Division by zero in Write_State.F90

[gnikit]

In line 91 https://github.com/FluidityProject/fluidity/blob/2ad88a413ca999d4bd37b50e8ddb2588866335c1/femtools/Write_State.F90#L91

A comparison for int_dump_period == 0 is done followed by a mod(,int_dump_period), mod() however is incapable of handling divisions by zero (it should throw and arithmetic exception if divided by zero). The actual problem arises due to the fact that the Fortran standard does not evaluate logic statements in any particular direction and does not have short-circuit evaluation either.

7.1.5.4.2 Evaluation of logical intrinsic operations Once the interpretation of a logical intrinsic operation is established, the processor may evaluate any other expression that is logically equivalent, provided that the integrity of parentheses in any expression is not violated.

taken from this discussion at the Intel Forum

So it is up to the compiler and its optimiser to chose how such clauses are evaluated. I think GCC strictly follows the standard in newer versions (at least with debugging symbols) so it will probably segfault in Ubuntu 20.04

[stephankramer]

That's right: fortran does not guarantee short-circuit logic

Maybe something like:

if (int_dump_period>0) then
   zero_modulus = mod(timestep, int_dump_period) == 0
else
   zero_modulus = True
end if
if (zero_modulus) then
   ...

TBH I find the logic in that part of the code a little hard to follow

[gnikit]

Closed via #301