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The C standard library uses Unix time for all date and time functions, and Unix time is sometimes referred to as time_t, the name of the data type used for timestamps in C and C++. C's Unix time functions are defined as the system time API in the POSIX specification. [ 15 ]
Many computer systems measure time and date using Unix time, an international standard for digital timekeeping.Unix time is defined as the number of seconds elapsed since 00:00:00 UTC on 1 January 1970 (an arbitrarily chosen time based on the creation of the first Unix system), which has been dubbed the Unix epoch.
Software timekeeping systems vary widely in the resolution of time measurement; some systems may use time units as large as a day, while others may use nanoseconds.For example, for an epoch date of midnight UTC (00:00) on 1 January 1900, and a time unit of a second, the time of the midnight (24:00) between 1 January 1900 and 2 January 1900 is represented by the number 86400, the number of ...
For example, the Unix system time 1 000 000 000 seconds since the beginning of the epoch translates into the calendar time 9 September 2001 01:46:40 UT. Library subroutines that handle such conversions may also deal with adjustments for time zones , daylight saving time (DST), leap seconds, and the user's locale settings.
Timestamps do not have to be based on some absolute notion of time, however. They can have any epoch, can be relative to any arbitrary time, such as the power-on time of a system, or to some arbitrary time in the past. A distinction is sometimes made between the terms datestamp, timestamp and date-timestamp:
POSIX.1-2001 (or IEEE Std 1003.1-2001) equates to the Single UNIX Specification, version 3 minus X/Open Curses. [12] This standard consisted of: the Base Definitions, Issue 6, the System Interfaces and Headers, Issue 6, the Commands and Utilities, Issue 6. IEEE Std 1003.1-2004 involved a minor update of POSIX.1-2001.
The 64-bit binary fixed-point timestamps used by NTP consist of a 32-bit part for seconds and a 32-bit part for fractional second, giving a time scale that rolls over every 2 32 seconds (136 years) and a theoretical resolution of 2 −32 seconds (233 picoseconds).
Unix permissions, POSIX ACL, POSIX Capabilities? ? ? ? GNU Mach: C: part of GNU Hurd, used in GNU/Hurd: GNU Project/Free Software Foundation: ELF: microkernel: No partial: No Yes chroot, Xen: Unix permissions, POSIX ACL, POSIX Capabilities? ? ? ? Inferno kernel: Inferno: Bell Labs / Vita Nuova Holdings? virtual machine? Yes Yes Yes chroot, Xen ...