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bugs - Return value of Import "MX" when file was saved with newer version


I just detected that Mathematica 10.4.1 on Windows will just quietly return Null when I try to Import an .mx file I exported with version 11.1.1. I understand and know that .mx files that were generated with newer versions will generally not load with older versions. Up to now I thought (and I believe I have seen that for older verions) it would give some message and return $Failed, but obviously this is not the case here. As often, the documentation page for the MX format is not giving detailed information about that specific case.


My question is whether that is the expected behavior and whether there is some documentation about that or that would be something to report to WRI...



Answer



This is indeed a bug. A careless tweak was made to the format, so that MX files created by Version 11.1 or newer aren't even recognized as MX files by pre-11.1.0 versions. So Get is succeeding, reading the binary file as text and returning Null because it encounters a zero byte, interpreted as end of file, early on. If for some reason it is important to you that old versions properly recognize and reject newer MX files, you can use the program below after you DumpSave to tweak the MX file. This version will be loaded by V11.1+, and properly rejected as too new by older versions.


On the plus side, one thing we did in the forth coming 11.2 is improve cross-version compatibility of MX files. So, unless a new feature forces an incompatible change in the format, 11.2 will be able to read MX files from later versions. (Obviously, new features won't work in an older kernel).


newHeader = {40, 42, 84, 104, 105, 115, 32, 105, 115, 32, 97, 32, 77, 
   97, 116, 104, 101, 109, 97, 116, 105, 99, 97, 32, 98, 105, 110, 97,
    114, 121, 32, 100, 117, 109, 112, 32, 102, 105, 108, 101, 46, 32, 
   73, 116, 32, 99, 97, 110, 32, 98, 101, 32, 108, 111, 97, 100, 101, 

   100, 32, 119, 105, 116, 104, 32, 71, 101, 116, 46, 42, 41};

newFooter = {40, 42, 69, 110, 100, 32, 111, 102, 32, 77, 97, 116, 104,
    101, 109, 97, 116, 105, 99, 97, 32, 98, 105, 110, 97, 114, 121, 
   32, 100, 117, 109, 112, 32, 102, 105, 108, 101, 42, 41, 0};

fixUp111PlusMX[file_] := With[{bytes = BinaryReadList[file], path = AbsoluteFileName[file]},
   BinaryWrite[path, 
   Join[newHeader, Take[bytes, {79, -49}], newFooter]];
   Close[path]

]

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