Systems Programming/ C and UNIX

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1 Systems Programming/ C and UNIX A. Fischer CSCI 4547/6647 What is Time? November 6, 2017 A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

2 Outline 1 What is time? 2 Representations of Time Time Functions A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

3 What is time? What is time? Real Time and Elapsed Time Universal Time Time Accuracy A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

4 What is time? Real Time and Elapsed Time We use the word time in two ways: When: at what point in the history of the universe did an event take place. What time is it now? I will use time or clock time or real time to talk about when. How long: how many time units elapsed between the beginning and the end of an event? I will call this elapsed time. This seems like a simple subject but it is truly NOT simple. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

5 What is time? Universal Time We have to measure a physical phenomenon, like time, before we can talk about it, store it, or use it. The world has an official time standard, maintained by a standards committee. It is based on astronomical observations and an atomic clock: UT1 is one form of Universal time, established by measuring the Earth s movement relative to a distant quasar. It is the same if measured anywhere on earth. The problem is that Earth s rotation is slightly irregular. UTC, or Coordinated Universal Time is measured using a completely regular atomic clock. It approximates UT1 and is the basis for world-wide civil time. This is kept synchronized with UT1 by adding leap-seconds to the UTC clock when the drift reaches.9 seconds. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

6 What is time? Time Accuracy We set our clocks from some Universal Time reference clock, and hope they stay accurate. Any clock could be off in two ways: Drift: A clock could run a bit too fast or a bit too slow, or irregularly, and thus differ from the actual global time standard by some number of time units. Resolution: A time could be exactly on the global time standard, but be expressed in units that offer little precision. A measurement in seconds is less precise than a measurement in nanoseconds. When we record a real-time value, it might be recorded in an absolute frame of reference (UTC, or Universal Time) or in a relative frame of reference (Eastern Standard Time). A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

7 What is time? Drift Are our clocks always right? If a clock loses power and stops, it must be reset before it can be used at all. When the seasons change, clocks are set forward or back for daylight savings time. If a clock s time drifts gradually, it must be reset eventually. However, the drift may be small and not be noticed or corrected for a while. When a clock is corrected, its time suddenly jumps forward or back. People can cope with these changes. Computers cope less well. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

8 What is time? Resolution What units do we use to measure time? Years? Good enough for ages. Days? Good enough for due dates. Seconds? Good enough for most human work, and to time program execution on a Commadore-64 The system s software clock measures time in jiffies:.01 to.001 seconds, depending on the version of your kernel. Microseconds? Appropriate for timing events at the scale of today s CPU. Modern computers are able to measure time at this resolution. The resolution of our clocks and timers must be meaningful for the measurement task. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

9 System Time How Time is Used in Unix Representations of Time A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

10 System Time Most computers have a battery-powered hardware clock that the system reads at boot time in order to initialize the system s software clock. The software clock defines the system time. Unix systems run a daemon that constantly communicates using the SNTP protocol (Simple Network Time Protocol) to stay synchronized with the official network time authority. The system clock is adjusted to conform to the network clock. It could be set either forward or backward. It is probable that some of the protocols will gradually slow down a clock that is too fast, until it comes into synchrony with the standard. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

11 System Time Star time Global time International Telecommunication Union Network Time System Time A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

12 How Time is Used in Unix Unix uses an absolute coordinate system that is based on global time. Each file has a creation time, a time of last modification, and a time of last access, recorded in system time. File times are the basis for maintaining consistency and currency among related files and file systems. cron jobs are executed automatically to do tasks such as backup at specific system times. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

13 How Intervals are Used in Unix Each process maintains a total elapsed time, the total execution time of system calls, and the total execution time of the user code, including all daughter threads, and excluding system calls. Execution time is used for time-sharing the CPU, for measuring performance and for charging customers on shared commercial systems. Modern systems support measurement of microsecond intervals, some smaller. Intervals are used to end timeouts (sleep). A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

14 Mirrors and Backups These programs use date the of last modification of a file. rsync is a brilliant algorithm developed by Andrew Tridgell. It is capable of maintaining a backup mirror of a directory either locally or remotely over the network. The number of bytes that must be transmitted is minimized by sending differences, not whole files. I use the rsync command constantly to move files from one to another of my four personal machines. rsync machine1:source/ machine1:destination/ I can push files from here to elsewhere, or pull from there to here. rsync. machine2:destination/ rsync machine1:source/. My home file directory is backed up automatically each night by a cron job running rdiff-backup, which is based on the rsync library. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

15 Problems with Time Because our time tools are not perfect Recorded Unix times are absolute, but they are displayed in local time coordinates. A translation must therefore be done each time a date is displayed. A time recorded by non-unix-aware software might be in a relative, not absolute, coordinate system. The real time, recorded simultaneously on two different Unix machines, might be different if one is not connected to the internet. Systems like rsync rely on the accuracy of the timestamps of files on two different machines. If one machine is off, the software cannot work properly. When the local time is adjusted to synchronize with network time, it might need to go backwards. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

16 Representations of Time Representations of Time and Date The basic Unix times representation is an integer type, time_t, giving milliseconds since Jan 1, 1970 Negative date values go back to December 13, On February 13, 2009 at 23:31:30 (UTC) the Unix time was Worldwide, people had parties to celebrate this event. Currently, type time_t is the same length as a long int. This representation will be good until January 19, Before then, the Unix world will need to begin using more than 4 bytes for a date. Already, our hardware is beginning to support 8-byte integers. Also, we already need a finer mesh than milliseconds. So... it will change and we don t know when. So... don t rely on any particular implementation use type time_t and library functions to process dates.. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

17 Representations of Time The Time Structure A secondary time representation is broken down into readable fields: #include <time.h> struct tm { int tm_sec; // seconds int tm_min; // minutes int tm_hour; // hours int tm_mday; // day of the month int tm_mon; // month int tm_year; // year int tm_wday; // day of the week int tm_yday; // day in the year int tm_isdst; // daylight saving time }; Standard functions are used to convert between time_t and struct tm. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

18 Representations of Time High-resolution Time The timespec structure is normally used in combination with time_t to provide finer resolution for binary times. struct timeval { // defined in <sys/time.h> time_t tv_sec; // seconds since Jan suseconds_t tv_usec; // and microseconds }; struct timespec { // defined in <time.h> time_t tv_sec; // Seconds since 00:00:00 GMT Jan 1, 1970 long tv_nsec; // Additional nanoseconds since then } timespec_t; A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

19 Representations of Time More Time Definitions useconds_t // defined in <unistd.h> an integer rep for microseconds that depends on the hardware. struct timezone { int tz_minuteswest; int tz_dsttime; }; // defined in <sys/time.h> // of Greenwich // type of dst correction to use struct itimerval { // defined in <sys/time.h> struct timeval it_interval; // timer interval struct timeval it_value; // current value }; A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

20 Time Functions Time Functions All of these are in time.h. System time is stored as UTC (Coordinated Universal Time) time_t now = time( NULL ); // Read the system clock. Once you have a time_t, it can be converted to and from other forms using library functions: struct tm nowstruct = localtime( now ); struct tm nowstruct = gmtime( now ); These two formats can be converted to printable strings: char nowstring[25] = ctime( now ); char* now2 = asctime( &nowstruct ); tools contains a set of functions to extract the date (today()) and time (oclock()) separately. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

21 Time Functions Time Functions Use these variables: time_t time1, time2, elapsed; struct tm timestr; Convert from struct to integer format in current time zone: time1 = mktime( &timestr ); Convert from struct to integer format in UTC time: time1 = timegm( &timestr ); Interval from time1 to time2, in seconds. elapsed = difftime(time2, time1); A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

22 Time Functions Functions for Formatting Times Use these variables: int resultlength; struct tm timestr; char *st; char buffer[25]; Format the time, producing a string. (Like a Java tostring().) See man page for format details. resultlength = strftime(buffer, 25, format, &imestr); The opposite operation from strftime(). Parse the string st, according to the format given, and fill in the fields of timestr. Return a pointer to the first character in st that has not been converted. st = strptime( st1, "%H:%M:%S", &timestr); A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

23 Time Functions Hi-res Time Functions useconds_t ualarm( useconds_t, useconds_t ); A simplified interface to setitimer The first argument is the total lifetime of this timer. The second argument is the frequency at which the alarm should ring. int usleep( useconds_t usecs ); suspend thread execution for usecs microseconds int nanosleep( const struct timespec *rqtp, struct timespec *rmtp); suspend thread execution for rqtp nanoseconds The interval may be slightly longer, but will be as close as the system hardware can measure the interval. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

24 Time Functions Adding and Subtracting Hi-Res Times The C libraries do not contain functions for adding and subtracting hi-res times. The code for working on microsecond-times is given at this website: libc Elapsed time This code can be easily modified to work on nanosecond-times. Basically, an addition or subtraction is done on the microsecond member of the timeval, and if that overflows, a carry is added to the member representing seconds. A. Fischer CSCI 4547/6647[1ex]What is Time? Systems Programming Lecture /24 November 6, / 24

Systems Programming/ C and UNIX

Systems Programming/ C and UNIX A. Fischer CSCI 4547 / 6647 November 1, 2013 A. Fischer CSCI 4547 / 6647 () Systems Programming Lecture 8... 1/41 November 1, 2013 1 / 41 Outline 1 Signals for Threads Signals