Question

I am messing around with the time header and have encountered an error:

invalid conversion from 'tm*' to 'time_t'

You can see the where the error is in the code below all the way at the bottom when I try to use the difftime function. I know im just doing something stupid and illegal but I cant see how I can get around this. If anyone has a solution let me know. Thanks!

The program is supposed to get the current time, take time input from a user, and then find the time difference.

Code:

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

int main ()
{
  time_t rawtime;
  struct tm * timeinfo;
  struct tm * cur_timeinfo_hold;
  int year, month ,day,hour,minute,second;
  double dif;

  /* prompt user for time to be compared*/
  printf ("Enter year: "); scanf ("%d",&year);
  printf ("Enter month: "); scanf ("%d",&month);
  printf ("Enter day: "); scanf ("%d",&day);
  printf ("Enter hour: "); scanf ("%d",&hour);
  printf ("Enter minute: "); scanf ("%d",&minute);
  printf ("Enter second: "); scanf ("%d",&second);

  /* get current timeinfo*/
  time ( &rawtime );
  timeinfo = localtime ( &rawtime );
  /* print it */
  printf("The present time is: "); printf(asctime (timeinfo));

 /* set current time into a new variable to use for difftime, since timeinfo, will be changed */
  cur_timeinfo_hold = timeinfo;

  /* modify current timeinfo to the user's choice */
  timeinfo->tm_year = year - 1900;
  timeinfo->tm_mon = month - 1;
  timeinfo->tm_mday = day;
  timeinfo->tm_hour = hour;
  timeinfo->tm_min = minute;
  timeinfo->tm_sec = second;
  mktime ( timeinfo );
  /* and print it */
  printf ("time to compare: "); printf(asctime (timeinfo));

  /* find time difference */
 //dif = difftime (cur_timeinfo_hold,timeinfo); //error: invalid conversion from 'tm*' to 'time_t'

  return 0;
}

Answer 1

mktime is a function. It accepts one parameter, a tm pointer, and returns a value, a time_t. It does not transform its argument from one type into another. Therefore, after you call mktime(timeinfo), your timeinfo variable is still a tm pointer.

The difftime function expects to receive two time_t values, so it won’t accept cur_timeinfo_hold or timeinfo; they’re the wrong type. The first variable’s value came from converting rawtime to a tm pointer with localtime, so use that for the first parameter. When you later called mktime(timeinfo), it returned a time_t value, but you ignored the return value. Assign the return value to a variable so you can use it for the second difftime parameter.

time_t info = mktime(timeinfo);
// ...
dif = difftime(rawtime, info);