Describe the bug

The r3.gwflow module crashes with a segmentation fault when the sink parameter is not provided, despite being marked as optional in the documentation and parser.

To reproduce

1. Start GRASS and set a 3D region:

   g.region res=10 res3=10 n=100 s=0 e=100 w=0 t=50 b=0

2. Create minimal input maps:

   r3.mapcalc expression="phead = 50" --o
   r3.mapcalc expression="status = 1" --o
   r3.mapcalc expression="hydcond = 0.0001" --o
   r3.mapcalc expression="syield = 0.001" --o

3. Run r3.gwflow without providing the sink parameter:

   r3.gwflow phead=phead status=status hc_x=hydcond hc_y=hydcond \
            hc_z=hydcond yield=syield output=result velocity_x=vx \
            velocity_y=vy velocity_z=vz budget=budget

4. Observe the segmentation fault.

Expected behavior

If sink is not provided, the module should:

• Either gracefully fall back to using a zero-valued sink field internally,
• Or document sink as a required parameter and fail with a clear error message if omitted.

Screenshots

System description

• Operating System: Windows Subsystem for Linux (WSL)
• GRASS GIS version: 8.4
• details about further software components
  • GRASS 8.5.0dev
  • Python verison: 3.10.12
  • wxPython version: 4.2.2

Possible Root Cause

The issue appears to come from the unconditional access of param.q->answer in the C source (main.c), even when sink is not set by the user. This leads to dereferencing a NULL pointer and crashing the module.

I would be happy to submit a patch for this, either to:

• Add null-checks and initialize a zero-valued map internally if unset, or
• Update the parser and docs to mark sink as required.

Looking forward to making the preferred change.