how gnu make tells if file's been changed?

This is a discussion on how gnu make tells if file's been changed? within the C Programming forums, part of the General Programming Boards category; I am writing a program to test if the file has been modified recently, and am thinking about how to ...

  1. #1
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    how gnu make tells if file's been changed?

    I am writing a program to test if the file has been modified recently, and am thinking about how to implement it.

    To know if a file has been changed, there has to be a reference to compared. My primitive idea was to check the modified stamp and the accessed stamp of the file, if they were not the same, probabaly the file has been changed. But after some experiments, it was not like what I supposed.

    I also thought to keep a copy of the file, so I could compare the time stamps between them, but if the volume of the files were large, that certainly causes some overhead.

    As I know, GNU make only compile those source files that have been modified. How does it do to tell if a file's changed or not?

    thanks.

  2. #2
    Crazy Fool Perspective's Avatar
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    AFAIK it compares the timestamp of the source file to the object file. If the source file is newer than the object file, it rebuilds it.

  3. #3
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    All versions of Make uses file modification time to determine which is newer/older. [This is sometimes a problem if your file lives on a drive that is shared between machines, and the time on the machines aren't in sync].

    Note also that a target file that doesn't exist is treated as "really old".

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    Please don't PM me for help - and no, I don't do help over instant messengers.

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