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Debugging Guide

VS Code Setup

Create .vscode/launch.json:

{
    "version": "0.2.0",
    "configurations": [{
        "name": "Debug Admittance",
        "type": "cppdbg",
        "request": "launch",
        "miDebuggerServerAddress": "localhost:3000",
        "program": "${workspaceFolder}/../../install/ur_admittance_controller/lib/ur_admittance_controller/admittance_node",
        "cwd": "${workspaceFolder}"
    }]
}

Build with Debug Symbols

cd ~/ros2_ws
colcon build --packages-select ur_admittance_controller --cmake-args -DCMAKE_BUILD_TYPE=Debug
source install/setup.bash

Start Debugging

# Launch with GDB server
ros2 run --prefix 'gdbserver localhost:3000' ur_admittance_controller admittance_node

# In VS Code:
# - Set breakpoints in C++ code
# - Press F5 to start debugger
# - Use F10 (step over) or F11 (step into)

Terminal Debugging with GDB

Quick Start - Just Run and See the Error

# Simplest way - just run with GDB
cd ~/ros2_ws
ros2 run --prefix 'gdb' ur_admittance_controller admittance_node

# GDB starts, type 'run' to start
(gdb) run

# When it crashes, see where
(gdb) backtrace

Level 1: Basic Debugging - Set One Breakpoint

Want to stop at a specific line? Here's how:

# Start GDB with our node
ros2 run --prefix 'gdb' ur_admittance_controller admittance_node

# Set a breakpoint at the FK error
(gdb) break kinematics.hpp:42

# Run the program
(gdb) run

# When it stops, print a value
(gdb) print fk_result

# Continue running
(gdb) continue

Level 2: Essential Commands - The Fab Four

Now let's learn the four commands you'll use 90% of the time:

run         # Start your program
next        # Go to next line (like F10 in VS Code)
print       # Show a variable's value
continue    # Keep running after a breakpoint

Example session:

(gdb) break admittance_computations.cpp:300
(gdb) run
# Stops at line 300
(gdb) print kdl_chain_.getNrOfSegments()
$1 = 7
(gdb) next
# Now at line 301
(gdb) continue

Level 3: Stepping Through Code

Three ways to move through your code:

next     # Execute current line, don't go into functions
step     # Go INTO the function being called
finish   # Run until current function returns

Example - debugging our FK error:

(gdb) break get_X_BP_current
(gdb) run
# Stops at function start
(gdb) next      # Skip over simple lines
(gdb) next      # Until you reach line 132
(gdb) step      # Step INTO computeForwardKinematics

Level 4: Inspecting Complex Objects

For our KDL chain debugging:

# Make output pretty
(gdb) set print pretty on

# Print entire objects
(gdb) print components
(gdb) print kdl_chain_

# Call member functions
(gdb) print kdl_chain_.getNrOfSegments()
(gdb) print fk_pos_solver_->chain.getNrOfSegments()

# Show all local variables
(gdb) info locals

Level 5: Advanced - Multiple Breakpoints and Conditions

# Set multiple breakpoints
(gdb) break admittance_computations.cpp:298  # Before move
(gdb) break admittance_computations.cpp:304  # After move

# Conditional breakpoint
(gdb) break kinematics.hpp:42 if fk_result < 0

# See all breakpoints
(gdb) info breakpoints

# Delete breakpoint 2
(gdb) delete 2

Level 6: Complete Debug Session for KDL Error -4

Let's put it all together:

# 1. Build with debug symbols
cd ~/ros2_ws
colcon build --packages-select ur_admittance_controller --cmake-args -DCMAKE_BUILD_TYPE=Debug
source install/setup.bash

# 2. Start debugging
ros2 run --prefix 'gdb' ur_admittance_controller admittance_node

# 3. Set strategic breakpoints
(gdb) break admittance_computations.cpp:298   # Before std::move
(gdb) break admittance_computations.cpp:304   # After std::move
(gdb) break kinematics.hpp:42                 # Where error occurs

# 4. Run and inspect
(gdb) run

# At first breakpoint (line 298)
(gdb) print components.robot_chain.getNrOfSegments()
$1 = 7
(gdb) print components.fk_solver->chain.getNrOfSegments() 
$2 = 7

# Continue to after moves
(gdb) continue

# At second breakpoint (line 304)
(gdb) print kdl_chain_.getNrOfSegments()
$3 = 7
(gdb) print fk_pos_solver_->chain.getNrOfSegments()
$4 = 0    # <-- Problem found! Chain is empty in solver

# Continue to see the error
(gdb) continue
# FK failed: KDL error=-4

Pro Tips

  • Repeat last command: Just press Enter
  • Tab completion: Type part of variable name and press Tab
  • Interrupt running program: Ctrl+C
  • Exit GDB: quit or Ctrl+D
  • Command shortcuts: b for break, p for print, c for continue
  • See source code: list shows code around current line

Quick Reference Card

What you want Command Example
Start debugging gdb --args ros2 run pkg node gdb --args ros2 run ur_admittance_controller admittance_node
Run program run (gdb) run
Set breakpoint break file:line (gdb) break admittance_computations.cpp:300
Next line next (gdb) next
Step into step (gdb) step
Print value print variable (gdb) print kdl_chain_.getNrOfSegments()
Continue continue (gdb) continue
See stack backtrace (gdb) backtrace
Quit quit (gdb) quit