Question

It seems remarkably difficult to find out what a valid range of values is for time_t.

It is on some platforms 32 bit, on most 64 bit, and so can easily enough be set to LONG_MAX. However trying to then use that value doesn’t really work properly. For instance you can’t pass it to localtime and change it into a struct tm.

A quick test program to binary search the value tells me it is 67768036191676799. That corresponds to the end of year 2147483647, so that makes sense as a value. But is it this specified anywhere, and is there any reasonable, platform independent value for a maximum usable time_t?

Answer 1

Indeed, the specification for time_t and clock_t are implementation defined (C99 7.23.1).

This is one of those things where I would recommend not generating these values yourself, but rely on the implementation to create them for you such as with mktime(), and use the struct tm for directly manipulating the time. -1 is the only value of time_t that is a “good” value you can use on your own.

I would specifically suggest that you not treat it as a 32 bit value of any type, as jgm suggests. You just never know if some strange embedded compiler will want to use a 16 bit time or 18 or who knows.