??? FAQ: Crash "Segmentation fault" ????

  * **Q:** WOMBAT crashes with a "Segmentation fault" with a heap of unintelligible numbers to follow -- this seems to happen quite early on, i.e. during the set-up phase before any likelihoods are printed to the screen. What is wrong?
   * **A:** A `segmentation fault' (or 'access violation') message is a run-time error message generated by the FORTRAN compiler. In contrast, the majority of error messages in WOMBAT are programmed in to catch mistakes made by the programs user -- you. It usually means that an array has the wrong dimensions, but it can also be a consequence of other `bugs' somewhere else in the program. Further, it can reflect a RAM or stack size problem 
      * If you are working under Linux and have a larg(ish) analysis, make sure you have set the stack size as large as possible. The command is **''unlimit -s''** (or similar, depending on the flavour of Linux). NB: Putting this into your login file may be a good idea.
      * There is a **known problem** with WOMBAT that it can crash with a segmentation error  or simply hang up during the set-up phase when trying to establish the optimum ordering of equations. Fortunately, this is rare -- it occurs most often for larger problems and/or when using the **<nowiki>MeTiS</nowiki>** option, either implicitly (program default for large problems) or explicitly specified, in particular when the model of analysis includes a fixed effect with very few levels or covariables. 
      * WOMBAT incorporates three different ordering strategies, all using subroutines available in the public domain. Unfortunately, none of them are totally reliable. In particular, <nowiki>MeTiS</nowiki> can misbehave and does not provide a `graceful' exit when problems occur, i.e. there is no facility to fault-trap this and shut down WOMBAT with an explanatory message.
      *  So what can you do in this case? Strategies to try:
        - Use a different ordering strategy, i.e. the approximate minimum degree ordering invoked with the **''−−amd''**  run time option.
        - Use different options for <nowiki>MeTis</nowiki>, e.g. reduce the number of graph separators considered (option 1), switch from random to standard edge matching (a "3" for the third option), or change from two-sided to one-sided node refinement (a "2" for the fourth option) -- see the [[http://agbu.une.edu.au/~kmeyer/WOMBAT/WWW/manualse13.html|manual]] for details.  
        - WOMBAT has a guess at the space required in the set-up phase. Usually, this is quite appropriate and tends to err on the large side. Occasionally, however, it is too small for <nowiki>MeTiS</nowiki> to work properly. So, you could try to give it some extra space using the ''−−choozhz'' run time option (this will print out a screen message giving a the default value WOMBAT would use and allow you to type in a different value  overriding the default).
~~UP~~