clock_gettime and time are inconsistent with each other

[Anthony Williams]

When I compile and run the attached program, the output shows an inconsistency between the time returned by clock_gettime() and time(). Even though the call to time() is after the call to clock_gettime(), it can return an earlier timestamp. This is particularly apparent after waiting until a particular time, as shown in the example code. Sample output is:

Now (clock_gettime) time is 1574670740847614452 (timespec)
=>1574670740(time_t)
=>8:32:20
Now (time(nullptr)) time is 1574670740(time_t)
=>8:32:20
Now (clock_gettime) time is 1574670742000185727 (timespec)
=>1574670742(time_t)
=>8:32:22
Now (time(nullptr)) time is 1574670741(time_t)
=>8:32:21

Compiled with g++ v7 from the ubuntu repositories. Output of g++ version:

g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-1064-oem 4.15.0-1064.73
ProcVersionSignature: Ubuntu 4.15.0-1064.73-oem 4.15.18
Uname: Linux 4.15.0-1064-oem x86_64
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Mon Nov 25 08:28:21 2019
DistributionChannelDescriptor:
 # This is the distribution channel descriptor for the OEM CDs
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-bionic-amd64-20180608-47+beaver-turtlebay+X57
InstallationDate: Installed on 2019-08-12 (104 days ago)
InstallationMedia: Ubuntu 18.04 "Bionic" - Build amd64 LIVE Binary 20180608-09:38
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: linux-signed-oem
UpgradeStatus: No upgrade log present (probably fresh install)

[Anthony Williams (anthony-justsoftwaresolutions)]

This bug also occurs on Fedora.