[Octave-bug-tracker] [bug #29786] Octave does not save sparse matrices correctly in -mat format

[David Bateman]

Update of bug #29786 (project octave):

                  Status:               Confirmed => In Progress            
             Assigned to:                    None => dbateman               

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Follow-up Comment #4:

The matlab file format is ugly in that the size of ech block of data is
stored at the start of the block. However to calculate the size of the block
you basically have to write the file and figure out the size post-hoc. This
leaves two choices. Firstly you write the block once to a temporary location,
calculate its size from there and then write the size to the correct location.
Or secondly, you write a blank to the block size, write the data to the file,
then use fseek to come back and correct the block size. Octave chooses the
second option for performance reasons. 

However, if the size of the block calculation is incorrect, then the seeks
will be misperformed and the data of the following variables will be
corruptted. I strongly suspect this is what is happening. I'm however
surprised that I never noticed this issue.

I'll take this one on later in the week

D.


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