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

First, I think you need to decide if you want to deal with the issue that the C standard leaves the meaning of time_t overly vague (it's not necessarily represented in seconds and doesn't even have any meaningful numerical properties like order/comparison). This is contrary to the behavior of every existing and historical implementation, where time_t is in seconds. Both C and POSIX also allow time_t to be a floating point type; as far as I know, no implementations make use of this, and on POSIX it would be rather harmful since the value of time_t has to be an integer the way it's used in struct timespec, etc.

If you decide you're happy assuming time_t is always an integral number of seconds since the epoch, i.e. the values are meaningful for interchange between systems, then it's just a matter of formatting them. The safest thing to do would be to simply cast to a integer type that's large enough to store any meaningful value and that's the same size on all systems: that would be int64_t. Then use whatever normal means you use for serializing int64_t in a way that's immune to endian differences.

If on the other hand you want to be "absolutely" portable, you should compute your own time_t value for "the epoch" (either the standard one or your own choice of epoch), then use difftime to convert to a double representing "seconds since the epoch", and format the double with snprintf(buf, sizeof buf, "%.0f", diff). Note that computing a time_t value for the epoch in portable C is actually quite difficult because most of the standard functions work in local time, whereas you need universal time. There are tricks you can do with the gmtime, mktime, and localtime functions to figure it out, but it's nontrivial...