[Octave-bug-tracker] [bug #49699] Octave crashes when using fread to load more than 2 GB from a file

[anonymous]

Follow-up Comment #2, bug #49699 (project octave):

Thanks, Rik, that explains what to do to fix the problem (though Octave should
just produce an error, not crash).

By the way, that wiki article is very misleading. The limitation is *not* 2 GB
of memory, but rather 2^31 *elements*.

In an installation not compiled with <code>--enable-64</code> this produces an
error as expected:

<pre>
>> a = zeros (1024*1024*2048, 1, 'int8');
error: out of memory or dimension too large for Octave's index type
</pre>

If we reduce the volume by just one slice, we do not hit the limit. But if we
also change the data type a larger one the memory used by the array can be
much larger:

<pre>
>> a = zeros (1024*1024*2047, 1, 'int8');
>> b = zeros (1024*1024*2047, 1, 'double');
>> whos
Variables in the current scope:

   Attr Name            Size                     Bytes  Class
   ==== ====            ====                     =====  =====
        a      2146435072x1                 2146435072  int8
        b      2146435072x1                17171480576  double

Total is 4292870144 elements using 19317915648 bytes
</pre>

Both arrays have a slightly less than 2^31  elements. But while array
<code>a</code> is just under 2 GB, <code>b</code> is close to 16 GB.

That is the reason why I'm able to workaround the limitation by reading the
file in batches of less than 2 GB: my files and arrays are larger than 2 GB,
but the number of elements is less than 2^31 because I use float32 and int16.

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