strftime issues day of the week - c

Question

char buffer[800];
struct tm str_time;
str_time.tm_mday = Cur_Day;
str_time.tm_mon = Cur_Month - 1;
str_time.tm_year = entries[i].Year_Start - 1900;
int len = strftime(buffer, 100, "%A, %d %B %Y", &str_time);
printf("\n%s\n", buffer);
What about the above results in the day of the week always being Sunday, regardless of values of Cur_Day and Cur_Month?
Sample output:
Sunday, 23 November 2012
------------------------
stuff
Sunday, 25 November 2012
------------------------
stuff
Sunday, 26 November 2012
------------------------
stuff

Answer 1

Your str_time structure (if, as it seems to be, it's a local variable) has indeterminate values in its fields unless you explicitly set them. All strftime does is use the values it has, it doesn't first adjust values to conform other fields.
Since you're not setting tm_wday, it will stay with whatever it was originally (0 by the looks of it, since it's always Sunday).
If you do want to adjust fields based on other fields, you should look into mktime().
From the standard (ISO C99):
The mktime function converts the broken-down time, expressed as local time, in the
structure pointed to by timeptr into a calendar time value with the same encoding as
that of the values returned by the time function.
The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to the ranges indicated above.
On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to the ranges indicated above; the final value of tm_mday is not set until tm_mon and tm_year are determined.
Your best bet is to use time() and localtime() to populate a tm structure, then change the fields you want to change before calling mktime(). That way, you're guaranteed that all fields will have sensible values.
The following program shows one way to do this:
#include <stdio.h>
#include <time.h>
int main (void) {
char buffer[100];
time_t now;
struct tm *ts;
// Get today in local time and output it.
now = time (NULL);
struct tm *ts = localtime (&now);
strftime (buffer, 100, "%A, %d %B %Y", ts);
printf ("Now = %s\n", buffer);
// Advance day-of-month and make new date.
// Probably need to intelligently handle month rollover.
ts->tm_mday++;
mktime (ts);
strftime (buffer, 100, "%A, %d %B %Y", ts);
printf ("Tomorrow = %s\n", buffer);
return 0;
}
The output of that program is:
Now = Tuesday, 09 October 2012
Tomorrow = Wednesday, 10 October 2012
For what it's worth, here's a complete program that uses that method to give you the day of the week for a given date (defaults to today).
You can change the year, month and day of month with the optional -y, -m and -d arguments in any order you want, and as many times as you want, though only the last for each type counts.
#include <stdio.h>
#include <time.h>
static int makeError (char *argVal, char *errStr) {
printf ("Error with argument '%s': %s\n", argVal, errStr);
printf ("Usage: dow [-y<year>] [-m<month>] [-d<day>]\n");
return 1;
}
int main (int argc, char *argv[]) {
int idx, intVal;
char chVal;
char buff[100];
time_t now = time (NULL);
struct tm *nowStr = localtime (&now);
for (idx = 1; idx < argc; idx++) {
chVal = (*argv[idx] != '-') ? '\0' : *(argv[idx] + 1);
if ((chVal != 'y') && (chVal != 'm') && (chVal != 'd'))
return makeError (argv[idx], "does not start with '-y/m/d'");
intVal = atoi (argv[idx] + 2);
if (intVal < 0)
return makeError (argv[idx], "suffix is negative");
sprintf (buff, "%d", intVal);
if (strcmp (buff, argv[idx] + 2) != 0)
return makeError (argv[idx], "suffix is not numeric");
switch (chVal) {
case 'y': nowStr->tm_year = intVal - 1900; break;
case 'm': nowStr->tm_mon = intVal - 1; break;
case 'd': nowStr->tm_mday = intVal; break;
}
}
mktime (nowStr);
strftime (buff, sizeof (buff), "%A, %d %B %Y", nowStr);
printf ("%s\n", buff);
return 0;
}
A sample transcript:
pax> ./dow
Tuesday, 09 October 2012
pax> ./dow -y2011
Sunday, 09 October 2011
pax> ./dow -y2000 -m1 -d1
Saturday, 01 January 2000

Answer 2

The most probable explanation is that your strftime requires tm_wday to have a meaningful value if you're going to ask it to print the day of the week.
This is the simplest available way to avoid having to compute it yourself:
struct tm tm;
memset(&tm, 0, sizeof(struct tm));
tm.tm_mday = Cur_Day;
tm.tm_mon = Cur_Month - 1;
tm.tm_year = entries[i].Year_Start - 1900;
tm.tm_hour = 12;
(void) timegm(&tm); /* fills in the rest of `tm` as a side effect */
/* now call strftime */
If you don't have timegm you MAY be able to get away with using mktime instead (the problems with doing this calculation in 'local' time are largely irrelevant if you only want to print the date). DO NOT use the "portable version of timegm" described in the Linux manpage for timegm, it has portability gotchas waiting to bite on almost every line!

Answer 3

You need to set str_time.tm_wday so that strftime() can convert it.
See localtime(3) and strftime(3) for examples.