[gmx-developers] Native endianess in TPR body

[Jonathan Barnoud]

Hello everyone,

I upgraded the code of MDAnalysis to read the latest TPR version. To add 
to Len's comments, it appears indeed that the new TPR body is 4 times as 
big as it use to be for the same content, and is not portable between 
architectures. gmx dump does fail at reading a file with a different 
byte order than native, and there is no obvious way to determine the 
endianness of the body. While the TPR format is not meant to really be 
portable, it seemed commonly agreed that it was a good file to share 
(https://pubs.acs.org/doi/abs/10.1021/acs.jcim.9b00665), it is for sure 
a good input file in MDAnalysis. TPR files are commonly produced on a 
local machine before being actually run on a cluster, that may use a 
different byte order.

 > Second the individual bytes of a value are padded to 4 bytes per 
original bytes (each byte is packed as `char`).

To be noted that the in-file XDR decoder in gromacs (used for the header 
and prior to gromacs 2020) uses 4 bytes for "char", hence the padding. 
The in-memory one reads 1 padded byte (1 byte of information, 4 bytes in 
the file).

As my use case for noticing these differences is fairly niche, I may be 
missing the reason for them. In such case, I would be curious to read 
about them.

Best regards,
Jonathan


On 12/26/19 7:39 PM, Len Kimms wrote:
> Hello everyone,
>
> while fooling around with the new (i.e. version 2020 rc1) TPR file format I noticed some strange behaviors that I don’t understand. As far as I understand the body of the new format is written by the `gmx::InMemorySerializer`. My following questions are basically about this module.
>
> First it seems that the memory serializer writes the values in native byte order. This means that the body of TPR files differ between big- and little-endian systems. The XDR standard used before requires big-endian data. For me, a novice user, the new implementation seems to be less portable and robust. Endian swapping seems to be implemented but not currently used for TPR files.
> Is this intentional, if so, why?
>
> Second the individual bytes of a value are padded to 4 bytes per original bytes (each byte is packed as `char`). Therefore the size increases accordingly.
> Do those padding bytes serve a special purpose?
> Also regarding the padding bytes: Some bytes are not, like most others, padded with zeros. In some places they are padded with ones. At first glance this seem to happen to the second byte (big-endian) of a float. From some initial testing my best guess is, that this is caused by the union conversion in `CharBuffer`. With an `unsigned char` in the private union `u` those values would be zero padded.
>
> In the attachment one could find example files from a big- and little-endian system as well as a file created with GROMACS 2019.
> I also brought this to the attention of the MDAnalysis devs here:
> https://github.com/MDAnalysis/mdanalysis/issues/2428
>
> Best regards,
>     Len
>

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