I need to be able to count the seconds between now and a specific time (like the next time it's 3pm). I saw some similar questions but I wasn't able to use any of these.
C standard library has two representation of time: time_t is a seconds since Unix Epoch and struct tm where you can individially set seconds, minutes, etc.
So to get next moment in time when wall clock will show 3 p.m. you'll need take current time in seconds time(NULL), convert it to struct tm, advance time to 3 p.m. by setting structure fields, convert it back to time_t and calculate the difference:
#include <time.h>
#include <string.h>
#include <stdio.h>
int main() {
time_t now, next3pm;
struct tm threepm;
// Get current time (now)
now = time(NULL);
// Copy current date to a `threepm`, and set time
memcpy(&threepm, gmtime(&now), sizeof(struct tm));
if(threepm.tm_hour > 15) {
// Advance to a next day
++threepm.tm_mday;
}
threepm.tm_hour = 15;
threepm.tm_min = threepm.tm_sec = 0;
printf("%.f seconds till 3:00 PM\n", difftime(mktime(&threepm), now));
return 0;
}
I used UTC conversion functions gmtime()/mktime(). Since there is no timezoned version of mktime() you may need to convert time on your own. Using UTC time may cause trouble with advancing to a next day when it shouldn't do that (because it is already 15:00 according to UTC, but not yet 15:00 according to a local time).
Or, a classic joke version:
int main() {
time_t t;
struct tm* tm;
do {
t = time(NULL);
tm = gmtime(&t);
usleep(1000000);
} while(tm->tm_hour != 15 && tm->tm_min != 0);
puts("0 seconds till 3:00 PM");
}
Related
my server uses local time of Prague (+ 2 hours) and visitor's request uses GMT time.
In code I want to compare these times but for that I need to convert them to the same time zone. How to do it? When I try to use gmtime() and localtime() they returns same result.
struct tm time;
struct stat data;
time_t userTime, serverTime;
// this time will send me user in GMT
strptime("Thu, 15 Apr 2021 17:20:21 GMT", "%a, %d %b %Y %X GMT", &time)
userTime = mktime(&time); // in GMT
// this time I will find in my server in another time zone
stat("test.txt", &data);
serverTime = data.st_mtimespec.tv_sec; // +2 hours (Prague)
// it's not possible to compare them (2 diferrent time zones)
if(serverTime < userTime) {
// to do
}
Thank you for answer.
On linux with glibc you can just use %Z with strptime to read GMT.
#define _XOPEN_SOURCE
#define _DEFAULT_SOURCE
#include <time.h>
#include <assert.h>
#include <string.h>
#include <sys/stat.h>
#include <stdio.h>
int main() {
// this time will send me user in GMT
struct tm tm;
char *buf = "Thu, 15 Apr 2021 17:20:21 GMT";
char *r = strptime(buf, "%a, %d %b %Y %X %Z", &tm);
assert(r == buf + strlen(buf));
time_t userTime = timegm(&tm);
// this time represents time that has passed since epochzone
struct stat data;
stat("test.txt", &data);
// be portable, you need only seconds
// see https://pubs.opengroup.org/onlinepubs/007904875/basedefs/sys/stat.h.html
time_t serverTime = data.st_mtime;
// it's surely is possible to compare them
if (serverTime < userTime) {
// ok
}
}
// it's not possible to compare them (2 diferrent time zones)
But it is!
The time that has passed since an event can't be in a timezone. Count of seconds since epoch is how many seconds have passed since that event, it's relative time that has passed, it's distance in time. No matter in which timezone you are, no matter if daylight saving time or not, the time that has passed since an event is the same in every location (well, excluding relativistic effects, which we don't care about). Timezone is irrelevant. mktime returns the number of seconds since epoch. stat returns timespec which represents time that have passed since epoch. Timezone has nothing to do here. Once you represent time as relative to some event (ie. since epoch), then just compare them.
I'm trying to run the below code. I'm compiling like so.
gcc -o time time.c
Instead of getting my desired output, the current time of day, hor minute, year, etc.
Today is : Thu Oct 31 02:01:37 2019
Time is : 02:01:37 am
Date is : 31/10/2019
I'm getting this instead. I've tried a variety of different time functions similar to this and I keep getting this below output instead of what I want.
real 0m0.000s
user 0m0.000s
sys 0m0.000s
I'm sure this is really simple, can anyone point me in the right direction?
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
// Print current date and time in C
int main(void){
// variables to store date and time components
int hours, minutes, seconds, day, month, year;
// time_t is arithmetic time type
time_t now;
// Obtain current time
// time() returns the current time of the system as a time_t value
time(&now);
// Convert to local time format and print to stdout
printf("Today is : %s", ctime(&now));
// localtime converts a time_t value to calendar time and
// returns a pointer to a tm structure with its members
// filled with the corresponding values
struct tm *local = localtime(&now);
hours = local->tm_hour; // get hours since midnight (0-23)
minutes = local->tm_min; // get minutes passed after the hour (0-59)
seconds = local->tm_sec; // get seconds passed after minute (0-59)
day = local->tm_mday; // get day of month (1 to 31)
month = local->tm_mon + 1; // get month of year (0 to 11)
year = local->tm_year + 1900; // get year since 1900
// print local time
if (hours < 12) // before midday
printf("Time is : %02d:%02d:%02d am\n", hours, minutes, seconds);
else // after midday
printf("Time is : %02d:%02d:%02d pm\n", hours - 12, minutes, seconds);
// print current date
printf("Date is : %02d/%02d/%d\n", day, month, year);
return 0;
}
It seems to me you are calling the time command somehow, rather than your program.
Typically you will want to do:
./time
Instead of:
time
I'm writing a function that, among other things, must print out an elapsed time counter. It receives by reference a start time _start, and compares it to current, both typed as time_t. I want to use strftime() to print out the observed time delta in ISO 8601 format. Here's what I attempted to do:
// Negative start time implies program has not begun
if (*_start > 0) {
time_t elapsed = current - *_start;
strftime(time_str, sizeof(time_str) - 1, "%T", localtime(&elapsed));
printf("%s\n", time_str);
}
And here is the output I get immediately after running the program
01:00:00
01:00:00
01:00:01
01:00:01
01:00:01
The seconds work fine, if I let it run longer they get incremented as expected, so do the minutes, however the hour starts as 01 as opposed to 00. Why is this happening? How can I get the hours to start zeroed, like the minutes?
time_t typically (see edit) stores absolute timestamps (the number of seconds since midnight UTC, January 1, 1970). By calculating current - *_start, you're getting elapsed time in seconds (as desired), but by then passing that to localtime and strftime, you're telling the computer to take the time elapsed since the start of your program and treat it as the time elapsed since midnight UTC 1-1-1970.
I'm guessing that happens to be 01:00:00 in your system's local time zone.
I'm not aware of a C99 function to print elapsed time, but it's not hard to write one yourself.
void format_elapsed_time(char *time_str, int total_seconds_elapsed) {
int hours_elapsed = total_seconds_elapsed / 3600;
int minutes_elapsed = (total_seconds_elapsed % 3600) / 60;
int seconds_elapsed = total_seconds_elapsed % 60;
sprintf(time_str, "%02i:%02i:%02i", hours_elapsed, minutes_elapsed, seconds_elapsed);
}
Edit: As #chux points out, time_t doesn't have to store timestamps as seconds since 1-1-1970. See this answer for details.
To portably find the number of elapsed seconds between 2 time_t, use difftime() as subtracting 2 time_t values is not specified in C to be a difference in seconds.
double difftime(time_t time1, time_t time0); C11dr ยง7.26.2.2
The difftime function returns the difference expressed in seconds as a double.
double elapsed_seconds = difftime(current, *_start);
printf("elapsed_seconds: %f\n", elapsed_seconds);
If you want to use this method, you should be aware that time() returns UTC. If you then subtract that from your local time (as returned by localtime()), time zones will have an effect on the result (it's likely your particular time zone is removed from UTC by one hour).
Consider the following complete program, similar to your snippet:
#include <stdio.h>
#include <time.h>
#include <unistd.h>
int main(void) {
time_t start = time(0);
char time_str[100];
for (int i = 5; i > 0; i--) {
sleep (1);
time_t elapsed = time(0) - start;
strftime(time_str, sizeof(time_str) - 1, "%T", localtime(&elapsed));
printf("%s\n", time_str);
}
return 0;
}
Also consider that my particular time zone is removed from UTC by eight hours:
pax> date ; date -u
Thursday 30 March 10:25:57 AWST 2017
Thursday 30 March 02:25:57 UTC 2017
When I run it, I see:
08:00:01
08:00:02
08:00:03
08:00:04
08:00:05
You can see there that the eight-hour time difference from UTC affects the value. The fix for that is actually quite simple: don't use local time at all. If you replace the localtime() call with gmtime(), you get the output you expect:
00:00:01
00:00:02
00:00:03
00:00:04
00:00:05
I realize this question already has an answer, but I thought I might shift the focus a little and expand on the accepted answer. Since the OP specified that they are dealing with elapsed times, one can avoid dealing with absolute timestamps altogether using the clock() function.
clock_t start = clock();
.
.
clock_t end = clock();
double elapsed = (end - start) / (double)CLOCKS_PER_SEC;
then you can call the notionally revised format_elapsed_time() function, like so:
void format_elapsed_time(char *time_str, double elapsed) {
int h, m, s, ms;
h = m = s = ms = 0;
ms = elapsed * 1000; // promote the fractional part to milliseconds
h = ms / 3600000;
ms -= (h * 3600000);
m = ms / 60000;
ms -= (m * 60000);
s = ms / 1000;
ms -= (s * 1000);
sprintf(time_str, "%02i:%02i:%02i.%03i", h, m, s, ms);
}
Is there any way of converting milliseconds to date in C?
What I am trying to do is write a small application in C that can return the financial year and the like(quarter, week) given the start month and isCurentYear bool, where the input might be milliseconds or a date!
In the first place, is there any way by which this can be achieved in C?
And if so, in the process of finding out a way of converting milliseconds to date
I have found out that the use of time_t takes the current millis of our system and by creating a structure pointing to it,it permits us to extract the year,month, date, sec etc!
Refer the below code:
#include <sys/time.h>
#include<stdio.h>
#include<time.h>
void main()
{
time_t t = time(000);
//time_t t = time(0);
struct tm tm = *localtime(&t);
printf("now: %d-%d-%d %d:%d:%d\n", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
tm.tm_hour, tm.tm_min, tm.tm_sec);
}
And also, can time_t be used to store millisecond values so that it can be converted to date using tm struct?
Function time_t time(time_t* timer) returns the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC. In addition, if the input argument timer != NULL, then the function also sets this argument to the same value (so you probably have no reason to call it with anything else but NULL).
Function struct tm* localtime(const time_t* timer) takes the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC, and returns a structure that represents the equivalent time & date. If you're working on a multi-threaded application, then please note that this function is not thread safe.
As to your question - is there any way for converting milliseconds to time & date - yes, but:
Take into consideration that the milliseconds will be considered as of 00:00 hours, Jan 1, 1970 UTC.
Since the time_t type is 32-bit long, you will not be able to convert 4G*1000 milliseconds or more.
Here is a function for converting milliseconds to time & date:
struct tm* GetTimeAndDate(unsigned long long milliseconds)
{
time_t seconds = (time_t)(milliseconds/1000);
if ((unsigned long long)seconds*1000 == milliseconds)
return localtime(&seconds);
return NULL; // milliseconds >= 4G*1000
}
For those of us who were searching the web for an answer to apply to embedded c applications, think pic 32 programming here is the mathematical calculation:
Date in Epoch_seconds = ( (epoch_seconds / 1000) / 86400 ) + 25569
Resulting in a 5 digit answer which is 10 bits long format dd/MM/yyyy
(Note: the slashes are encoded in the result here so when converting to human readable date please account for it)
Where one day = 86400 ms
and the date 1970/1/1 = 25569
example:=( (1510827144853/1000) / 86400 ) + 25569 = 43055
put 43055 in excel and format cell to date dd/MM/yyyy and it gives you 16/11/2017
Perhaps, you are looking for strftime function.
char text[100];
time_t now = time(NULL);
struct tm *t = localtime(&now);
strftime(text, sizeof(text)-1, "%d %m %Y %H:%M", t);
printf("Current Date: %s", text);
How do I do the above? There is mktime function but that treats the input as expressed in local time but how do i perform the conversion if my input tm variable happens to be in UTC.
Use timegm() instead of mktime()
for those on windows, the below function is available:
_mkgmtime
link for more info: https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/mkgmtime-mkgmtime32-mkgmtime64
Here is a solution I use (Can't recall where I found it) when it isn't a windows platform
time_t _mkgmtime(const struct tm *tm)
{
// Month-to-day offset for non-leap-years.
static const int month_day[12] =
{0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334};
// Most of the calculation is easy; leap years are the main difficulty.
int month = tm->tm_mon % 12;
int year = tm->tm_year + tm->tm_mon / 12;
if (month < 0) { // Negative values % 12 are still negative.
month += 12;
--year;
}
// This is the number of Februaries since 1900.
const int year_for_leap = (month > 1) ? year + 1 : year;
time_t rt = tm->tm_sec // Seconds
+ 60 * (tm->tm_min // Minute = 60 seconds
+ 60 * (tm->tm_hour // Hour = 60 minutes
+ 24 * (month_day[month] + tm->tm_mday - 1 // Day = 24 hours
+ 365 * (year - 70) // Year = 365 days
+ (year_for_leap - 69) / 4 // Every 4 years is leap...
- (year_for_leap - 1) / 100 // Except centuries...
+ (year_for_leap + 299) / 400))); // Except 400s.
return rt < 0 ? -1 : rt;
}
The answer of Loki Astari was a good start, timegm is one of the possible solutions. However, the man page of timegm gives a portable version of it, as timegm is not POSIX-compliant. Here it is:
#include <time.h>
#include <stdlib.h>
time_t
my_timegm(struct tm *tm)
{
time_t ret;
char *tz;
tz = getenv("TZ");
if (tz)
tz = strdup(tz);
setenv("TZ", "", 1);
tzset();
ret = mktime(tm);
if (tz) {
setenv("TZ", tz, 1);
free(tz);
} else
unsetenv("TZ");
tzset();
return ret;
}
timegm() works, but is not present on all systems.
Here's a version that only uses ANSI C. (EDIT: not strictly ANSI C! I'm doing math on time_t, assuming that the units are in seconds since the epoch. AFAIK, the standard does not define the units of time_t.) Note, it makes use of a hack, so-to-speak, to determine the machine's time zone and then adjusts the result from mktime accordingly.
/*
returns the utc timezone offset
(e.g. -8 hours for PST)
*/
int get_utc_offset() {
time_t zero = 24*60*60L;
struct tm * timeptr;
int gmtime_hours;
/* get the local time for Jan 2, 1900 00:00 UTC */
timeptr = localtime( &zero );
gmtime_hours = timeptr->tm_hour;
/* if the local time is the "day before" the UTC, subtract 24 hours
from the hours to get the UTC offset */
if( timeptr->tm_mday < 2 )
gmtime_hours -= 24;
return gmtime_hours;
}
/*
the utc analogue of mktime,
(much like timegm on some systems)
*/
time_t tm_to_time_t_utc( struct tm * timeptr ) {
/* gets the epoch time relative to the local time zone,
and then adds the appropriate number of seconds to make it UTC */
return mktime( timeptr ) + get_utc_offset() * 3600;
}
The following implementation of timegm(1) works swimmingly on Android, and probably works on other Unix variants as well:
time_t timegm( struct tm *tm ) {
time_t t = mktime( tm );
return t + localtime( &t )->tm_gmtoff;
}
POSIX page for tzset, describes global variable extern long timezone which contains the local timezone as an offset of seconds from UTC. This will be present on all POSIX compliant systems.
In order for timezone to contain the correct value, you will likely need to call tzset() during your program's initialization.
You can then just subtract timezone from the output of mktime to get the output in UTC.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
time_t utc_mktime(struct tm *t)
{
return (mktime(t) - timezone) - ((t->tm_isdst > 0) * 3600);
}
int main(int argc, char **argv)
{
struct tm t = { 0 };
tzset();
utc_mktime(&t);
}
Note: Technically tzset() and mktime() aren't guaranteed to be threadsafe.
If a thread accesses tzname, [XSI] [Option Start] daylight, or timezone [Option End] directly while another thread is in a call to tzset(), or to any function that is required or allowed to set timezone information as if by calling tzset(), the behavior is undefined.
...but the majority of implementations are. GNU C uses mutexes in tzset() to avoid concurrent modifications to the global variables it sets, and mktime() sees very wide use in threaded programs without synchronization. I suspect if one were to encounter side effects, it would be from using setenv() to alter the value of TZ as done in the answer from #liberforce.
I was troubled by the issue of mktime() as well. My solution is the following
time_t myTimegm(std::tm * utcTime)
{
static std::tm tmv0 = {0, 0, 0, 1, 0, 80, 0, 0, 0}; //1 Jan 1980
static time_t utcDiff = std::mktime(&tmv0) - 315532801;
return std::mktime(utcTime) - utcDiff;
}
The idea is to get the time difference by calling std::mktime() with a known time (in this case 1980/01/01) and subtract its timestamp (315532801). Hope it helps.
Here's my take, which is based exclusively on time_t/tm conversion functions, and the only presumption it makes about time_t is that it is linear:
Pretending against better knowledge the tm structure holds local time (non-DST if anyone asks; it doesn't matter, but must be consistent with step 3), convert it to time_t.
Convert the date back into a tm structure, but this time in UTC representation.
Pretending against better knowledge that tm structure to also hold local (non-DST if anyone asks, but more importantly consistent with step 1), and convert it to time_t once more.
From the two time_t results I can now compute the difference between local time (non-DST if anyone asks) and UTC in time_t units.
Adding that difference to the first time_t result gives me the proper time in UTC.
Note that computation of the difference can conceivably be done once, and then applied later to as many dates as desired; this might be a way to solve issues arising from the lack of thread-safety in gmtime.
(Edit: Then again, this might cause issues if the time zone is changed between the date used to compute the offset, and the date to be converted.)
tm tt;
// populate tt here
tt.tm_isdst = 0;
time_t tLoc = mktime(&tt);
tt = *gmtime(&tLoc);
tt.tm_isdst = 0;
time_t tRev = mktime(&tt);
time_t tDiff = tLoc - tRev;
time_t tUTC = tLoc + tDiff;
Caveat: If the system uses a TAI-based time_t (or anything else that does respect leap seconds), the resulting time may be off by 1 second if applied to a point in time close to a leap second insertion.
This is really a comment with code to address the answer by Leo Accend:
Try the following:
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
/*
* A bit of a hack that lets you pull DST from your Linux box
*/
time_t timegm( struct tm *tm ) { // From Leo's post, above
time_t t = mktime( tm );
return t + localtime( &t )->tm_gmtoff;
}
main()
{
struct timespec tspec = {0};
struct tm tm_struct = {0};
if (gettimeofday(&tspec, NULL) == 0) // clock_gettime() is better but not always avail
{
tzset(); // Not guaranteed to be called during gmtime_r; acquire timezone info
if (gmtime_r(&(tspec.tv_sec), &tm_struct) == &tm_struct)
{
printf("time represented by original utc time_t: %s\n", asctime(&tm_struct));
// Go backwards from the tm_struct to a time, to pull DST offset.
time_t newtime = timegm (&tm_struct);
if (newtime != tspec.tv_sec) // DST offset detected
{
printf("time represented by new time_t: %s\n", asctime(&tm_struct));
double diff = difftime(newtime, tspec.tv_sec);
printf("DST offset is %g (%f hours)\n", diff, diff / 3600);
time_t intdiff = (time_t) diff;
printf("This amounts to %s\n", asctime(gmtime(&intdiff)));
}
}
}
exit(0);
}
For all timezones and at all times would be exceedingly difficult if not impossible. You would need an accurate record of all the various arbitrary timezone and daylight savings time (DST) decrees. Sometimes, it is not clear who the local authority is, never mind what was decreed and when. Most systems, for example, are off by one second for uptime (time system has been up) or boottime (timestamp system booted), if a leap second was spanned. A good test would be a date that was once in DST but now is not (or vis versa). (It was not too long ago in the US that it changed.)