Question

I’ve the following piece of code to find the difference between the UTC and local time zone.

struct tm *local_time, *gmt_time;
    time_t t = time(NULL);
    local_time = localtime(&t);
    gmt_time = gmtime(&t);
    int y = mktime(local_time);
    int x = mktime(gmt_time);       
    tzone_diff = y - x;

This doesn’t work. However if i move the statement gmt_time = gmtime(&t) below int y = mktime(local_time); it works. That is the following piece of code works:

struct tm *local_time, *gmt_time;
time_t t = time(NULL);
local_time = localtime(&t);
int y = mktime(local_time);
gmt_time = gmtime(&t);
int x = mktime(gmt_time);       
tzone_diff = y - x;

This seems wierd to me.. Any clues?

Answer 1

localtime and gmtime both returns a pointer to a struct tm. This struct tm is defined somewhere in the C library, and can easily be the same for both functions. Something like the below.

struct tm temp;

struct tm * gmtime() {
  ...
  return &temp;
}

struct tm * localtime() {
  ...
  return &temp;
}