How does std::chrono::steady_clock::now report errors?

Question

This is related to my previous question where I asked if std::chrono::steady_clock::now should be noexcept. Now that I know that it should I wonder how does this function report errors? For example, a common implementation of this function on Linux uses clock_gettime which can return an error.

Answer 1

All of the errors that could be reported by the Linux clock_gettime facility are not possible in a debugged implementation of std::chrono::steady_clock::now().

The errors are:

int clock_gettime(clockid_t clk_id, struct timespec *tp);


EFAULT

tp points outside the accessible address space.

There is no user-supplied tp to get wrong in std::chrono::steady_clock::now(). If std::chrono::steady_clock::now() passes the wrong tp to clock_gettime, it is just a bug in steady_clock that is easily fixed.

EINVAL

The clk_id specified is not supported on this system.

There is no user-supplied clk_id to get wrong in std::chrono::steady_clock::now(). If std::chrono::steady_clock::now() passes the wrong clk_id to clock_gettime, it is just a bug in steady_clock that must be fixed.

Note that an implementation of steady_clock isn't guaranteed to be portable, and may have to be ported to any given platform and be tested for correctness. It might turn out that steady_clock can not be implemented in terms of clock_gettime on some platforms.

EPERM

clock_settime() does not have permission to set the clock indicated.

Not applicable to steady_clock as there is no API to set the time.