c/c++ - safest way to send time_t over socket

Question

I\'ve set up a C++ server/client environment, and am trying to send a time_t value from the server to the client (an useful thing in any server). But I\'m coming accross a heada

Answer 1

Update: There seems to be a library that provides exactly what you are looking for, check out Apache Portable Runtime, specifically this page on time routines. Other than that, I would say my answer still provides a way to implement this manually, provided all systems are POSIX.1-2001 compliant.

I'm having a similar problem right now and I think it might be nice to have some instructions on how to go through with this. Note that this solution here should be POSIX.1-2001 compatible (on Ubuntu 14.04, man tzset and man localtime gives such information, and I haven't used other sources, really).

Use localtime to convert the data obtained from your call to time into a struct tm (see time.h):

struct tm {
           int tm_sec;         /* seconds */
           int tm_min;         /* minutes */
           int tm_hour;        /* hours */
           int tm_mday;        /* day of the month */
           int tm_mon;         /* month */
           int tm_year;        /* year */
           int tm_wday;        /* day of the week */
           int tm_yday;        /* day in the year */
           int tm_isdst;       /* daylight saving time */
};

Note that localtime will set the variable declared as (again, see time.h)

extern long timezone; //seconds West of UTC 

Since all types are now known, you can write your own conversion tool to make this data network-portable. Things to consider:

• (Preferrably in your software init), you will have to determine how many bytes your int and long have (use sizeof, of course)
• Based on that information, you could do a host-to-network conversion on each integer field of the struct tm (use htonl or htons - for 64bit types, you will have to write your own) and send that information to the client (in whatever way you like).
  If you have a time zone difference, you will also need to convert/send time zone information, i.e. the data stored in the variable timezone (see above).

• The client will then have to

  1. Convert the received data into client byte order
  2. Write it into a struct tm (if time zones are involved, write the received time zone data into a long)
  3. If time zones are involved, write the received timezone data into a long and compute the time zone difference (compute the difference of the received timezone datum and what you get by calling tzset locally...), then adjust the struct tm variable created in step 2.
  4. Having obtained a struct tm adjusted to the client's local time zone, use mktime to convert data back into a time_t
  5. Account for time zone difference, if there is any.