CODEWITHSUNDEEP
cdatetime
paul lanken
paul lanken

Reputation: 197

baffled by output result from strftime()

After closely looking at time.h more than a few times I wrote the following func : 

void output_date ( int day, int month, int year ) {
    char buffer[64] = "";
    struct tm *e_time = calloc( (size_t) 1, sizeof(struct tm) );

    e_time->tm_year = year - 1900;
    e_time->tm_mon = month - 1;
    e_time->tm_mday = day;
    e_time->tm_hour = 0;
    e_time->tm_min = 0;
    e_time->tm_sec = 0;
    e_time->tm_isdst = -1;

    /* strftime ( buffer, 64, (char *)0, e_time ); */
    strftime ( buffer, 64, "%a %b %e %H:%M:%S %Z (%z) %Y", e_time );

    printf ( "%s\n", buffer );

    free(e_time);
    e_time = NULL;

}

I then called this function with a range of possible inputs and saw nothing but bizarre output like so : 

Sun Jul  8 00:00:00  () 2013
Sun Jul  9 00:00:00  () 2013
Sun Jul 10 00:00:00  () 2013
Sun Jul 11 00:00:00  () 2013
Sun Jul 12 00:00:00  () 2013

When I test my format string to strftime I see good results thus : 

$ date -u "+%a %b %e %H:%M:%S %Z (%z) %Y"
Sat Oct 12 00:40:05 GMT (+0000) 2013

I even went so far as to single step via a debugger and saw the same bizarre results : 

stopped in main at line 27 in file "flight.c"
   27                       output_date ( day+1, month+1, year );
(dbx) print day+1, month+1, year
day+1 = 1
month+1 = 1
year = 1977

(dbx) step                      
stopped in output_date at line 43 in file "flight.c"
   43       char buffer[64] = "";
(dbx) step
stopped in output_date at line 44 in file "flight.c"
   44       struct tm *e_time = calloc( (size_t) 1, sizeof(struct tm) );
(dbx) step
stopped in output_date at line 46 in file "flight.c"
   46       e_time->tm_year = year - 1900;
(dbx) print e_time
e_time = 0x100101640
(dbx) step        
stopped in output_date at line 47 in file "flight.c"
   47       e_time->tm_mon = month - 1;
(dbx) step
stopped in output_date at line 48 in file "flight.c"
   48       e_time->tm_mday = day;
(dbx) step
stopped in output_date at line 49 in file "flight.c"
   49       e_time->tm_hour = 0;
(dbx) step
stopped in output_date at line 50 in file "flight.c"
   50       e_time->tm_min = 0;
(dbx) step
stopped in output_date at line 51 in file "flight.c"
   51       e_time->tm_sec = 0;
(dbx) step
stopped in output_date at line 52 in file "flight.c"
   52       e_time->tm_isdst = -1;
(dbx) step
stopped in output_date at line 55 in file "flight.c"
   55       strftime ( buffer, 64, "%a %b %e %H:%M:%S %Z (%z) %Y", e_time );
(dbx) print *e_time
*e_time = {
    tm_sec   = 0
    tm_min   = 0
    tm_hour  = 0
    tm_mday  = 1
    tm_mon   = 0
    tm_year  = 77
    tm_wday  = 0
    tm_yday  = 0
    tm_isdst = -1
}
(dbx) step        
stopped in output_date at line 57 in file "flight.c"
   57       printf ( "%s\n", buffer );
(dbx) print buffer
buffer = "Sun Jan  1 00:00:00  () 1977"
(dbx) quit

In fact, all I ever get is a day of the week being Sunday and the correct month with the correct day and year. Not much else seems correct.

Am I missing something obvious ?

Upvotes: 1

Views: 339

Answers (2)

Yu Hao
Yu Hao

Reputation: 122383

You gave the field tm_isdst the value of -1, which means the daylight saving time is not available, change it to 1 if the daylight saving time is in effect, or 0 if it's not.

As @Paul Griffiths points out, calling mktime(e_time) is the better option here, the mktime function will fill the broken down e_time, as it will fix the fileds like tm_wday and tm_day as well. For the field of tm_isdst, it follows this rule:

a positive or zero value for tm_isdst causes the mktime function to presume initially that Daylight Saving Time, respectively, is or is not in effect for the specified time. A negative value causes it to attempt to determine whether Daylight Saving Time is in effect for the specified time.

Upvotes: 1

Crowman
Crowman

Reputation: 25908

You should call mktime(e_time) after populating your struct, but before calling strftime(). You have some other struct members that are not currently being completed. Passing a struct with uninitialized values (or initialized to incorrect values, here, due to your calloc() call) is generally a Bad Thing to do, and mktime() modifies the struct you pass to fill them in for you.

Like so:

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

void output_date ( int day, int month, int year ) {
    char buffer[64] = "";
    struct tm *e_time = calloc( (size_t) 1, sizeof(struct tm) );

    e_time->tm_year = year - 1900;
    e_time->tm_mon = month - 1;
    e_time->tm_mday = day;
    e_time->tm_hour = 0;
    e_time->tm_min = 0;
    e_time->tm_sec = 0;
    e_time->tm_isdst = -1;

    if ( mktime(e_time) < 0 ) {
        fprintf(stderr, "Error getting calendar time.\n");
        exit(EXIT_FAILURE);
    }

    int n = strftime ( buffer, 64, "%a %b %e %H:%M:%S %Z (%z) %Y", e_time );
    printf("Return from strftime() is %d\n", n);

    printf ( "%s\n", buffer );

    free(e_time);
    e_time = NULL;

}

int main(void) {
    output_date(12, 6, 2013);
    return 0;
}

yields:

paul@local:~/src/c/scratch$ ./st
Return from strftime() is 36
Wed Jun 12 00:00:00 EDT (-0400) 2013
paul@local:~/src/c/scratch$

Upvotes: 1

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