# man > TIME_NAMESPACES(7) [TIME_NAMESPACES(7)](https://www.chedong.com/phpMan.php/man/TIMENAMESPACES/7/markdown) Linux Programmer's Manual [TIME_NAMESPACES(7)](https://www.chedong.com/phpMan.php/man/TIMENAMESPACES/7/markdown) ## NAME time_namespaces - overview of Linux time namespaces ## DESCRIPTION Time namespaces virtualize the values of two system clocks: • **CLOCK**___**MONOTONIC** (and likewise **CLOCK**___**MONOTONIC**___**COARSE** and **CLOCK**___**MONOTONIC**___**RAW**), a nonset‐ table clock that represents monotonic time since—as described by POSIX—"some unspeci‐ fied point in the past". • **CLOCK**___**BOOTTIME** (and likewise **CLOCK**___**BOOTTIME**___**ALARM**), a nonsettable clock that is identical to **CLOCK**___**MONOTONIC**, except that it also includes any time that the system is suspended. Thus, the processes in a time namespace share per-namespace values for these clocks. This affects various APIs that measure against these clocks, including: **clock**___**[gettime**(2)](https://www.chedong.com/phpMan.php/man/gettime/2/markdown), **clock**___**[nanosleep**(2)](https://www.chedong.com/phpMan.php/man/nanosleep/2/markdown), [**nanosleep**(2)](https://www.chedong.com/phpMan.php/man/nanosleep/2/markdown), **timer**___**[settime**(2)](https://www.chedong.com/phpMan.php/man/settime/2/markdown), **timerfd**___**[settime**(2)](https://www.chedong.com/phpMan.php/man/settime/2/markdown), and _/proc/uptime_. Currently, the only way to create a time namespace is by calling [**unshare**(2)](https://www.chedong.com/phpMan.php/man/unshare/2/markdown) with the **CLONE**___**NEWTIME** flag. This call creates a new time namespace but does _not_ place the calling process in the new namespace. Instead, the calling process's subsequently created children are placed in the new namespace. This allows clock offsets (see below) for the new namespace to be set before the first process is placed in the namespace. The _/proc/[pid]/ns/time_for_children_ symbolic link shows the time namespace in which the children of a process will be created. (A process can use a file descriptor opened on this symbolic link in a call to [**setns**(2)](https://www.chedong.com/phpMan.php/man/setns/2/markdown) in order to move into the namespace.) **/proc/PID/timens**___**offsets** Associated with each time namespace are offsets, expressed with respect to the initial time namespace, that define the values of the monotonic and boot-time clocks in that namespace. These offsets are exposed via the file _/proc/PID/timens_offsets_. Within this file, the off‐ sets are expressed as lines consisting of three space-delimited fields: The _clock-id_ is a string that identifies the clock whose offsets are being shown. This field is either _monotonic_, for **CLOCK**___**MONOTONIC**, or _boottime_, for **CLOCK**___**BOOTTIME**. The remaining fields express the offset (seconds plus nanoseconds) for the clock in this time namespace. These offsets are expressed relative to the clock values in the initial time namespace. The _offset-secs_ value can be negative, subject to restrictions noted below; _offset-nanosecs_ is an unsigned value. In the initial time namespace, the contents of the _timens_offsets_ file are as follows: $ **cat** **/proc/self/timens**___**offsets** monotonic 0 0 boottime 0 0 In a new time namespace that has had no member processes, the clock offsets can be modified by writing newline-terminated records of the same form to the _timens_offsets_ file. The file can be written to multiple times, but after the first process has been created in or has en‐ tered the namespace, [**write**(2)](https://www.chedong.com/phpMan.php/man/write/2/markdown)s on this file fail with the error **EACCES**. In order to write to the _timens_offsets_ file, a process must have the **CAP**___**SYS**___**TIME** capability in the user name‐ space that owns the time namespace. Writes to the _timens_offsets_ file can fail with the following errors: **EINVAL** An _offset-nanosecs_ value is greater than 999,999,999. **EINVAL** A _clock-id_ value is not valid. **EPERM** The caller does not have the **CAP**___**SYS**___**TIME** capability. **ERANGE** An _offset-secs_ value is out of range. In particular; • _offset-secs_ can't be set to a value which would make the current time on the corre‐ sponding clock inside the namespace a negative value; and • _offset-secs_ can't be set to a value such that the time on the corresponding clock inside the namespace would exceed half of the value of the kernel constant **KTIME**___**SEC**___**MAX** (this limits the clock value to a maximum of approximately 146 years). In a new time namespace created by [**unshare**(2)](https://www.chedong.com/phpMan.php/man/unshare/2/markdown), the contents of the _timens_offsets_ file are inherited from the time namespace of the creating process. ## NOTES Use of time namespaces requires a kernel that is configured with the **CONFIG**___**TIME**___**NS** option. Note that time namespaces do not virtualize the **CLOCK**___**REALTIME** clock. Virtualization of this clock was avoided for reasons of complexity and overhead within the kernel. For compatibility with the initial implementation, when writing a _clock-id_ to the _/proc/[pid]/timens_offsets_ file, the numerical values of the IDs can be written instead of the symbolic names show above; i.e., 1 instead of _monotonic_, and 7 instead of _boottime_. For redability, the use of the symbolic names over the numbers is preferred. The motivation for adding time namespaces was to allow the monotonic and boot-time clocks to maintain consistent values during container migration and checkpoint/restore. ## EXAMPLES The following shell session demonstrates the operation of time namespaces. We begin by dis‐ playing the inode number of the time namespace of a shell in the initial time namespace: $ **readlink** **/proc/$$/ns/time** time:[4026531834] Continuing in the initial time namespace, we display the system uptime using [**uptime**(1)](https://www.chedong.com/phpMan.php/man/uptime/1/markdown) and use the _clock_times_ example program shown in **clock**___**[getres**(2)](https://www.chedong.com/phpMan.php/man/getres/2/markdown) to display the values of various clocks: $ **uptime** **--pretty** up 21 hours, 17 minutes $ **./clock**___**times** CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 36m 41s) CLOCK_TAI : 1585989438.972 (18356 days + 8h 37m 18s) CLOCK_MONOTONIC: 56338.247 (15h 38m 58s) CLOCK_BOOTTIME : 76633.544 (21h 17m 13s) We then use [**unshare**(1)](https://www.chedong.com/phpMan.php/man/unshare/1/markdown) to create a time namespace and execute a [**bash**(1)](https://www.chedong.com/phpMan.php/man/bash/1/markdown) shell. From the new shell, we use the built-in **echo** command to write records to the _timens_offsets_ file adjusting the offset for the **CLOCK**___**MONOTONIC** clock forward 2 days and the offset for the **CLOCK**___**BOOTTIME** clock forward 7 days: $ **PS1="ns2#** **"** **sudo** **unshare** **-T** **--** **bash** **--norc** ns2# **echo** **"monotonic** **$((2*24*60*60))** **0"** **>** **/proc/$$/timens**___**offsets** ns2# **echo** **"boottime** **$((7*24*60*60))** **0"** **>** **/proc/$$/timens**___**offsets** Above, we started the [**bash**(1)](https://www.chedong.com/phpMan.php/man/bash/1/markdown) shell with the **--norc** options so that no start-up scripts were executed. This ensures that no child processes are created from the shell before we have a chance to update the _timens_offsets_ file. We then use [**cat**(1)](https://www.chedong.com/phpMan.php/man/cat/1/markdown) to display the contents of the _timens_offsets_ file. The execution of [**cat**(1)](https://www.chedong.com/phpMan.php/man/cat/1/markdown) creates the first process in the new time namespace, after which further attempts to update the _timens_offsets_ file produce an error. ns2# **cat** **/proc/$$/timens**___**offsets** monotonic 172800 0 boottime 604800 0 ns2# **echo** **"boottime** **$((9*24*60*60))** **0"** **>** **/proc/$$/timens**___**offsets** bash: echo: write error: Permission denied Continuing in the new namespace, we execute [**uptime**(1)](https://www.chedong.com/phpMan.php/man/uptime/1/markdown) and the _clock_times_ example program: ns2# **uptime** **--pretty** up 1 week, 21 hours, 18 minutes ns2# **./clock**___**times** CLOCK_REALTIME : 1585989457.056 (18356 days + 8h 37m 37s) CLOCK_TAI : 1585989494.057 (18356 days + 8h 38m 14s) CLOCK_MONOTONIC: 229193.332 (2 days + 15h 39m 53s) CLOCK_BOOTTIME : 681488.629 (7 days + 21h 18m 8s) From the above output, we can see that the monotonic and boot-time clocks have different val‐ ues in the new time namespace. Examining the _/proc/[pid]/ns/time_ and _/proc/[pid]/ns/time_for_children_ symbolic links, we see that the shell is a member of the initial time namespace, but its children are created in the new namespace. ns2# **readlink** **/proc/$$/ns/time** time:[4026531834] ns2# **readlink** **/proc/$$/ns/time**___**for**___**children** time:[4026532900] ns2# **readlink** **/proc/self/ns/time** # Creates a child process time:[4026532900] Returning to the shell in the initial time namespace, we see that the monotonic and boot-time clocks are unaffected by the _timens_offsets_ changes that were made in the other time name‐ space: $ **uptime** **--pretty** up 21 hours, 19 minutes $ **./clock**___**times** CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 38m 51s) CLOCK_TAI : 1585989438.972 (18356 days + 8h 39m 28s) CLOCK_MONOTONIC: 56338.247 (15h 41m 8s) CLOCK_BOOTTIME : 76633.544 (21h 19m 23s) ## SEE ALSO [**nsenter**(1)](https://www.chedong.com/phpMan.php/man/nsenter/1/markdown), [**unshare**(1)](https://www.chedong.com/phpMan.php/man/unshare/1/markdown), **clock**___**[settime**(2)](https://www.chedong.com/phpMan.php/man/settime/2/markdown), [**setns**(2)](https://www.chedong.com/phpMan.php/man/setns/2/markdown), [**unshare**(2)](https://www.chedong.com/phpMan.php/man/unshare/2/markdown), [**namespaces**(7)](https://www.chedong.com/phpMan.php/man/namespaces/7/markdown), [**time**(7)](https://www.chedong.com/phpMan.php/man/time/7/markdown) ## COLOPHON This page is part of release 5.10 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 . Linux 2020-06-09 [TIME_NAMESPACES(7)](https://www.chedong.com/phpMan.php/man/TIMENAMESPACES/7/markdown)