Adding two epoch millseconds in C++ - c++

Question

My goal is to determine expiry of an item to when it was acquired(bought) and when it is sold.There is a TTL value associated with each of the item.
I am doing following :
time_t currentSellingTime;
long currentSystemTime = time(&currentSellingTime); // this gives me epoch millisec of now()
long TTL = <some_value>L;
long BuyingTime = <some_value> // this is also in epoch millsec
if(currentSystemTime > TTL+BuyingTime))
{
//throw exception
// item is expired
}
My question is how to sum two epoch millisec and compare it with another epoch millsec in C++

Answer 1

There may be some misconceptions on how time() works:
epoch time as given by time() is expressed in seconds, not millseconds
time returns the current time value and can optionally set current time in the variable given as its sole argument. This means that
long currentSystemTime = time(&currentSellingTime);
will set both currentSystemTime and currentSellingTime to the current time, and that's probably not what you intend to do... You should probably do
long currentSystemTime = time(NULL);
or
time(&currentSellingTime);
but the "double form" you are using is quite suspicious. For completeness' sake the MS Help reference for time()

Answer 2

You want to use another function, as as previously pointed out, time() returns seconds. Try:
#include <time.h>
long current_time() {
struct timespec t;
clock_gettime(CLOCK_REALTIME, &t);
return t.tv.sec * 1000l + t.tv_nsec / 1000000l;
}
Your code should work then. This approach is also POSIX compatible. Example usage:
const long TTL = 100;
long start_time = current_time();
while (!(current_time() > start_time + TTL))
{
// do the stuff that can expire
}
note: I know that the condition in the while loop can be constructed differently, but this way it is more like "until not expired".