I am working on an Linux based router. I am working on a C application. I want to get current time in my application continuously.
The problem is, it gives me time according to the application launch timezone although I have changed timezone after the application started. The timezone of the system has been changed. The date command on Linux terminal shows different timezone and date/time.
time_t currTm;
struct tm *loctime;
char udrTime[50];
while (1)
{
currTm = time(NULL);
loctime = localtime(&currTm);
strftime(udrTime, sizeof(udrTime), "%Y-%m-%d %H:%M:%S", loctime);
printf("udr_time = %s\n", udrTime);
usleep(10000);
}
I expect output according to timezone changes.
To change the timezone from within the application just set TZ environment variable, nothing else is necessary:
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
void print_time(time_t t) {
char buf[256];
strftime(buf, sizeof buf, "%H:%M:%S", localtime(&t));
printf("%s %s\n", getenv("TZ"), buf);
}
int main() {
time_t t = time(NULL);
setenv("TZ", "Europe/London", 1);
print_time(t);
setenv("TZ", "America/New_York", 1);
print_time(t);
return 0;
}
Outputs:
Europe/London 15:48:58
America/New_York 10:48:58
Related
I have written the following code to convert date to timestamp.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
int main()
{
struct tm date_time;
char date_time_buf[255];
char date_time_hdr[255]={0};
strncpy(date_time_hdr,"Thu, 02 Mar 2017 05:54:28 GMT",255);
memset(&date_time, 0, sizeof(struct tm));
strptime(date_time_hdr, "%a, %d %b %Y %H:%M:%S %Z", &date_time);
memset(date_time_buf, 0, 255);
strftime(date_time_buf, 255, "%s", &date_time);
int p=atoi(date_time_buf);
printf("time is %d \r\n", p);
return 0;
}
I am able to convert date to timestamp. But facing an issue.
The timestamp is offset by 5 hrs 30 minutes which is the timezone of my linux machine. But I don't want that. Is there a way to ignore system timezone?
Instead of using strftime() to format the broken down time struct tm as an integer, and parsing it using atoi(), you can use the simple but nonstandard timegm() function.
Even though timegm() is not in POSIX, it is provided by most POSIXy systems. If you want your code to be portable across POSIXy systems, you can use a workaround (as described in some versions of the timegm man page):
#define _POSIX_C_SOURCE 200809L
#define _XOPEN_SOURCE
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <errno.h>
time_t alt_timegm(struct tm *from)
{
char *tz;
time_t result;
int saved_errno;
tz = getenv("TZ");
if (tz)
tz = strdup(tz);
setenv("TZ", "", 1); /* TZ empty refers to UTC */
tzset();
errno = 0;
result = mktime(from);
saved_errno = errno;
setenv("TZ", tz, 1);
free(tz);
errno = saved_errno;
return result;
}
This workaround does have the drawback that it temporarily modifies the current timezone, which affects other threads executing any timezone-related functions. In a single-threaded process this is not an issue (as the timezone-related functions are not async-signal safe, so they aren't to be used in signal handlers).
In multithreaded programs, one should serialize all accesses to the timezone related functions -- alt_timegm() above, mktime(), localtime(), and so on --, to ensure each function is run with the proper timezone set.
The timegm() implementations in Linux and BSDs are thread-safe; that is, they do not modify the timezone.
Using the above, parsing a string using an strptime() format to an Unix timestamp is easy:
const char *parse_utc(const char *s, const char *format, time_t *to)
{
char *result;
struct tm t_parts;
time_t t;
if (!s || !format) {
errno = EINVAL;
return NULL;
}
result = strptime(s, format, &t_parts);
if (!result) {
errno = EINVAL;
return NULL;
}
errno = 0;
t = alt_timegm(&t_parts);
if (to)
*to = result;
return (const char *)result;
}
The above parse_utc() parses a string s using strptime() format format, saving the UTC timestamp to to if not NULL, returning the pointer to the first unparsed character in string s.
Setting errno = 0 in the above functions may look strange, but it is because the mktime() (and timegm()) function may or may not set errno in case of an error; it just returns (time_t)-1. This means that it is impossible to reliably determine whether a (time_t)-1 (corresponding to last second of year 1969 in UTC`) is an actual valid timestamp, or an error return.
In other words, if errno != 0 after a call to parse_utc(), there was an error. (Note that if the string or format was invalid, parse_utc() returns NULL, with errno == EINVAL.) If errno == 0 but *to == (time_t)-1, it depends on the architecture whether the time string actually referred to the last second of year 1969 in UTC, or if it was an error; we simply do not know.
Using C, I want to convert a UNIX Timestamp number to several usual date data.
How do I convert a UNIX timestamp like 12997424 to different numbers representing seconds, minutes, hours and days while using C?
Use gmtime or localtime from standard library. Prototypes are defined in time.h.
ADDED & EDITED:
So for example, the following code prints current timestamp, hour and minute:
#include <stdio.h>
#include <time.h>
void main() {
time_t t;
struct tm ttm;
t = time(NULL);
printf("Current timestamp: %d\n", t);
ttm = * localtime(&t);
printf("Current time: %02d:%02d\n", ttm.tm_hour, ttm.tm_min);
}
Here's an example of how to use localtime to convert time_t to tm as local time (credit goes to www.cplusplusreference.com):
#include <stdio.h>
#include <time.h>
int main() {
time_t rawtime;
struct tm * timeinfo;
time (&rawtime);
timeinfo = localtime (&rawtime);
printf ("Current local time and date: %s", asctime(timeinfo));
return 0;
}
In shell scripting, when ever I want the local time I do something like
date +%s
from the command line and it returns me the current date and time in this format "1343221713"
I am wondering whether there is a way to achieve the same result in C
Use time.h library in C
example:
#include <stdio.h>
#include <time.h>
int main()
{
/* Obtain current time as seconds elapsed since the Epoch. */
time_t clock = time(NULL);
/* Convert to local time format and print to stdout. */
printf("Current time is %s", ctime(&clock));
return 0;
}
see more examples:
http://en.wikipedia.org/wiki/C_date_and_time_functions
More flexible than time(3) is gettimeofday(3) as declared in sys/time.h
#include <sys/time.h>
#include <stdio.h>
int main(void)
{
struct timeval tv = {0};
gettimeofday(&tv, NULL);
printf("seconds since epoch %ld, microseconds %ld\n", tv.tv_sec, tv.tv_usec);
return 0;
}
I am wondering is there any function that would return the current time in seconds, just 2 digits of seconds? I'm using gcc 4.4.2.
The following complete program shows you how to access the seconds value:
#include <stdio.h>
#include <time.h>
int main (int argc, char *argv[]) {
time_t now;
struct tm *tm;
now = time(0);
if ((tm = localtime (&now)) == NULL) {
printf ("Error extracting time stuff\n");
return 1;
}
printf ("%04d-%02d-%02d %02d:%02d:%02d\n",
tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday,
tm->tm_hour, tm->tm_min, tm->tm_sec);
return 0;
}
It outputs:
2010-02-11 15:58:29
How it works is as follows.
it calls time() to get the best approximation to the current time (usually number of seconds since the epoch but that's not actually mandated by the standard).
it then calls localtime() to convert that to a structure which contains the individual date and time fields, among other things.
at that point, you can just de-reference the structure to get the fields you're interested in (tm_sec in your case but I've shown a few of them).
Keep in mind you can also use gmtime() instead of localtime() if you want Greenwich time, or UTC for those too young to remember :-).
A more portable way to do this is to get the current time as a time_t struct:
time_t mytime = time((time_t*)0);
Retrieve a struct tm for this time_t:
struct tm *mytm = localtime(&mytime);
Examine the tm_sec member of mytm. Depending on your C library, there's no guarantee that the return value of time() is based on a number of seconds since the start of a minute.
You can get the current time with gettimeofday (C11), time (Linux), or localtime_r (POSIX); depending on what calendar & epoch you're interested. You can convert it to seconds elapsed after calendar epoch, or seconds of current minute, whichever you are after:
#include <stdio.h>
#include <string.h>
#include <time.h>
int main() {
time_t current_secs = time(NULL);
localtime_r(¤t_secs, ¤t_time);
char secstr[128] = {};
struct tm current_time;
strftime(secstr, sizeof secstr, "%S", ¤t_time);
fprintf(stdout, "The second: %s\n", secstr);
return 0;
}
You want to use gettimeofday:
man 2 gettimeofday
#include <stdio.h>
#include <sys/time.h>
int main (int argc, char **argv)
{
int iRet;
struct timeval tv;
iRet = gettimeofday (&tv, NULL); // timezone structure is obsolete
if (iRet == 0)
{
printf ("Seconds/USeconds since epoch: %d/%d\n",
(int)tv.tv_sec, (int)tv.tv_usec);
return 0;
}
else
{
perror ("gettimeofday");
}
return iRet;
}
This is better to use then time(0), because you get the useconds as well, atomically, which is the more common use case.
I represent dates using seconds (and microseconds) since 1970 as well as a time zone and dst flag. I want to print a representation of the date using strftime, but it uses the global value for timezone (extern long int timezone) that is picked up from the environment. How can I get strftime to print the zone of my choice?
The following program sets the UNIX environment variable TZ with your required timezone and then prints a formatted time using strftime.
In the example below the timezone is set to U.S. Pacific Time Zone .
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main (int argc, char *argv[])
{
struct tm *mt;
time_t mtt;
char ftime[10];
setenv("TZ", "PST8PDT", 1);
tzset();
mtt = time(NULL);
mt = localtime(&mtt);
strftime(ftime,sizeof(ftime),"%Z %H%M",mt);
printf("%s\n", ftime);
}
Change timezone via setting timezone global variable and use localtime to get the time you print via strftime.