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22.17 Make says "foo has modification time in the future"

Q: I keep getting messages about file modification time in the future when I compile my programs using Make...

A: This happens on fast machines running Windows/NT and Windows 9X. (However, somebody even reported such a problem on a FAT32 drive under plain DOS.) It is evidently due to a misfeature in the way Windows reports the time a file was last modified. Current versions of Windows 9X store this time with 2-second granularity, but the file creation time is stored with a 100-nanosecond granularity. It seems that Windows blindly adds 2 seconds to the system clock reading when it calculates the file modification time, apparently to prevent it from being older than the file creation time, which could happen because of the greater accuracy used to store the creation time. On a fast machine, this 2-second add-on can very easily make the file modification time be ahead of the system clock when Make checks the time stamp of a file it has just created/updated.

GNU Make reports such cases because inconsistencies in file times could easily defeat its decisions about which files need to be rebuilt. In particular, if some of the files reside on a networked drive, and there's a clock skew between the machine where Make runs and the one which exports the drive, Make could really fail to rebuild some files.

DJGPP ports of GNU Make v3.77 and later allow for up to 3 seconds of positive difference between the file timestamp and the system clock (that is, the file is allowed to be up to 3 seconds into the future), before the above warning is printed. So upgrading to the latest version of Make should eliminate such bogus warnings and leave you only with messages due to real clock skews.


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