How to change local time using C

[akvseiji]

How to change date and time in PC using C?

I found a function: settimeofday() but don't know how to use it.

Can someone help me pls..

[lloyd_b]

How to change date and time in PC using C?

I found a function: settimeofday() but don't know how to use it.

Can someone help me pls..

In a terminal window:
sudo apt-get install manpages-dev
man settimeofday
This will give you the documentation on the settimeofday() function.

Lloyd B.

[akvseiji]

In a terminal window:
sudo apt-get install manpages-dev
man settimeofday
This will give you the documentation on the settimeofday() function.

Lloyd B.

I've seen the manual of settimeofday().
But still can't understand how it works.
Can someone give me some example on how to use it.

From what I see, timeval struct in settimeofday() only provide seconds and microseconds.
But I want to change the whole system time including date.

[lloyd_b]

I've seen the manual of settimeofday().
But still can't understand how it works.
Can someone give me some example on how to use it.

From what I see, timeval struct in settimeofday() only provide seconds and microseconds.
But I want to change the whole system time including date.

What you need to understand is that the "seconds" in the timeval structure is the number of seconds since the "epoch" (00:00:00 UTC, January 1, 1970). This is the basic unit of time on the system - it is converted, via various functions, to the year/date/hours/minutes/seconds format that you normally see.

So here's a list of steps changing the system time to any arbitrary value:

1. Use gettimeofday() to get a struct timeval.
2. Use localtime() to convert the "seconds" element of timeval into a struct tm (which has all the of individual values you might want to tweak).
3. Change the values in the struct tm.
4. Use mktime() to convert the struct tm back into a time_t, and assign it to the "seconds" element of the struct timeval.
5. Use settimeofday() to apply the changes to the system clock.

I don't have any code handy to show this being done (and I don't feel like writing any at the moment), but between the instructions above and the manpages for those functions you should have enough to work with.

Lloyd B.

[akvseiji]

I've found another function that can change date and time; execl().
Before I use this function, I have to disable sudo password.

When I run, it change my date/time successfully.

This is my code:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <errno.h>

int main()
{
//MMDDhhmmYY.ss
char dTime[13] = "0820102508.30"; //20 Aug 2008 10:25:30am
int n;

n = execl("/usr/bin/sudo", "/bin/date", "date", dTime, NULL);
if (n == -1)
{
fprintf(stdout, "ERRNO[%d]:", errno);
perror(" ");
}

fprintf(stdout, "Date Changed\n\n"); //Confirmation!!

return 0;
}


However, after debugger execute execl(),
it will terminated automatically,
without execute the fprintf() function ("Date Change\n\n").
This is a big problem to me.

I don't understand why it happen.
How to counter this problem?
Can someone help me please...

TQ.

[lloyd_b]

Any "exec...()" function will do the same. They do NOT return to the calling program, the *replace* the calling program with the program that is being executed.

Since you're just using a wrapper for the "date" command, I'd suggest using system() rather than execl():
#include <stdio.h>

int main()
{
system("date 0820102508.30");
printf("Date changed!\n");

return 0;
}

Lloyd B.

[akvseiji]

Hi Lloyd B.

Thank you very much for your help.
I've solve it.

Here is my code:

char dTime[14] = "0915175309.00";
char dInput[24];

sprintf(dInput, "sudo date %s", dTime);

system(dInput);

fprintf(stdout, "Date Changed\n\n");

It works successfully.
Thanks again.

[dwhitney67]

Since you originally asked about settimeofday(), I thought it would (finally) be appropriate to post a short/simple piece of code.


#include <sys/time.h>
#include <time.h>

int main()
{
time_t theTimeInSeconds = time(0);

theTimeInSeconds += (3 * 60 * 60); // add 3 hours (3 * 60 mins * 60 secs) to current time

struct timeval tv = { theTimeInSeconds, 0 };

settimeofday(&tv, 0);

return 0;
}

An attempt to use settimeofday() will fail unless the user running the application has super-user privileges. Thus to run the app above, login as root or use sudo.

P.S. I probably should have used gettimeofday() instead of time(). Here's how:


#include <sys/time.h>

int main()
{
struct timeval tv;

gettimeofday(&tv, 0);

tv.tv_sec += (3 * 60 * 60); // add 3 hours (3 * 60 mins * 60 secs) to current time

settimeofday(&tv, 0);

return 0;
}

[akvseiji]

Hi dwhitney,

From what I see of your post,
settimeofday() cannot change to the specific date,
unless we know what is our current time.
Am I right?

Let say,
If I want to change my date to 14 Sept 2008 - 13h45m20s,
but I don't know my current time,
so, when I use settimeofday(),
how much time I must put to tv.tv_sec?
tv.tv_sec += ????

Can settimeofday() do that?
Or I just miss understand..?

[lloyd_b]

Hi dwhitney,

From what I see of your post,
settimeofday() cannot change to the specific date,
unless we know what is our current time.
Am I right?

Let say,
If I want to change my date to 14 Sept 2008 - 13h45m20s,
but I don't know my current time,
so, when I use settimeofday(),
how much time I must put to tv.tv_sec?
tv.tv_sec += ????

Can settimeofday() do that?
Or I just miss understand..?

The proper way to handle that is to create a struct tm with the values you want, and then pass it to mktime() to get the correct value for tv_sec:

#include <stdio.h>
#include <time.h>
#include <sys/time.h>

int main()
{
struct tm mytime;
struct timeval systime;
char * text_time;

mytime.tm_sec = 20; // Seconds
mytime.tm_min = 45; // Minutes
mytime.tm_hour = 13; // Hours
mytime.tm_mday = 14; // Day of Month
mytime.tm_mon = 8; // Month
mytime.tm_year = 108; // Year


systime.tv_sec = mktime(&mytime);
systime.tv_usec = 0;

settimeofday(&systime, NULL);

/* Time of day is set. Now, let's get it, and verify that it looks right */
gettimeofday(&systime, NULL);

text_time = ctime(&systime.tv_sec);

printf("The system time is set to %s\n", text_time);

return 0;

}
Note: like any program that sets the system time, this must be run with "sudo", or it won't have any effect.

For details on the mktime() command and struct tm, "man mktime" in a terminal window.

Lloyd B.

[akvseiji]

Hi Lloyd B and dwhitney,

Thank you for your help.
Very appreciate that.