[gmx-developers] broadcast of zero-length arrays

[Mark Abraham]

Hi,

During src/gmxlib/mvdata.c bc_grpopts(), my BlueGene/L segfaults during 
the broadcasts of the QMMM stuff. The lines that break are attempts to 
broadcast arrays of zero length. Adding a check for non-zero length into 
the definition of nblock_bc fixes the problem. Presumably a null pointer 
is being dereferenced inside the MPI library.

I'm not sure whether this observation is indicative of (this version of) 
IBM's MPI library not having implemented the full standard, the standard 
not specifying behaviour in this case, or GROMACS not being sufficiently 
defensive. I haven't found anything useful in the MPI documentation I 
have to hand. You could argue cases either way - the implementors of the 
library want to avoid such checks to speed performance, and the users of 
the library expect it either to take care of such housekeeping for them, 
or not dereference pointers unnecessarily (think buffering)...

Does anyone know what expected behaviour is here?

Cheers,

Mark