Compilation

This page covers configuring and building CosmicGrain itself — the steps you repeat every time you change which physics modules are active, adjust resolution, or switch between production and debug builds. It assumes Installation is already done.

Configuring Config.sh

CosmicGrain's physics modules are enabled at compile time via Config.sh. The following flags are required for a standard dust-enabled run:

Flag	Purpose
DUST	Enables the CosmicGrain dust physics module (creation, transport, growth, destruction, radiation pressure).
FEEDBACK	Enables stellar feedback (Type II SNe and AGB winds).
STARFORMATION	Required for metallicity- and SFR-dependent dust gating (e.g. molecular fraction, clumping).
IDS_64BIT	Required. 32-bit particle IDs silently overflow once dust superparticle counts grow large — this does not raise a compile or runtime error, it just corrupts particle bookkeeping. Always enable this for dust-enabled runs.

Optional flags, useful for debugging stellar feedback energy deposition:

Flag	Purpose
FEEDBACK_LIMIT_DULOG	Clamps |Δlog₁₀ u| per heating event — useful when diagnosing runaway temperatures from feedback.
FEEDBACK_T_CAP	Applies a soft temperature cap for debugging; not intended for production runs.

Building

CosmicGrain compiles with mpicxx, requiring C++11 and the libraries set up in Installation. A representative per-file compile invocation:

mpicxx -std=c++11 -O3 -march=native -mtune=native \
    -ffast-math -fno-math-errno -fno-trapping-math \
    -funroll-loops -Wall -Wno-format-security -Wno-unused-result -flto \
    -I/usr/include/hdf5/serial \
    -I/path/to/gsl/build/include \
    -I/path/to/fftw3/build/include \
    -Ibuild -Isrc \
    -c src/<file>.cc -o build/<file>.o

In practice you won't invoke this directly — the top-level make handles it for every source file:

make -j <!-- TODO: recommended parallelism, e.g. 8 -->

The executable is produced at <!-- TODO: path, e.g. build/Gadget4 -->.

Optimization flag note: -ffast-math, -fno-math-errno, and -fno-trapping-math relax IEEE floating-point compliance for speed. This is standard practice for production N-body/hydro codes and shouldn't affect the dust physics (which relies on exact exponential/analytical forms rather than delicate floating-point edge cases), but keep it in mind if you're ever debugging a numerical discrepancy that seems to depend on compiler flags.

Verifying the build

On startup, a successful build with dust physics enabled prints a one-time flag summary confirming the parameter file was read correctly:

[DUST_FLAGS|Step=0] Creation=1 Drag=1 Growth=1 Coagulation=1 Sputtering=1 ShockDestruction=1 Astration=1 RadPressure=1 Clumping=1 Cooling=1

Check that these flags match what you set in your .param file — a 0 where you expected 1 usually means a parameter name mismatch (e.g. a typo in DustEnableGrowth) rather than a compilation problem.