeared, and was replaced by a per-thread bounding set; see [**capabilities**(7)](https://www.chedong.com/phpMan.php/man/capabilities/7/markdown). _/proc/sys/kernel/core_pattern_ See [**core**(5)](https://www.chedong.com/phpMan.php/man/core/5/markdown). _/proc/sys/kernel/core_pipe_limit_ See [**core**(5)](https://www.chedong.com/phpMan.php/man/core/5/markdown). _/proc/sys/kernel/core_uses_pid_ See [**core**(5)](https://www.chedong.com/phpMan.php/man/core/5/markdown). _/proc/sys/kernel/ctrl-alt-del_ This file controls the handling of Ctrl-Alt-Del from the keyboard. When the value in this file is 0, Ctrl-Alt-Del is trapped and sent to the [**init**(1)](https://www.chedong.com/phpMan.php/man/init/1/markdown) program to handle a graceful restart. When the value is greater than zero, Linux's reaction to a Vulcan Nerve Pinch (tm) will be an immediate reboot, without even syncing its dirty buffers. Note: when a program (like dosemu) has the keyboard in "raw" mode, the Ctrl-Alt-Del is intercepted by the program before it ever reaches the kernel tty layer, and it's up to the program to decide what to do with it. _/proc/sys/kernel/dmesg_restrict_ (since Linux 2.6.37) The value in this file determines who can see kernel syslog contents. A value of 0 in this file imposes no restrictions. If the value is 1, only privileged users can read the kernel syslog. (See [**syslog**(2)](https://www.chedong.com/phpMan.php/man/syslog/2/markdown) for more details.) Since Linux 3.4, only users with the **CAP_SYS_ADMIN **capability may change the value in this file. _/proc/sys/kernel/domainname_ and _/proc/sys/kernel/hostname_ can be used to set the NIS/YP domainname and the hostname of your box in exactly the same way as the commands [**domainname**(1)](https://www.chedong.com/phpMan.php/man/domainname/1/markdown) and [**hostname**(1)](https://www.chedong.com/phpMan.php/man/hostname/1/markdown), that is: # **echo 'darkstar' > /proc/sys/kernel/hostname** # **echo 'mydomain' > /proc/sys/kernel/domainname** has the same effect as # **hostname 'darkstar'** # **domainname 'mydomain'** Note, however, that the classic darkstar.frop.org has the hostname "darkstar" and DNS (Internet Domain Name Server) domainname "frop.org", not to be confused with the NIS (Network Information Service) or YP (Yellow Pages) domainname. These two domain names are in general different. For a detailed discussion see the [**hostname**(1)](https://www.chedong.com/phpMan.php/man/hostname/1/markdown) man page. _/proc/sys/kernel/hotplug_ This file contains the pathname for the hotplug policy agent. The default value in this file is _/sbin/hotplug_. _/proc/sys/kernel/htab-reclaim_ (before Linux 2.4.9.2) (PowerPC only) If this file is set to a nonzero value, the PowerPC htab (see kernel file _Documentation/powerpc/ppc_htab.txt_) is pruned each time the system hits the idle loop. _/proc/sys/kernel/keys/_ This directory contains various files that define parameters and limits for the key- management facility. These files are described in [**keyrings**(7)](https://www.chedong.com/phpMan.php/man/keyrings/7/markdown). _/proc/sys/kernel/kptr_restrict_ (since Linux 2.6.38) The value in this file determines whether kernel addresses are exposed via _/proc_ files and other interfaces. A value of 0 in this file imposes no restrictions. If the value is 1, kernel pointers printed using the _%pK_ format specifier will be replaced with zeros unless the user has the **CAP_SYSLOG **capability. If the value is 2, kernel pointers printed using the _%pK_ format specifier will be replaced with zeros regardless of the user's capabilities. The initial default value for this file was 1, but the default was changed to 0 in Linux 2.6.39. Since Linux 3.4, only users with the **CAP_SYS_ADMIN **capability can change the value in this file. _/proc/sys/kernel/l2cr_ (PowerPC only) This file contains a flag that controls the L2 cache of G3 processor boards. If 0, the cache is disabled. Enabled if nonzero. _/proc/sys/kernel/modprobe_ This file contains the pathname for the kernel module loader. The default value is _/sbin/modprobe_. The file is present only if the kernel is built with the **CONFIG_MOD‐** **ULES **(**CONFIG_KMOD **in Linux 2.6.26 and earlier) option enabled. It is described by the Linux kernel source file _Documentation/kmod.txt_ (present only in Linux 2.4 and ear‐ lier). _/proc/sys/kernel/modules_disabled_ (since Linux 2.6.31) A toggle value indicating if modules are allowed to be loaded in an otherwise modular kernel. This toggle defaults to off (0), but can be set true (1). Once true, modules can be neither loaded nor unloaded, and the toggle cannot be set back to false. The file is present only if the kernel is built with the **CONFIG_MODULES **option enabled. _/proc/sys/kernel/msgmax_ (since Linux 2.2) This file defines a system-wide limit specifying the maximum number of bytes in a sin‐ gle message written on a System V message queue. _/proc/sys/kernel/msgmni_ (since Linux 2.4) This file defines the system-wide limit on the number of message queue identifiers. See also _/proc/sys/kernel/auto_msgmni_. _/proc/sys/kernel/msgmnb_ (since Linux 2.2) This file defines a system-wide parameter used to initialize the _msg_qbytes_ setting for subsequently created message queues. The _msg_qbytes_ setting specifies the maximum number of bytes that may be written to the message queue. _/proc/sys/kernel/ngroups_max_ (since Linux 2.6.4) This is a read-only file that displays the upper limit on the number of a process's group memberships. _/proc/sys/kernel/ns_last_pid_ (since Linux 3.3) See [**pid_namespaces**(7)](https://www.chedong.com/phpMan.php/man/pidnamespaces/7/markdown). _/proc/sys/kernel/ostype_ and _/proc/sys/kernel/osrelease_ These files give substrings of _/proc/version_. _/proc/sys/kernel/overflowgid_ and _/proc/sys/kernel/overflowuid_ These files duplicate the files _/proc/sys/fs/overflowgid_ and _/proc/sys/fs/overflowuid_. _/proc/sys/kernel/panic_ This file gives read/write access to the kernel variable _panic_timeout_. If this is zero, the kernel will loop on a panic; if nonzero, it indicates that the kernel should autoreboot after this number of seconds. When you use the software watchdog device driver, the recommended setting is 60. _/proc/sys/kernel/panic_on_oops_ (since Linux 2.5.68) This file controls the kernel's behavior when an oops or BUG is encountered. If this file contains 0, then the system tries to continue operation. If it contains 1, then the system delays a few seconds (to give klogd time to record the oops output) and then panics. If the _/proc/sys/kernel/panic_ file is also nonzero, then the machine will be rebooted. _/proc/sys/kernel/pid_max_ (since Linux 2.5.34) This file specifies the value at which PIDs wrap around (i.e., the value in this file is one greater than the maximum PID). PIDs greater than this value are not allocated; thus, the value in this file also acts as a system-wide limit on the total number of processes and threads. The default value for this file, 32768, results in the same range of PIDs as on earlier kernels. On 32-bit platforms, 32768 is the maximum value for _pid_max_. On 64-bit systems, _pid_max_ can be set to any value up to 2^22 (**PID_MAX_LIMIT**, approximately 4 million). _/proc/sys/kernel/powersave-nap_ (PowerPC only) This file contains a flag. If set, Linux-PPC will use the "nap" mode of powersaving, otherwise the "doze" mode will be used. _/proc/sys/kernel/printk_ See [**syslog**(2)](https://www.chedong.com/phpMan.php/man/syslog/2/markdown). _/proc/sys/kernel/pty_ (since Linux 2.6.4) This directory contains two files relating to the number of UNIX 98 pseudoterminals (see [**pts**(4)](https://www.chedong.com/phpMan.php/man/pts/4/markdown)) on the system. _/proc/sys/kernel/pty/max_ This file defines the maximum number of pseudoterminals. _/proc/sys/kernel/pty/nr_ This read-only file indicates how many pseudoterminals are currently in use. _/proc/sys/kernel/random/_ This directory contains various parameters controlling the operation of the file _/dev/random_. See [**random**(4)](https://www.chedong.com/phpMan.php/man/random/4/markdown) for further information. _/proc/sys/kernel/random/uuid_ (since Linux 2.4) Each read from this read-only file returns a randomly generated 128-bit UUID, as a string in the standard UUID format. _/proc/sys/kernel/randomize_va_space_ (since Linux 2.6.12) Select the address space layout randomization (ASLR) policy for the system (on archi‐ tectures that support ASLR). Three values are supported for this file: **0 **Turn ASLR off. This is the default for architectures that don't support ASLR, and when the kernel is booted with the _norandmaps_ parameter. **1 **Make the addresses of [**mmap**(2)](https://www.chedong.com/phpMan.php/man/mmap/2/markdown) allocations, the stack, and the VDSO page random‐ ized. Among other things, this means that shared libraries will be loaded at randomized addresses. The text segment of PIE-linked binaries will also be loaded at a randomized address. This value is the default if the kernel was configured with **CONFIG_COMPAT_BRK**. **2 **(Since Linux 2.6.25) Also support heap randomization. This value is the de‐ fault if the kernel was not configured with **CONFIG_COMPAT_BRK**. _/proc/sys/kernel/real-root-dev_ This file is documented in the Linux kernel source file _Documentation/admin-guide/ini‐_ _trd.rst_ (or _Documentation/initrd.txt_ before Linux 4.10). _/proc/sys/kernel/reboot-cmd_ (Sparc only) This file seems to be a way to give an argument to the SPARC ROM/Flash boot loader. Maybe to tell it what to do after rebooting? _/proc/sys/kernel/rtsig-max_ (Up to and including Linux 2.6.7; see [**setrlimit**(2)](https://www.chedong.com/phpMan.php/man/setrlimit/2/markdown)) This file can be used to tune the maximum number of POSIX real-time (queued) signals that can be outstanding in the sys‐ tem. _/proc/sys/kernel/rtsig-nr_ (Up to and including Linux 2.6.7.) This file shows the number of POSIX real-time sig‐ nals currently queued. _/proc/_pid_/sched_autogroup_enabled_ (since Linux 2.6.38) See [**sched**(7)](https://www.chedong.com/phpMan.php/man/sched/7/markdown). _/proc/sys/kernel/sched_child_runs_first_ (since Linux 2.6.23) If this file contains the value zero, then, after a [**fork**(2)](https://www.chedong.com/phpMan.php/man/fork/2/markdown), the parent is first scheduled on the CPU. If the file contains a nonzero value, then the child is sched‐ uled first on the CPU. (Of course, on a multiprocessor system, the parent and the child might both immediately be scheduled on a CPU.) _/proc/sys/kernel/sched_rr_timeslice_ms_ (since Linux 3.9) See [**sched_rr_get_interval**(2)](https://www.chedong.com/phpMan.php/man/schedrrgetinterval/2/markdown). _/proc/sys/kernel/sched_rt_period_us_ (since Linux 2.6.25) See [**sched**(7)](https://www.chedong.com/phpMan.php/man/sched/7/markdown). _/proc/sys/kernel/sched_rt_runtime_us_ (since Linux 2.6.25) See [**sched**(7)](https://www.chedong.com/phpMan.php/man/sched/7/markdown). _/proc/sys/kernel/seccomp/_ (since Linux 4.14) This directory provides additional seccomp information and configuration. See **sec‐** [**comp**(2)](https://www.chedong.com/phpMan.php/man/comp/2/markdown) for further details. _/proc/sys/kernel/sem_ (since Linux 2.4) This file contains 4 numbers defining limits for System V IPC semaphores. These fields are, in order: SEMMSL The maximum semaphores per semaphore set. SEMMNS A system-wide limit on the number of semaphores in all semaphore sets. SEMOPM The maximum number of operations that may be specified in a [**semop**(2)](https://www.chedong.com/phpMan.php/man/semop/2/markdown) call. SEMMNI A system-wide limit on the maximum number of semaphore identifiers. _/proc/sys/kernel/sg-big-buff_ This file shows the size of the generic SCSI device (sg) buffer. You can't tune it just yet, but you could change it at compile time by editing _include/scsi/sg.h_ and changing the value of **SG_BIG_BUFF**. However, there shouldn't be any reason to change this value. _/proc/sys/kernel/shm_rmid_forced_ (since Linux 3.1) If this file is set to 1, all System V shared memory segments will be marked for de‐ struction as soon as the number of attached processes falls to zero; in other words, it is no longer possible to create shared memory segments that exist independently of any attached process. The effect is as though a [**shmctl**(2)](https://www.chedong.com/phpMan.php/man/shmctl/2/markdown) **IPC_RMID **is performed on all existing segments as well as all segments created in the future (until this file is reset to 0). Note that existing segments that are attached to no process will be immediately destroyed when this file is set to 1. Setting this option will also destroy segments that were cre‐ ated, but never attached, upon termination of the process that created the segment with [**shmget**(2)](https://www.chedong.com/phpMan.php/man/shmget/2/markdown). Setting this file to 1 provides a way of ensuring that all System V shared memory seg‐ ments are counted against the resource usage and resource limits (see the description of **RLIMIT_AS **in [**getrlimit**(2)](https://www.chedong.com/phpMan.php/man/getrlimit/2/markdown)) of at least one process. Because setting this file to 1 produces behavior that is nonstandard and could also break existing applications, the default value in this file is 0. Set this file to 1 only if you have a good understanding of the semantics of the applications using Sys‐ tem V shared memory on your system. _/proc/sys/kernel/shmall_ (since Linux 2.2) This file contains the system-wide limit on the total number of pages of System V shared memory. _/proc/sys/kernel/shmmax_ (since Linux 2.2) This file can be used to query and set the run-time limit on the maximum (System V IPC) shared memory segment size that can be created. Shared memory segments up to 1 GB are now supported in the kernel. This value defaults to **SHMMAX**. _/proc/sys/kernel/shmmni_ (since Linux 2.4) This file specifies the system-wide maximum number of System V shared memory segments that can be created. _/proc/sys/kernel/sysctl_writes_strict_ (since Linux 3.16) The value in this file determines how the file offset affects the behavior of updating entries in files under _/proc/sys_. The file has three possible values: -1 This provides legacy handling, with no printk warnings. Each [**write**(2)](https://www.chedong.com/phpMan.php/man/write/2/markdown) must fully contain the value to be written, and multiple writes on the same file descriptor will overwrite the entire value, regardless of the file position. 0 (default) This provides the same behavior as for -1, but printk warnings are writ‐ ten for processes that perform writes when the file offset is not 0. 1 Respect the file offset when writing strings into _/proc/sys_ files. Multiple writes will _append_ to the value buffer. Anything written beyond the maximum length of the value buffer will be ignored. Writes to numeric _/proc/sys_ entries must always be at file offset 0 and the value must be fully contained in the buffer provided to [**write**(2)](https://www.chedong.com/phpMan.php/man/write/2/markdown). _/proc/sys/kernel/sysrq_ This file controls the functions allowed to be invoked by the SysRq key. By default, the file contains 1 meaning that every possible SysRq request is allowed (in older kernel versions, SysRq was disabled by default, and you were required to specifically enable it at run-time, but this is not the case any more). Possible values in this file are: 0 Disable sysrq completely 1 Enable all functions of sysrq > 1 Bit mask of allowed sysrq functions, as follows: 2 Enable control of console logging level 4 Enable control of keyboard (SAK, unraw) 8 Enable debugging dumps of processes etc. 16 Enable sync command 32 Enable remount read-only 64 Enable signaling of processes (term, kill, oom-kill) 128 Allow reboot/poweroff 256 Allow nicing of all real-time tasks This file is present only if the **CONFIG_MAGIC_SYSRQ **kernel configuration option is en‐ abled. For further details see the Linux kernel source file _Documentation/ad‐_ _min-guide/sysrq.rst_ (or _Documentation/sysrq.txt_ before Linux 4.10). _/proc/sys/kernel/version_ This file contains a string such as: #5 Wed Feb 25 21:49:24 MET 1998 The "#5" means that this is the fifth kernel built from this source base and the date following it indicates the time the kernel was built. _/proc/sys/kernel/threads-max_ (since Linux 2.3.11) This file specifies the system-wide limit on the number of threads (tasks) that can be created on the system. Since Linux 4.1, the value that can be written to _threads-max_ is bounded. The minimum value that can be written is 20. The maximum value that can be written is given by the constant **FUTEX_TID_MASK **(0x3fffffff). If a value outside of this range is written to _threads-max_, the error **EINVAL **occurs. The value written is checked against the available RAM pages. If the thread struc‐ tures would occupy too much (more than 1/8th) of the available RAM pages, _threads-max_ is reduced accordingly. _/proc/sys/kernel/yama/ptrace_scope_ (since Linux 3.5) See [**ptrace**(2)](https://www.chedong.com/phpMan.php/man/ptrace/2/markdown). _/proc/sys/kernel/zero-paged_ (PowerPC only) This file contains a flag. When enabled (nonzero), Linux-PPC will pre-zero pages in the idle loop, possibly speeding up get_free_pages. ## SEE ALSO [**proc**(5)](https://www.chedong.com/phpMan.php/man/proc/5/markdown), [**proc_sys**(5)](https://www.chedong.com/phpMan.php/man/procsys/5/markdown) Linux man-pages 6.7 2023-09-30 [_proc_sys_kernel_(5)](https://www.chedong.com/phpMan.php/man/procsyskernel/5/markdown)