[Octave-bug-tracker] [bug #53991] Octave crashes using fsolve when x0 is a stationary point

[Rik]

Follow-up Comment #4, bug #53991 (project octave):

I think there are two separate bug reports here.  The first is that calling
balance can lead to a segfault on Windows platforms.

Using the test code from comment #1 on a Linux machine I get


 ** On entry to DGEBAL parameter number  3 had an illegal value
A =

     0     1     0     0
     0     0     1     0
     0     0     0     1
     0     0   NaN     0

error: balance: unknown error in fortran subroutine


The segfault is probably back to the inability to overide the XERBLA handler
on Windows (bug #39000).  The question is how far we want to keep patching
things.  It seems like possibly every routine which ever interacts with the
BLAS or LAPACK library would potentially need to look for NaN values before
continuing.  At the very least, it would seem that this should be restricted
to Windows platforms which would require putting in #ifdef tests.

The second bug report is about fsolve not returning a correct error code when
it encounters a stationary point.  This should be fixed, and it shouldn't be
that difficult. 


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