Calculating time

ok as a second part of my program i want to make. I want it to check how many weeks has passes since the program was first run. I've been reading the c library time header file to try and get an idea how to do this.

firstly i was going to do this

Code:

time_t tim;
struct tm *currtim;
time(&tim);
currtim = localtime(&time);

However i believe this only assigns the value of how many days months hours etc there has been since 1st of jan 1970 to the values in the currtim structure.

So i thought about using

Code:

char timestr = ascitime(currtim);

This sets the current date and time. However it gives it in a string format. Which means i cannot use difftime() to calculate the difference in times.

finally i thought about using just

Code:

  time_t start;
  time(&start);

Then writing the value to a file. However im not sure how to write something that is a data type time_t to a file correctly, then reading it again and comparting it with endtime.

Any ideas?

Unfortunately, time_t is an implementation dependent type, and not a defined size or format. You could store it as some sort of string, but it would not be portable to other machine / OS / compiler architectures.

You can, however, convert it to a struct tm type, which has well-defined components, and use the "mktime" function to make it back into the integral type of time_t.

Have a look at:
http://www.hmug.org/man/3/ctime.php

--
Mats

Ok i understand what you mean about assigning it to a struct tm type and then using mktime() to make it into a time_t

But the problem i have is how to assign the values of the current date and time to struct tm. any functions ive seen so far only assing how many days months years etc have passed since 1st jan 1970.

Basically the challenge is to work how to get todays date stored in the struct tm structure.

difftime() will take two time_t's and tell you the difference in seconds.

I know that salem. But thats not what im stuck at. The bit im stuck at is getting the current month, year and day into the struct tm structure. If i can get them in there i understand how to use the mktime and difftime functions to get the result.

question is how do i get the current date in the struct tm structure.

gmtime is your friend!

Hey Elysia. I used your advise about gmtime and have come up with this. I dont have a compiler here in work so i'm not sure if works.

Code:

void storeTime(FILE *p1)
{
	time_t tim;
	struct tm *curtim;
	time(&tim);
	curtim =gmtime(&tim);

	p1 = fopen("timestore.txt",w);

        if(p1 != NULL)
		{
			fprintf(p1,"%d %d %d",curtim->tm_mday,curtim->tm_mon,curtim->tm_year);
                        fclose(p1);
		}
	
	
}



void readtime ()       //reads the time which was stored in the file by storetime
{
        FILE *p2       //file pointer
        tm *newtm;     // new tm structure
        time_t timer;  //used with mktime()
        double diff;   //used to store the difference in times

	p2 = fopen("timestore.txt",r);
        
        if(p2 != NULL)
		{
		        fscanf(p2,%d %d %d,newtm->tm_dat,newtm->tm_mon,newtm->tm_year); //scam the times from file
			fclose(p2);
			
		}
        timer = mktime(newtm); // give timer a value


	//create a time_t for current time to compare

	time_t timeNow;
	time(&timeNow);
        diff = difftime(timeNow,timer);
	
	printf("%d : Time difference in seconds",diff);

}

Code:

void storeTime()
{
	FILE* p1;
	time_t tim;
	struct tm* curtim;
	time(&tim);
	curtim = gmtime(&tim);

	p1 = fopen("timestore.txt", "w");
        if(p1 != NULL)
	{
		fprintf(p1,"%d %d %d", curtim->tm_mday, curtim->tm_mon, curtim->tm_year);
		fclose(p1);
	}
}

void readtime()       //reads the time which was stored in the file by storetime
{
	FILE* p2;       //file pointer
	tm* newtm;     // new tm structure
	time_t timer;  //used with mktime()
	double diff;   //used to store the difference in times

	p2 = fopen("timestore.txt", "r");
	if (p2 != NULL)
	{
	        fscanf(p2,"%d %d %d", newtm->tm_mday, newtm->tm_mon, newtm->tm_year); //scam the times from file
		fclose(p2);
	}
        timer = mktime(newtm); // give timer a value

	//create a time_t for current time to compare
	time_t timeNow;
	time(&timeNow);
        diff = difftime(timeNow, timer);
	
	printf("%d : Time difference in seconds",diff);
}

Corrections in red.
Questionable part of the code in blue. If you fail to read the date from the file, you can't compare the dates, can you? Yet, the code will continue even if it fails.

The code compiles. I don't know if it works.

Oh and you don't need to indent the braces and it's a bad idea to mix spaces and tabs. Use one.

Code:

	        fscanf(p2,"%d %d %d", &newtm->tm_mday, &newtm->tm_mon, &newtm->tm_year); //scam the times from file

You need the ADDRESS of the data you are reading from the file.

And are you actually scamming or scanning?

--
Mats

Thanks guys for your help. I'll see if it works when i get home. only 7 hours away lol.