CODEWITHSUNDEEP
c
dhein
dhein

Reputation: 6555

mktime returns -1 but I'm expecting my tm struct to be valid

timeStruct  @0x13b123b0 tm
    tm_hour     0   int
    tm_isdst    0   int
    tm_mday     1   int
    tm_min      0   int
    tm_mon      0   int
    tm_sec      33  int
    tm_wday     4   int
    tm_yday     0   int
    tm_year     70  int

This is my struct as represented by QTcreators memory watch.

with this as parameter to a call of mktime()

struct tm *timeStruct;
//[...]
time_t timeOrigin = mktime(timeStruct);

timeOrigin becomes -1.

There are questions out here asking for the same.

But they all were solved by the hint, that the tm_year field is not years since 1970 but years since 1900. I'm aware of this and also respecting it.

What confuses me further is:

man pages like: http://linux.die.net/man/3/mktime

explain that mktime doesn't change the structs members if it returns -1. In my case it did adjust in previous cases tm_wday and tm_yday, while returning -1.

I can't find any error regarding this by reading stderr, after mktime returning -1 aswell.

So whats going on here?

Upvotes: 2

Views: 772

Answers (2)

chqrlie
chqrlie

Reputation: 144695

mktime returns -1 because the tm structure values, interpreted as a time and date expressed in local time, fall outside the valid range. As quoted from the man page: If the specified broken-down time cannot be represented as calendar time (seconds since the Epoch), mktime() returns (time_t)-1 and does not alter the members of the broken-down time structure.

You are probably located east of Greenwich, meaning that 1/1/1970 0:00:00 local time falls before the beginning of the Unix Epoch that started on 1/1/1970 0:00:00 UTC.

Upvotes: 3

cwschmidt
cwschmidt

Reputation: 1204

mktime expects a pointer of type struct tm as it's first argument.

Here is an example, how you can use it:

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

int main(void)
{
    struct tm tm = *localtime(&(time_t){time(NULL)});
    printf("Today is           %s", asctime(&tm));
    tm.tm_mon -= 100;  // tm_mon is now outside its normal range
    time_t t = mktime(&tm); // recalculate tm
    printf("100 months ago was %s", asctime(&tm));
}

Upvotes: 0

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