Compiling C on remote Linux machine - clock skew detected warning

Question

I m connected to my university s small Linux cluster via PuTTY and WinSCP  transferring files using the latter and compiling and running them with the former   My work so far has been performed in the university s labs  but today I have been doing some work at home that generated an interesting warning   I uploaded an entire folder of stuff and  upon running the make command  I get this as the last line of output      make  warning   Clock skew detected   Your build may be incomplete    The resulting binary works correctly  and there doesn t seem to be any other unexpected errors in the build process   I seem to be able to trigger the error by building after uploading some new   replacement files  I edit everything locally then upload the new version   so I m wondering if it s something just as simple as mismatched file modification times  Or something more concerning   So  should I be worried   How do I fix prevent this

User · Answer

This is usually simply due to mismatching times between your host and client machines  You can try to synchronize the times on your machines using ntp

User · Answer

That message is usually an indication that some of your files have modification times later than the current system time  Since make decides which files to compile when performing an incremental build by checking if a source files has been modified more recently than its object file  this situation can cause unnecessary files to be built  or worse  necessary files to not be built   However  if you are building from scratch  not doing an incremental build  you can likely ignore this warning without consequence

User · Answer

According to user m9dhatter on LinuxQuestions org       make  uses the time stamp of the file to determine if the file   it is trying to compile is old or new  if your clock is bonked  it may have problems compiling       if you try to modify files at another machine with a clock time ahead by a few minutes and transfer them to your machine and then try to compile it may cough up a warning that says the file was modified from the future  clock may be skewed or something to that effect   cant really remember    you could just ls to the offending file and do this        touch  lt filename of offending file gt

User · Answer

I have had this in the past - due to the clocks being out on the machines  Consider setting up NTP so that all machines have the same time

User · Answer

Simple solution     touch filename   will do all OK   For more info  http   embeddedbuzz blogspot in 2012 03 make-warning-clock-skew-detected-your html

User · Answer

This happened to me  It s because I ran make -j 4 and some jobs finished out of order  This warning should be expected when using the -j option

User · Answer

Install the Network Time Protocol This also happened to me when running make on a Samba SMB CIFS share on a server  A durable solution consists in installing the ntp daemon on both the server and the client   Please  note that this problem is not solved by running ntpdate  This would resolve the time difference only temporarily  but not in the future   For Ubuntu and Debian-derived systems  simply type the following line at the command line    sudo apt install ntp  Moreover  one will still need to issue the command touch   once  and only once  in the affected directory to correct the file modification times once and for all    touch    For more information about the differences between ntp and ntpdate  please refer to   Time Synchronisation with NTP How To Set Up Time Synchronization on Ubuntu 16 04

User · Answer

The other answers here do a good job of explaining the issue  so I won t repeat that here  But there is one solution that can resolve it that isn t listed yet  simply run make clean  then rerun make   Having make remove any already compiled files will prevent make from having any files to compare the timestamps of  resolving the warning

User · Answer

Replace the watch battery in your computer  I have seen this error message when the coin looking battery on the motherboard was in need of replacement

User · Answer

Just in case anyone lands here  If you have sudo rights one option is to synchronize the system time  sudo date -s    wget -qSO- --max-redirect 0 google com 2 gt  amp 1   grep Date    cut -d    -f5-8 Z

User · Answer

type in the terminal and it will solve the issue  find   -type f   xargs -n 5 touch  make clean  clean

User · Answer

Typically this occurs when building in a NFS mounted directory  and the clocks on the client and the NFS server are out of sync   The solution is to run an NTP client on both the NFS server and all clients

User · Answer

The solution is to run an NTP client   just run the command as below    ntpdate 172 16 12 100   172 16 12 100 is the ntp server

User · Answer

Make checks if the result of the compilation  e g  somefile o  is older than the source  e g  somefile c  The warning above means that something about the timestaps of the files is strange  Probably the system clocks of the University server differs from your clock and you e g  push at 1 pm a file with modification date 2 pm  You can see the time at the console by typing date

[linux] Compiling C++ on remote Linux machine - "clock skew detected" warning

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