This file documents known bugs in Octave and describes where and how to
report any bugs that you may find.

   Copyright (C) 1996-2019 John W. Eaton.  You may copy, distribute, and
modify it freely as long as you preserve this copyright notice and
permission notice.

1 Known Causes of Trouble with Octave
*************************************

This section describes known problems that affect users of Octave.  Most
of these are not Octave bugs per se--if they were, we would fix them.
But the result for a user may be like the result of a bug.

   Some of these problems are due to bugs in other software, some are
missing features that are too much work to add, and some are places
where people's opinions differ as to what is best.

1.A Actual Bugs We Haven't Fixed Yet
====================================

   * Output that comes directly from Fortran functions is not sent
     through the pager and may appear out of sequence with other output
     that is sent through the pager.  One way to avoid this is to force
     pending output to be flushed before calling a function that will
     produce output from within Fortran functions.  To do this, use the
     command

          fflush (stdout)

     Another possible workaround is to use the command

          page_screen_output (false);

     to turn the pager off.

   A list of ideas for future enhancements is distributed with Octave.
See the file 'PROJECTS' in the top level directory in the source
distribution.

1.B Reporting Bugs
==================

Your bug reports play an essential role in making Octave reliable.

   When you encounter a problem, the first thing to do is to see if it
is already known.  *Note Trouble::.  If it isn't known, then you should
report the problem.

   Reporting a bug may help you by bringing a solution to your problem,
or it may not.  In any case, the principal function of a bug report is
to help the entire community by making the next version of Octave work
better.  Bug reports are your contribution to the maintenance of Octave.

   In order for a bug report to serve its purpose, you must include the
information that makes it possible to fix the bug.

1.B.1 Have You Found a Bug?
---------------------------

If you are not sure whether you have found a bug, here are some
guidelines:

   * If Octave gets a fatal signal, for any input whatever, that is a
     bug.  Reliable interpreters never crash.

   * If Octave produces incorrect results, for any input whatever, that
     is a bug.

   * Some output may appear to be incorrect when it is in fact due to a
     program whose behavior is undefined, which happened by chance to
     give the desired results on another system.  For example, the range
     operator may produce different results because of differences in
     the way floating point arithmetic is handled on various systems.

   * If Octave produces an error message for valid input, that is a bug.

   * If Octave does not produce an error message for invalid input, that
     is a bug.  However, you should note that your idea of "invalid
     input" might be my idea of "an extension" or "support for
     traditional practice".

   * If you are an experienced user of programs like Octave, your
     suggestions for improvement are welcome in any case.

1.B.2 Where to Report Bugs
--------------------------

To report a bug in Octave, submit a bug report to the Octave bug tracker
<https://bugs.octave.org>.

   *Do not send bug reports to 'help-octave'*.  Most users of Octave do
not want to receive bug reports.

1.B.3 How to Report Bugs
------------------------

Submit bug reports for Octave to the Octave bug tracker
<https://bugs.octave.org>.

   The fundamental principle of reporting bugs usefully is this: *report
all the facts*.  If you are not sure whether to state a fact or leave it
out, state it!

   Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter.  Thus, you
might assume that the name of the variable you use in an example does
not matter.  Well, probably it doesn't, but one cannot be sure.  Perhaps
the bug is a stray memory reference which happens to fetch from the
location where that name is stored in memory; perhaps, if the name were
different, the contents of that location would fool the interpreter into
doing the right thing despite the bug.  Play it safe and give a
specific, complete example.

   Keep in mind that the purpose of a bug report is to enable someone to
fix the bug if it is not known.  Always write your bug reports on the
assumption that the bug is not known.

   Sometimes people give a few sketchy facts and ask, "Does this ring a
bell?"  This cannot help us fix a bug.  It is better to send a complete
bug report to begin with.

   Try to make your bug report self-contained.  If we have to ask you
for more information, it is best if you include all the previous
information in your response, as well as the information that was
missing.

   To enable someone to investigate the bug, you should include all
these things:

   * The version of Octave.  You can get this by noting the version
     number that is printed when Octave starts, or running it with the
     '-v' option.

   * A complete input file that will reproduce the bug.

     A single statement may not be enough of an example--the bug might
     depend on other details that are missing from the single statement
     where the error finally occurs.

   * The command arguments you gave Octave to execute that example and
     observe the bug.  To guarantee you won't omit something important,
     list all the options.

     If we were to try to guess the arguments, we would probably guess
     wrong and then we would not encounter the bug.

   * The type of machine you are using, and the operating system name
     and version number.

   * The command-line arguments you gave to the 'configure' command when
     you installed the interpreter.

   * A complete list of any modifications you have made to the
     interpreter source.

     Be precise about these changes--show a context diff for them.

   * Details of any other deviations from the standard procedure for
     installing Octave.

   * A description of what behavior you observe that you believe is
     incorrect.  For example, "The interpreter gets a fatal signal," or,
     "The output produced at line 208 is incorrect."

     Of course, if the bug is that the interpreter gets a fatal signal,
     then one can't miss it.  But if the bug is incorrect output, we
     might not notice unless it is glaringly wrong.

     Even if the problem you experience is a fatal signal, you should
     still say so explicitly.  Suppose something strange is going on,
     such as, your copy of the interpreter is out of sync, or you have
     encountered a bug in the C library on your system.  Your copy might
     crash and the copy here would not.  If you said to expect a crash,
     then when the interpreter here fails to crash, we would know that
     the bug was not happening.  If you don't say to expect a crash,
     then we would not know whether the bug was happening.  We would not
     be able to draw any conclusion from our observations.

     Often the observed symptom is incorrect output when your program is
     run.  Unfortunately, this is not enough information unless the
     program is short and simple.  It is very helpful if you can include
     an explanation of the expected output, and why the actual output is
     incorrect.

   * If you wish to suggest changes to the Octave source, send them as
     context diffs.  If you even discuss something in the Octave source,
     refer to it by context, not by line number, because the line
     numbers in the development sources probably won't match those in
     your sources.

   Here are some things that are not necessary:

   * A description of the envelope of the bug.

     Often people who encounter a bug spend a lot of time investigating
     which changes to the input file will make the bug go away and which
     changes will not affect it.  Such information is usually not
     necessary to enable us to fix bugs in Octave, but if you can find a
     simpler example to report _instead_ of the original one, that is a
     convenience.  Errors in the output will be easier to spot, running
     under the debugger will take less time, etc.  Most Octave bugs
     involve just one function, so the most straightforward way to
     simplify an example is to delete all the function definitions
     except the one in which the bug occurs.

     However, simplification is not vital; if you don't want to do this,
     report the bug anyway and send the entire test case you used.

   * A patch for the bug.  Patches can be helpful, but if you find a
     bug, you should report it, even if you cannot send a fix for the
     problem.

1.B.4 Sending Patches for Octave
--------------------------------

If you would like to write bug fixes or improvements for Octave, that is
very helpful.  When you send your changes, please follow these
guidelines to avoid causing extra work for us in studying the patches.

   If you don't follow these guidelines, your information might still be
useful, but using it will take extra work.  Maintaining Octave is a lot
of work in the best of circumstances, and we can't keep up unless you do
your best to help.

   * Send an explanation with your changes of what problem they fix or
     what improvement they bring about.  For a bug fix, just include a
     copy of the bug report, and explain why the change fixes the bug.

   * Always include a proper bug report for the problem you think you
     have fixed.  We need to convince ourselves that the change is right
     before installing it.  Even if it is right, we might have trouble
     judging it if we don't have a way to reproduce the problem.

   * Include all the comments that are appropriate to help people
     reading the source in the future understand why this change was
     needed.

   * Don't mix together changes made for different reasons.  Send them
     _individually_.

     If you make two changes for separate reasons, then we might not
     want to install them both.  We might want to install just one.

   * Use 'diff -c' to make your diffs.  Diffs without context are hard
     for us to install reliably.  More than that, they make it hard for
     us to study the diffs to decide whether we want to install them.
     Unified diff format is better than contextless diffs, but not as
     easy to read as '-c' format.

     If you have GNU diff, use 'diff -cp', which shows the name of the
     function that each change occurs in.

   * Write the change log entries for your changes.

     Read the 'ChangeLog' file to see what sorts of information to put
     in, and to learn the style that we use.  The purpose of the change
     log is to show people where to find what was changed.  So you need
     to be specific about what functions you changed; in large
     functions, it's often helpful to indicate where within the function
     the change was made.

     On the other hand, once you have shown people where to find the
     change, you need not explain its purpose.  Thus, if you add a new
     function, all you need to say about it is that it is new.  If you
     feel that the purpose needs explaining, it probably does--but the
     explanation will be much more useful if you put it in comments in
     the code.

     If you would like your name to appear in the header line for who
     made the change, send us the header line.

1.C How To Get Help with Octave
===============================

The mailing list <help@octave.org> exists for the discussion of matters
related to using and installing Octave.  If would like to join the
discussion, please send a short note to <help-request@octave.org>.

   *Please do not* send requests to be added or removed from the mailing
list, or other administrative trivia to the list itself.

   If you think you have found a bug in Octave or in the installation
procedure, however, you should submit a complete bug report to the
Octave bug tracker at <https://bugs.octave.org>.  But before you submit
a bug report, please read <https://www.octave.org/bugs.html> to learn
how to submit a useful bug report.

1.D How to Distinguish Between Octave and Matlab
================================================

Octave and MATLAB are very similar, but handle Java slightly different.
Therefore it may be necessary to detect the environment and use the
appropriate functions.  The following function can be used to detect the
environment.  Due to the persistent variable it can be called repeatedly
without a heavy performance hit.

   Example:

     %%
     %% Return: true if the environment is Octave.
     %%
     function retval = isOctave
       persistent cacheval;  % speeds up repeated calls

       if isempty (cacheval)
         cacheval = (exist ("OCTAVE_VERSION", "builtin") > 0);
       end

       retval = cacheval;
     end
