sys/newlib: gettimeofday() returns time since boot, not current wall time.

[jcarrano]

Description

PR #4286 added support for gettimeofday() by implementing it on top of xtimer_now_usec64().

This definition is in conflict with the specification, which says:

The gettimeofday() function shall obtain the current time, expressed as seconds and microseconds since the Epoch, and store it in the timeval structure pointed to by tp. The resolution of the system clock is unspecified.

Expected results

The current wall time is returned (if the board has a RTC). Otherwise, either the function should be made unavailable (this triggering a linker error if it is used), or it should always fail.

Actual results

gettimeofday() returns time since boot if xtimer was compiled in. If not, it fails with ENOSYS.

Related to

issue: #10523

[virajsahai]

@jcarrano just saw the implementation, it uses xtimer_now64() which returns the current timestamp as a 64 bit value. And according to that, the calculations are correct. Not sure what might be going wrong here.

[jcarrano]

@virajsahai As far as I know, xtimer_now() returns a timestamp since an unspecified time in the past (since last reset, maybe?). The docs are not 100% clear, but I would assume that, as measuring absolute time would require a RTC or some other time-setting mechanism.
gettimeofday(), on the other hand, requires time since Epoch.