Development FAQ

What does _rebuild=true do in backend() ?

Q: I’ve a question concerning the _rebuild=true option for solve what does this option do exactly ? (Because we’ve got a problem when we try to create 2 independent system, we get a PETSC error with wrong matrix dimension if we don’t do a (preconditionner?) rebuild.

A: For a given backend PETSc stores all kind of information and among them the preconditioner. If you try to solve with the same backend two problems of different sizes then PETSc will crash saying that you have wrong dimensions. Either you rebuild the backend (using _rebuild=true) or define two separate ones. The later is usually better since it allows you to fine tune the choice of solvers/preconditioners depending on the problem whereas with the first one you have to have a solver/preconditioner that works for in both cases.

How to print values taken by expressions in, say, variational formulations?

Q: I would like to know what the values taken by expressions and sub-expressions in Feel++.

A: There is a print(expr,"text") function that can wrap expressions and print their values when the expressions are evaluated.

Here is an example:

auto val1 = integrate(_range=elements(mesh),_expr=print(idv(myScalar),"myScalar") );
auto val2 = integrate(_range=markedfaces(mesh,"markerName"),_expr=print(idv(myScalar),"myScalar") );
auto val2 = integrate(_range=markedfaces(mesh,"markerName"),_expr=print(trans(idv(myVector)),"myVectorTrans")*print(idv(myVector),"myVector") );

How to debug testsuite applications?

Q: Feel++ testsuite uses the Boost.Test framework. When used in a debugger and an exception is triggered, the debugger won’t catch the exception and provide the backtrace. It is actually Boost.Test that catches all exception which were uncaught which means that the debugger will stop with no backtrace to be displayed. eventually some texts that an exception was thrown is going to be displayed.

A: It suffices to tell the Debugger to catch events before running the Feel++ application. Start the debugger then issue the following command : catch throw in gdb or break set -E c++ in lldb, then the debugger will stop if an exception is thrown by Feel++ or a third party library and you get a nice backtrace to understand why the exception was thrown.