# Debugging

Debugging is an art. Everyone has their own favorite method. Here we offer a few tips we have found to be useful.

Compiling in debug mode (e.g., `make DEBUG=TRUE` for gmake users; `cmake -DDEBUG` for cmake users) and running with `amrex.fpe_trap_invalid=1` in the inputs file can be helpful. In debug mode, many compiler debugging flags are turned on and all `MultiFab` s are initialized to signaling NaNs. The `amrex.fpe_trap_invalid` parameter will result in backtrace files when a floating point exception occurs. One can then examine those files to track down the origin of the issue.

Several other ways to look at the data include:

1. Writing a `MultiFab` to disk with

    VisMF::Write(const FabArray<FArrayBox>& mf, const std::string& name);

and examining it with `Amrvis` (see the Amrvis section in the AMReX documentation).

2. You can also use the `print_state` routine:

    void print_state(const MultiFab& mf, const IntVect& cell, const int n=-1);

which outputs the data for a single cell.

3. If you want to compare old and new plotfiles,

    fcompare --infile1 plt00000_run1 --infile2 plt00000_run2 --diffvar u_g

will print out the maximum absolute and relative differences between the two plotfiles for each variable and will also create a new plotfile “diffs” that contains the difference in u_g (in this case) between the two plotfiles.

The `fcompare` executable can be built in AMReX (go to amrex/Tools/Plotfile and type “make”).

4) Valgrind is another useful debugging tool. Note that for runs using more than one MPI process, one can tell valgrind to output to different files for different processes. For example,

    mpiexec -n 4 valgrind --leak-check=yes --track-origins=yes --log-file=vallog.%p ./mfix.exe ...