Chapter 9 Times and Timers Source: Robbins and Robbins, UNIX Systems Programming, Prentice Hall, 2003.

2 Introduction Operating systems use timers for such tasks as –Process scheduling –Timeouts for network protocols –Periodic updates of system statistics Applications access system time and timer functions to –Measure performance –Identify the time when events occur –Implement protocols –Control interaction with users

9.1a POSIX Time

4 time_t Type and the time() Function In POSIX, the Epoch is the start point for measuring time –It is defined as 00:00 (midnight) on January 1, 1970 The POSIX base standard supports only a time resolution of seconds It expresses time since the Epoch using a time_t type, which is usually a long integer A program can access the system time (expressed in seconds since the Epoch) by calling the time() function #include time_t time(time_t *timeLocation); If timeLocation is not NULL, the function also stores the time in the location pointed to by timeLocation If successful, time() returns the number of seconds; otherwise it returns –1 If time_t is implemented as a 32-bit long integer, the number will overflow in 2038 If time_t is implemented as an unsigned 32-bit long integer, the number will overflow in 2106, but this implementation would not allow the time() function to return –1 If time_ t is implemented as a 64-bit long integer, the overflow will not occur for another 292 billion years

5 difftime() Function The difftime() function computes the difference between two time values of type time_t #include double difftime(time_t timeA, time_t timeB); The timeB parameter is subtracted from the timeA parameter (i.e., A – B) The time resolution for time_t is one second, which may not be accurate enough for some calculations The program shown on the next two slides calculates the wall-clock (i.e., elapsed) time for a person to answer an addition problem

6 Example use of time() and difftime() #include #define MAX_NUMBER 100 // ******************************* int main(void) { time_t startTime; time_t stopTime; int valueX; int valueY; int guess; int answer; printf("\nAddition Drill Program \n\n"); srand(time(NULL)); valueX = rand() % MAX_NUMBER; valueY = rand() % MAX_NUMBER; answer = valueX + valueY; (More on next slide)

7 Example use of time() and difftime() (continued) printf(" %2d\n", valueX); printf(" + %2d\n", valueY); printf(" ----\n"); if (answer >= MAX_NUMBER) printf(" "); // Two spaces else printf(" "); // Three spaces startTime = time(NULL); scanf("%d", &guess); stopTime = time(NULL); if (guess == answer) printf("\nCorrect answer!\n"); else printf("\nWrong answer. The correct answer is %d\n", answer); printf("\nElapsed time: %.0f seconds\n", difftime(stopTime, startTime)); return 0; } // End main uxb2% a.out Addition Drill Program Correct answer! Elapsed time: 7 seconds uxb2% Sample Program Run

9.1b Date and Time

9 Displaying the Date and Time The localtime() function takes a parameter specifying the seconds since the Epoch and returns a pointer to a struct tm structure containing the components of time (such as day, month, and year) adjusted for local requirements The asctime() function converts the structure returned by localtime() to a 26-character English-language string Example: Mon Aug 21 10:00: \n\0 The ctime() function is equivalent to asctime(localtime(EpochTime)) #include struct tm *localtime(const time_t *EpochTime); char *asctime(const struct tm *timeRecordPtr); char *ctime(const time_t *EpochTime);

10 Example use of localtime(), asctime() and ctime() #include int main(void) { time_t currentTime; struct tm *localPtr; currentTime = time(NULL); localPtr = localtime(&currentTime); printf("Local time (using asctime): %s", asctime(localPtr)); printf("Local time (using ctime) : %s", ctime(&currentTime)); return 0; } // End main

11 struct tm Structure The struct tm structure has the members shown below int tm_sec;// Seconds after the minute [0, 60] int tm_min;// Minutes after the hour [0, 59] int tm_hour;// Hours since midnight [0, 23] int tm_mday;// Day of the month [1, 31] int tm_mon;// Months since January [0, 11] int tm_year;// Years since 1900 int tm_wday;// Days since Sunday [0, 6] int tm_yday;// Days since January 1 st [0, 365] int tm_isdst;// Flag indicating daylight savings time

12 Example use of struct tm Structure #include int main(void) { struct tm *localPtr; time_t currentTime; currentTime = time(NULL); localPtr = localtime(&currentTime); printf("Local time : %s\n", ctime(&currentTime)); printf("Seconds after minute: %d\n", localPtr->tm_sec); printf("Minutes after hour : %d\n", localPtr->tm_min); printf("Hours since midnight: %d\n", localPtr->tm_hour); printf("Day of the month : %d\n", localPtr->tm_mday); printf("Months since January: %d\n", localPtr->tm_mon); printf("Years since 1900 : %d\n", localPtr->tm_year); printf("Days since Sunday : %d\n", localPtr->tm_wday); printf("Days since Jan 1st : %d\n", localPtr->tm_yday); printf("DST Flag : %d\n", localPtr->tm_isdst); return 0; } // End main

13 Sample output for struct tm uxb2% a.out Local time : Wed Jul 26 19:36: Seconds after minute: 37 Minutes after hour : 36 Hours since midnight: 19 Day of the month : 26 Months since January: 6 Years since 1900 : 106 Days since Sunday : 3 Days since Jan 1st : 206 DST Flag : 1 uxb2%

14 gettimeofday() Function A time resolution of seconds is too coarse for timing programs or controlling program events The getttimeofday() function retrieves the system time in seconds and microseconds since the Epoch #include int gettimeofday(struct timeval *timePtr, void *); The struct timeval structure pointed to by timePtr receives the retrieved time The programmer should set the second parameter to NULL The function returns zero if successful; otherwise, some implementations return –1 and set errno The struct timeval structure contains two members time_t tv_sec; // Seconds since the Epoch time_t tv_usec; // Microseconds after the second

15 Example#1: use of gettimeofday() #include int main(void) { struct timeval timeRecord; int status; status = gettimeofday(&timeRecord, NULL); printf("Seconds since the Epoch : %d\n", timeRecord.tv_sec); printf("Microseconds after seconds: %d\n", timeRecord.tv_usec); return 0; } // End main uxb2% a.out Seconds since the Epoch : Microseconds after seconds: uxb2% Sample Program Run

16 Example#2: use of gettimeofday() #include #define MILLION L void functionToTime(void); int main(void) { struct timeval timeStart; struct timeval timeStop; long timeDifference; gettimeofday(&timeStart, NULL); functionToTime(); gettimeofday(&timeStop, NULL); timeDifference = MILLION * (timeStop.tv_sec - timeStart.tv_sec) + timeStop.tv_usec - timeStart.tv_usec; printf("The function took %ld microseconds\n", timeDifference); return 0; } // End main void functionToTime(void) { // Do some I/O or some calculations } // End functionToTime

9.2 Sleep Function

18 sleep() Function A process that voluntarily blocks for a specified time is said to sleep The sleep() function causes the calling thread to be suspended either until the specified number of seconds has elapsed or until the calling thread catches a signal #include unsigned int sleep(unsigned int seconds); In some implementations, the function returns 0 if the requested time has elapsed; otherwise, it returns the amount of unslept time if interrupted The function interacts with SIGALRM, so the programmer should avoid using them concurrently in the same process

19 Example use of sleep() #include int main(void) { int sleepTime; printf("\nPID: %d\n", getpid()); printf("\nHow long should I sleep (sec)? "); scanf("%d", &sleepTime); printf("\nGood night :)...\n"); sleep(sleepTime); printf("\nThat was a good rest!\n"); return 0; } // End main uxb2% a.out PID: How long should I sleep (sec)? 20 Good night :)... That was a good rest! uxb2% Sample Program Run