what's wrong with my program ?

[sugaray]

hi, below is my program for doing exercise with library time functions,
something is not right with it, and for the time being i couldn't figure
out what's wrong and where, thanx for your help.

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

double TimeDifference(struct tm *t1,struct tm *t2)
{
return difftime(mktime(t1),mktime(t2));
}


char *ConstructTime(struct tm *time,int month,int day,int year)
{
time->tm_mon=month-1;
time->tm_mday=day;
time->tm_year=year-1900;
mktime(time);
return asctime(time);
}

int main(int argc,char **argv)
{
struct tm now,t1,t2;
char *tmstr;
double diff;
int month,day,year;

scanf("%d%d%d",&month,&day,&year);
tmstr=ConstructTime(&now,month,day,year);
printf("%s",tmstr);
ConstructTime(&t1,6,7,2004);
ConstructTime(&t2,6,9,2004);
diff=TimeDifference(&t1,&t2);
printf("%f\n",diff);


return 0;
}

 

[T.M. Sommers]

hi, below is my program for doing exercise with library time functions,
something is not right with it, and for the time being i couldn't figure
out what's wrong and where, thanx for your help.

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

double TimeDifference(struct tm *t1,struct tm *t2)
{
return difftime(mktime(t1),mktime(t2));
}


char *ConstructTime(struct tm *time,int month,int day,int year)
{
time->tm_mon=month-1;
time->tm_mday=day;
time->tm_year=year-1900;

At this point, all of the members of time except the 3 you just
set contain garbage. You need to set them to some rational
values before going on.

 

[Vijay Kumar R Zanvar]

hi, below is my program for doing exercise with library time functions,
something is not right with it, and for the time being i couldn't figure
out what's wrong and where, thanx for your help.

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

double TimeDifference(struct tm *t1,struct tm *t2)
{
return difftime(mktime(t1),mktime(t2));
}


char *ConstructTime(struct tm *time,int month,int day,int year)
{
time->tm_mon=month-1;
time->tm_mday=day;
time->tm_year=year-1900;
mktime(time);
return asctime(time);
}

int main(int argc,char **argv)
{
struct tm now,t1,t2;
char *tmstr;
double diff;
int month,day,year;

/* adding these removed the misbehaviour */
memset ( &now, 0, sizeof now );
memset ( &t1, 0, sizeof t1 );
memset ( &t2, 0, sizeof t2 );

 

[Richard Bos]

/* adding these removed the misbehaviour */
memset ( &now, 0, sizeof now );
memset ( &t1, 0, sizeof t1 );
memset ( &t2, 0, sizeof t2 );

Not a good solution. First of all, the problem really is in
ConstructTime(). With a name like that, one can expect it to construct a
complete time struct, so it should do so. Second, if you use this
solution, you need to use it for every single time you create, not just
once in the function itself; forget one, and the problem comes back.
Third, this sets the structs to all-bits-zero, not to sensible initial
members; in particular, it does not set tm_isdst to something negative,
which is what I would want it to be.
Oh, and another small bug: it is never a good idea to use the same name
for one of your structs as for a closely related Library function...
A better solution is this:

char *ConstructTime(struct tm *the_time, int month,int day,int year)
{
the_time->tm_mon=month-1;
the_time->tm_mday=day;
the_time->tm_year=year-1900;
the_time->tm_sec=the_time->tm_min=the_time->tm_hour=0;
the_time->tm_wday=the_time->tm_yday=0;
the_time->tm_isdst=-1;
mktime(the_time);
return asctime(the_time);
}

Note also that asctime() returns a pointer to a static array, so if you
want to use two of those strings, copy the first _string_, not the first
pointer, before you get the second one. The second call will over-write
the string written by the first.

Richard

 

[Al Bowers]

hi, below is my program for doing exercise with library time functions,
something is not right with it, and for the time being i couldn't figure
out what's wrong and where, thanx for your help.

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

double TimeDifference(struct tm *t1,struct tm *t2)
{
return difftime(mktime(t1),mktime(t2));
}

Function mktime will return (time_t)-1 should it
fail to convert a calendar time. Therefore the
function arguments must be of the form that will
ensure a conversion. Or, you must modify the function.

char *ConstructTime(struct tm *time,int month,int day,int year)
{
time->tm_mon=month-1;
time->tm_mday=day;
time->tm_year=year-1900;
mktime(time);
return asctime(time);
}

I see two problems in the use of function ConstructTime.

First you only set three members of the struct. All the
other members are indeterminant.

Second, if function mktime is unable to determine a
calendar time, components are not set to represent the
specified calendar time, nor their values forced
to the normal ranges. Using this illegitmate struct
as argument to function asctime may be unpleasant.

A possible fix:

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

double TimeDifference(struct tm *t1,struct tm *t2)
{
return difftime(mktime(t1),mktime(t2));
}


char *ConstructTime(struct tm *time,int month,int day,int year)
{
struct tm tmp = {0};

*time = tmp;
time->tm_mon=month-1;
time->tm_mday=day;
time->tm_year=year-1900;
time->tm_isdst = -1;
return mktime(time)==(time_t)-1?NULL:asctime(time);
}

int main(int argc,char **argv)
{
struct tm now,t1,t2;
char *tmstr1, *tmstr2;
double diff;
int month,day,year;

printf("Enter the date (MM DD YYYY?): ");
fflush(stdout);
scanf("%d%d%d",&month,&day,&year);
tmstr1=ConstructTime(&now,month,day,year);
if(tmstr1) printf("%s\n",tmstr1);
tmstr1 = ConstructTime(&t1,6,7,2004);
if(tmstr1) printf("Time 1: %s",tmstr1);
tmstr2 = ConstructTime(&t2,6,8,2004);
if(tmstr2) printf("Time 2: %s", tmstr2 );
if(tmstr1 && tmstr2)
{
diff=TimeDifference(&t2,&t1);
printf("Difference is %.f secconds\n",diff);
}
return 0;
}

 

[Martin Ambuhl]

hi, below is my program for doing exercise with library time functions,
something is not right with it, and for the time being i couldn't figure
out what's wrong and where, thanx for your help.

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

double TimeDifference(struct tm *t1,struct tm *t2)
{
return difftime(mktime(t1),mktime(t2));
}


char *ConstructTime(struct tm *time,int month,int day,int year)
{
time->tm_mon=month-1;
time->tm_mday=day;
time->tm_year=year-1900;
mktime(time);
return asctime(time);
}

You need to initialize the elements of *time (which should be spelt some
way other that "time"). Here's one approach a modern C compiler would like:

char *ConstructTime(struct tm *t, int month, int day, int year)
{
*t = (struct tm) {
.tm_mon = month - 1,.tm_mday = day,.tm_year =
year - 1900,.tm_isdst = -1};
mktime(t);
return asctime(t);
}

An approach that doesn't need the anonymous struct is to declare a local
struct tm t_tmp, with the proper base values, and copy it: *t = t_tmp;

 

[sugaray]

char *ConstructTime(struct tm *t, int month, int day, int year)
{
*t = (struct tm) {
.tm_mon = month - 1,.tm_mday = day,.tm_year =>
year - 1900,.tm_isdst = -1};
mktime(t);
return asctime(t);
}

An approach that doesn't need the anonymous struct is to declare a local
struct tm t_tmp, with the proper base values, and copy it: *t = t_tmp;

so, why use a temp variable rather set the values directly to *t ?