[Users] Meetig minutes

[Roland Haas]

Hello Erik,

> Regarding segfaults in Fortran: why don't unallocated variables lead
> to an error in Fortran? Do you refer to Fortran allocation or Cactus
> allocations here?
Cactus allocations. Variables for which no STORAGE statement is active 
(either global or in the schedule item).

The reason is that (see configs/*/bindings/include/cctk_Arguments.h) you 
cannot legally pass a NULL pointer as a FORTRAN array subroutine 
argument, there apparently *has* to be some storage (I guess FORTRAN 
does no allow zero-sized arrays). So for non-debug runs Cactus passes in 
the address of a dummy array. This is sufficient to prevent segfaults 
(even for rather larger grids). Caught me recently when adding the 
Multipatch hacks to support non-multipatch runs. I was counting on 
segfaults if I had forgotten any pointers (and for rather large grids, 
so it is not just a small grid issue that goes away in production runs). 
  I ended up unitialized memory which was mostly 0 so it did not show up 
in the test (since the variable without storage was kxx in Minkowski 
spacetime). :-(

Yours,
Roland

--8<-- configs/*/bindings/include/cctk_Arguments.h --8<--
/*
  * References to non-existing or non-allocated variables should be passed
  * as NULL pointers in order to catch any invalid access immediately
  * However, this runtime debugging feature may cause problems
  * with some fortran compilers which require all fortran routine arguments
  * to refer to a valid memory location (eg. to enable the code optimizer
  * to generate conditional load/store instructions if applicable).
  * For this reason, we pass NULL pointers only for debugging 
configurations,
  * and a pointer to a user-accessable memory location (a local dummy 
variable)
  * otherwise.
  */
--8<--


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