C++11 clocks: g++ steady_clock::is_steady == false?

steady_clock is supported for GCC 4.7 (as shown by the docs for the 4.7 release: http://gcc.gnu.org/onlinedocs/gcc-4.7.2/libstdc++/manual/manual/status.html#status.iso.2011) and steady_clock::is_steady is true but only if you build GCC with --enable-libstdcxx-time=rt

See https://stackoverflow.com/a/12961816/981959 for details of that configuration option.

For GCC 4.9 it will be enabled automatically if your OS and C library supports POSIX monotonic clocks for clock_gettime (which is true for GNU/Linux with glibc 2.17 or later and for Solaris 10, IIRC)

Here are the results with GCC 4.8 configured with --enable-libstdcxx-time=rt on an AMD Phenom II X4 905e, 2.5GHz but I think it's throttled to 800MHz right now, running Linux 3.6.11, glibc 2.15

$ ./hrc
clock::period: 0.001000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 0.069646 seconds. That's 69.645928 ns/iteration.
$ ./sys
clock::period: 0.001000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 0.062535 seconds. That's 62.534986 ns/iteration.
$ ./sc
clock::period: 0.001000 us.
clock::is_steady: yes
Calling clock::now() 1000000 times...
That took 0.065684 seconds. That's 65.683730 ns/iteration.

And with GCC 4.7 without --enable-libstdcxx-time (so the same results for all three clock types) on ARMv7 Exynos5 running Linux 3.4.0, glibc 2.16

clock::period: 1.000000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 1.089904 seconds. That's 1089.904000 ns/iteration.

If you can run the simple program on other platforms/compilers, I would be very interested to know the results.

Mac OS X 10.8, clang++ / libc++, -O3, 2.8 GHz Core i5:

High resolution clock

clock::period: 0.001000 us.
clock::is_steady: yes
Calling clock::now() 1000000 times...
That took 0.021833 seconds. That's 21.832827 ns/iteration.

System clock

clock::period: 1.000000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 0.041930 seconds. That's 41.930000 ns/iteration.

Steady clock

clock::period: 0.001000 us.
clock::is_steady: yes
Calling clock::now() 1000000 times...
That took 0.021478 seconds. That's 21.477953 ns/iteration.

steady_clock and system_clock are required to be distinct types. steady_clock::is_steady is required to be true. high_resolution_clock may be a distinct type or an alias of steady_clock or system_clock. system_clock::rep must be a signed type.

According to GNU's site, GNU libstdc++ doesn't support steady_clock yet. That's why steady_clock::is_steady is false.

Here is the relevant section of the support checklist:

20.11.7.1   Class system_clock           Y   
20.11.7.2   Class steady_clock           N   Support old monotonic_clock spec instead
20.11.7.3   Class high_resolution_clock  Y