Appendix A 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.

A.1 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.

• Control-C doesn’t work properly in the pager on DEC Alpha systems running OSF/1 3.0.

  This appears to be a bug in the OSF/1 3.0 Bourne shell. The problem doesn’t appear on systems running OSF/1 1.3.

• If you get messages like

  Input line too long
  

  when trying to plot many lines on one graph, you have probably generated a plot command that is too larger for gnuplot’s fixed-length buffer for commands. Splitting up the plot command doesn’t help because replot is implemented in gnuplot by simply appending the new plotting commands to the old command line and then evaluating it again.

  You can demonstrate this ‘feature’ by running gnuplot and doing something like

    plot sin (x), sin (x), sin (x), ... lots more ..., sin (x)
  

  and then

    replot sin (x), sin (x), sin (x), ... lots more ..., sin (x)
  

  after repeating the replot command a few times, gnuplot will give you an error.

  Also, it doesn’t help to use backslashes to enter a plot command over several lines, because the limit is on the overall command line length, once the backslashed lines are all pasted together.

  Because of this, Octave tries to use as little of the command-line length as possible by using the shortest possible abbreviations for all the plot commands and options. Unfortunately, the length of the temporary file names is probably what is taking up the most space on the command line.

  You can buy a little bit of command line space by setting the environment variable TMPDIR to be "." before starting Octave, or you can increase the maximum command line length in gnuplot by changing the following limits in the file plot.h in the gnuplot distribution and recompiling gnuplot.

  #define MAX_LINE_LEN 32768  /* originally 1024 */
  #define MAX_TOKENS 8192     /* originally 400 */
  

  Of course, this doesn’t really fix the problem, but it does make it much less likely that you will run into trouble unless you are putting a very large number of lines on a given plot.

• String handling could use some work.

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

A.2 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. See section Known Causes of Trouble with Octave. 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.

If you have Octave working at all, the easiest way to prepare a complete bug report is to use the Octave function bug_report. When you execute this function, Octave will prompt you for a subject and then invoke the editor on a file that already contains all the configuration information. When you exit the editor, Octave will mail the bug report for you.

A.3 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.

A.4 Where to Report Bugs

If you have Octave working at all, the easiest way to prepare a complete bug report is to use the Octave function bug_report. When you execute this function, Octave will prompt you for a subject and then invoke the editor on a file that already contains all the configuration information. When you exit the editor, Octave will mail the bug report for you.

If for some reason you cannot use Octave’s bug_report function, send bug reports for Octave to:

bug-octave@che.utexas.edu

Do not send bug reports to ‘help-octave’. Most users of Octave do not want to receive bug reports. Those that do have asked to be on ‘bug-octave’.

As a last resort, send bug reports on paper to:

Octave Bugs c/o John W. Eaton
Department of Chemical Engineering
The University of Texas at Austin
Austin, Texas 78712

A.5 How to Report Bugs

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 synch, 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 probalby 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.

A.6 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. Unidiff 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.

A.7 How To Get Help with Octave

If you need help installing, using or changing Octave, the mailing list

help-octave@che.utexas.edu

exists for the discussion of Octave matters related to using, installing, and porting Octave. If you would like to join the discussion, please send a short note to

help-octave-request@che.utexas.edu
            ^^^^^^^

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

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