steady_time module fails to compile on Windows

[phil-opp]

The steady_time module introduced in a0d8c8e causes compile errors on Windows:

error[E0425]: cannot find function `clock_gettime` in crate `libc`
Error:   --> C:\Users\runneradmin\.cargo\git\checkouts\ros2-client-1852ea698eaf7825\008b76c\src\steady_time.rs:35:27
   |
35 |       assert_eq!(0, libc::clock_gettime(libc::CLOCK_MONOTONIC, &mut ts));
   |                           ^^^^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `CLOCK_MONOTONIC` in crate `libc`
Error:   --> C:\Users\runneradmin\.cargo\git\checkouts\ros2-client-1852ea698eaf7825\008b76c\src\steady_time.rs:35:47
   |
35 |       assert_eq!(0, libc::clock_gettime(libc::CLOCK_MONOTONIC, &mut ts));
   |                                               ^^^^^^^^^^^^^^^ not found in `libc`

The clock_gettime POSIX function is not available on Windows.

[Michael-J-Ward]

rust's std::time::Instant uses the same system call on unix system's that steady_time is using: clock_gettime with CLOCK_MONOTINC.

It's guaranteed to be non-decreasing, and it seems like the implementation is as to monotonic as possible for the given hardware. The monotonicity section of the docs also states (emphasis added):

On all platforms Instant will try to use an OS API that guarantees monotonic behavior if available, which is the case for all tier 1 platforms. In practice such guarantees are – under rare circumstances – broken by hardware, virtualization or operating system bugs.

What does steady_time's implementation offer above what std::time::Instant does?

For reference - the unix implementation:

https://github.com/rust-lang/rust/blob/79424056b05eaa9563d16dfab9b9a0c8f033f220/library/std/src/sys/pal/unix/time.rs#L266-L294

[Michael-J-Ward]

Digging further down this rabbit hole because this if the first that I've encountered it.

Based on the stackoverflow discussion Does Rust have a steady clock, Rust's Instant and steady_time in c++ are equivalent on rust's tier 1 systems.

However, a big caveat is that CLOCK_MONOTONIC operates differently depending on the platform. On Linux and FreeBSD, it does not include time that the machine was suspended. On Darwin and OpenBSD, it does include time suspended.

This comment on rust's github summarizes the semantics of the different clocks available on the different platforms.

I am a novice to the robot domain, so I don't know how important that caveat is. If it is, someone published an Instant crate that unifies the semantics between the different platforms to always include suspended time - announced in that same rust-lang thread.

[jhelovuo]

Thank you for reporting.

We do not routinely test on Windows, so this was released by accident.

The latest master branch changes the steady_time implementation to be a wrapper around std::time::Instant. This should be more portable, as noted in the comments above. As a downside, we lose the ability to convert between steady_time::Time and other representations, as Instant is opaque.

Please check if this works for you and report the result here.

[jhelovuo]

Version 0.7.1 also contains the fix.

[phil-opp]

I can confirm that this is fixed. Thanks a lot for the quick fix & release!