Peter Samuelson wrote: >The way 'make' determines when a file needs to be rebuilt is by the >simple heuristic "is this file older than any of its dependencies?" >This works fine if you can trust your timestamps to increase >monotonically. Seems obvious - but there are in fact at least four >scenarios where you *don't* have monotonic timestamps: > >(1) If some files are on an NFS server and others on a local machine >(think separate source / object trees), the two system clocks may be >far enough from each other that a freshly built object file still shows >up as "older" than the source - or a stale object shows up as "newer". >(2) You can get similar effects one a single box by using ntpd to skew >your local clock. (3) Things like 'tar', plus certain (broken) source >management tools, can move a file's timestamp backward. (4) Finally, >if you use shadow trees, and you delete a file on a shadow layer - so >that a different source file is "visible" than was visible last time >you built the object - the "visible" timestamp is not indicative of >whether the object is up-to-date. > >Keith's kbuild2.5 fixes all four problems by tracking all source file >timestamps; it triggers an object rebuild *whenever* the source file's >timestamp changes - whether or not the source is "newer" than the >existing object, and whether its time moved forward or backward. > >Like I said, this is a feature the world has done without for the past >15 or 20 years, so it's kind of a hard sell - unless you use shadow >trees, where it is quite necessary because of case (4). > Makes sense now.
>>It's quite ironic but I bet Kai ends up with something similar to >>kbuild-2.5 at the end of the day. >> >> >Certainly Kai has studied kbuild2.5, but there really isn't any way to >use an incremental approach to produce the kbuild2.5 architecture from >the in-kernel kbuild architecture. It's a problem of "irreducible >complexity", as they say in evolution debates. > But surely, eventually it will need a rewrite if the build is to be "correct". A single makefile is surely required to get know the complete list of dependencies. How could this be incrementally introduced ??? >I hear ya. Remember the 2.1.1xx days? Linus looked at the relatively >mature "uusb" project and basically said "Wow, what a load of crap, the >usb spec isn't *nearly* this complex." Then he spent a weekend hacking >up an extremely simple but working usb keyboard / mouse driver. > >Fast-forward a few years and look at the kernel usb system now. I have >no idea whether it resembles "uusb" at all, but simple it ain't.... > > Yes that would be interesting. Cheers, Brendan Simon. ------------------------------------------------------- This sf.net email is sponsored by:ThinkGeek Welcome to geek heaven. http://thinkgeek.com/sf _______________________________________________ kbuild-devel mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/kbuild-devel
