Gdb

GNU Debugger (GDB) Skill

Non-blocking GDB debugging for AI agents. Trace live processes, set conditional breakpoints, and inspect program state while the agent stays responsive and can multitask.

How It Works

Agents are single-threaded — if an agent runs GDB directly and waits for a breakpoint, it blocks entirely. It can't respond to the user, answer questions, or change strategy until the breakpoint fires (which may take a very long time).

This skill avoids that by running GDB in the background via a named pipe. The agent sends commands by writing to the pipe and reads results from a log file, so it never blocks. Instead of traditional breakpoints, it uses dprintf (dynamic printf) — a GDB feature that prints formatted output when hit and automatically continues execution without stopping.

This means the agent can:

1. Start a debug session and immediately return to the user.
2. Sample the log file periodically to report progress.
3. Multitask (e.g., search the web, edit code) while the target process runs.
4. Interrupt and modify tracepoints dynamically without killing the session.

The skill includes helper scripts that encapsulate all the pipe management, signal handling, and quoting logic — so any agent that can run shell commands can use it.

Core Concepts

1. Background Execution: GDB runs in the background. You interact with it by writing to its standard input (stdin).
2. Non-Blocking: Use dprintf (dynamic printf) instead of break. This inserts a printf call and continues execution immediately.
3. Log Sampling: GDB output is redirected to a file (set logging file ...). You read this file to see the trace.
4. Dynamic Updates: proper interrupt handling is required to add/remove tracepoints on a running process.

Prerequisites & Setup

Requirements

• Linux OS
• gdb-multiarch (recommended) or gdb

Installation

# Debian/Ubuntu
sudo apt-get update && sudo apt-get install -y gdb-multiarch

# Fedora/RHEL
sudo dnf install -y gdb

# Arch
sudo pacman -S gdb

# Verify
gdb-multiarch --version || gdb --version

• Target process is running.
• If remote: gdbserver running on target.

Workflow

1. Start a Session (Universal Method)

This method works with any agent that can run shell commands, without needing special stdin handling.

Recommended — use the helper script:

./scripts/gdb_start.sh ./path/to/executable
# Creates: gdb_cmd_pipe (named pipe), .gdb_pid (PID file), trace.log (output)

Manual method (if you need more control):

1. Create a Named Pipe (FIFO):

   mkfifo gdb_cmd_pipe
   

2. Start GDB reading from the pipe:

   tail -f gdb_cmd_pipe | gdb-multiarch -q -x scripts/setup.gdb --args ./executable &
   echo $! > .gdb_pid
   

Interaction Protocol:

1. Send Commands: ./scripts/gdb_send.sh "command" (or echo "command" > gdb_cmd_pipe)
2. Interrupt: kill -INT $(cat .gdb_pid)
3. Resume: ./scripts/gdb_send.sh "continue"

2. Configure Logging (The Easy Way)

Use the provided scripts/setup.gdb to automatically configure non-blocking logging.

# Start GDB with the setup script
gdb-multiarch -q -x scripts/setup.gdb -p <PID>

If you need to configure it manually (e.g., if you forgot -x):

set pagination off
set logging file trace.log
set logging enabled on

3. Set Non-Blocking Tracepoints (dprintf)

Use dprintf to print variables or status without pausing the program.

Format: dprintf <LOCATION>, "<FORMAT_STRING>", <ARGUMENTS>

Examples:

dprintf main.c:120, "Iteration %d, Value: %s\n", i, some_string
dprintf process_request, "Got request ID: %d\n", req->id

Send continue to let the process run while tracing:

continue

4. Conditional Tracing

To trace only when a condition is true, use the helper script:

./scripts/gdb_trace.sh loop_function '"Hit 10: %d\n", iteration' 'iteration == 10'

Or manually via the pipe:

1. Set the dprintf:

   ./scripts/gdb_send.sh 'dprintf loop_function, "Hit 10: %d\n", iteration'
   # Output in trace.log: Dprintf 2 at ...
   

2. Apply the condition. Use $bpnum (GDB's convenience variable for the last breakpoint number). Important: Escape the $ so the shell doesn't expand it:

   ./scripts/gdb_send.sh 'condition $bpnum iteration == 10'
   

5. Sampling the Trace

Read the log file to see what's happening.

tail -n 20 trace.log

6. Modifying Tracepoints (The Interrupt Pattern)

Use the helper script to replace all tracepoints in one call:

# Replaces all existing tracepoints with a new one:
./scripts/gdb_trace.sh new_func '"New trace: %d\n", x'

# Or clear all tracepoints without setting new ones:
./scripts/gdb_trace.sh --clear

Manual method (if you need finer control):

1. Interrupt — kill -INT $(cat .gdb_pid)
2. Modify — ./scripts/gdb_send.sh "delete" then ./scripts/gdb_send.sh 'dprintf ...'
3. Resume — ./scripts/gdb_send.sh "continue"

Helper Scripts

The scripts/ directory provides helper scripts that handle quoting, timing, and PID tracking. Use these instead of raw echo commands — they save tokens and avoid shell quoting errors.

Example Session

Here is the complete workflow using the helper scripts. These are plain shell commands that work with any agent (Cursor, Claude Code, Gemini CLI, Antigravity, Aider, etc.).

1. Start GDB

./scripts/gdb_start.sh ./demo_app
# Output: GDB started. PID=12345, pipe=gdb_cmd_pipe, log=trace.log

2. Set a conditional tracepoint

./scripts/gdb_trace.sh loop_function '"iteration=%d\n", iteration' 'iteration == 10'

3. Run the program

./scripts/gdb_send.sh "run"

4. Sample the trace

tail -n 20 trace.log

5. Modify the tracepoint dynamically

./scripts/gdb_trace.sh loop_function '"new trace: %d\n", iteration' 'iteration == 50'

6. Clear all tracepoints (keep process running)

./scripts/gdb_trace.sh --clear

7. Stop GDB and clean up

./scripts/gdb_stop.sh
# Kills GDB + target + tail feeder, waits for exit, removes session files.

Agent Compatibility

The named pipe method was designed to work with any agent that can execute shell commands. It requires only three primitives: (1) write to a file, (2) send POSIX signals, (3) read a file.

Key design choice: The helper scripts (gdb_start.sh, gdb_send.sh, gdb_trace.sh) encapsulate all quoting and timing logic, so agents don't need to reason about shell escaping or sleep intervals. Every agent calls the same scripts the same way.

Limitations

1. Linux Only: This workflow relies on POSIX signals (SIGINT) and Linux process management. It will not work on Windows.
2. Permissions (ptrace):
   • Attaching to a running process (-p PID) often requires sudo or adjusting /proc/sys/kernel/yama/ptrace_scope (set to 0).
   • Workaround: Start the process via GDB (--args ./exe) to avoid this.
3. Signal Handling: If the target process masks SIGINT, interrupting it might be difficult. You may need to use kill -STOP / kill -CONT cautiously, but that stops GDB logic too.
4. Blind Spots: Since you are not seeing stdout in real-time (only the log file), you might miss immediate errors if you don't check the log frequently.

Common Pitfalls

These are real issues discovered in practice. Read them before starting a session.

1. Conditional tracepoint on a value that already passed

If the program has a monotonically increasing counter (e.g., iteration++) and you set condition $bpnum iteration == 20 after the counter is already at 50, the tracepoint will never fire. The counter will never be 20 again.

Fix: Before setting a conditional tracepoint on a changing value, interrupt and inspect the current value first. Then pick a target value that hasn't been reached yet.

# Interrupt, check current state, then set a future target
kill -INT $(cat .gdb_pid) && sleep 1
echo "frame 3" > gdb_cmd_pipe && sleep 0.5   # navigate to the right frame
echo "print counter" > gdb_cmd_pipe && sleep 1
tail -n 5 trace.log                           # read the value
# Now set condition for a value AHEAD of where the program is

2. Cleanup race: removing the pipe before GDB exits

If you delete gdb_cmd_pipe before GDB has finished processing kill/quit commands sent through that pipe, GDB and its child processes will linger as orphans. The pipe is gone, so GDB can no longer read commands and the tail -f feeder also hangs.

Fix: Use ./scripts/gdb_stop.sh which kills processes first, waits for them to die, and only then removes session files. Never rm the pipe as a first step.

3. Commands dropped after SIGINT

After sending kill -INT to GDB, it needs time to stop the inferior and return to the (gdb) prompt. Commands sent too soon (e.g., 0.3s later) may be silently dropped.

Fix: Wait at least 1 second after SIGINT before sending the next command. The helper scripts (gdb_trace.sh) already account for this.

4. GDB cannot process commands while the inferior is running

In all-stop mode (the default), GDB blocks on the running inferior. Commands like info breakpoints sent via the pipe are queued but not executed until the inferior stops.

Fix: Always interrupt first, then send your query command, read the log, and then continue. Example:

kill -INT $(cat .gdb_pid) && sleep 1
./scripts/gdb_send.sh "info breakpoints"
sleep 1 && tail -n 10 trace.log
./scripts/gdb_send.sh "continue"

5. Log output appears delayed

GDB's logging (set logging enabled on) can buffer output. After a dprintf fires or a command runs, the result may not appear in trace.log immediately.

Fix: Wait 1–2 seconds before reading the log after sending a command. Don't assume the log is broken just because it looks stale — check again after a brief delay.

Agent Guidance (Reducing Tool Calls)

Follow these patterns to handle common GDB tasks in fewer round-trips.

Before setting a conditional tracepoint on a running program

If the condition depends on a changing value (counter, timestamp, etc.), you MUST check the current value first to avoid setting a condition that will never fire. Do this in one shell call:

kill -INT $(cat .gdb_pid) && sleep 1 \
  && echo "frame 3" > gdb_cmd_pipe && sleep 0.5 \
  && echo "print counter" > gdb_cmd_pipe && sleep 1 \
  && tail -n 5 trace.log

Then set the tracepoint for a value ahead of the current one, and continue — again in one call:

./scripts/gdb_trace.sh loop_function '"trace: %d\n", iteration' 'iteration == <FUTURE_VALUE>'

Setting a tracepoint and immediately running

When the program hasn't been started yet (run not called), you can set the tracepoint and run in one call since there's nothing to interrupt:

./scripts/gdb_trace.sh loop_function '"iteration=%d\n", iteration' 'iteration == 10' \
  && ./scripts/gdb_send.sh "run"

Sampling the log after a tracepoint

Don't check the log immediately — give the program time to reach the tracepoint. Estimate the wait from the loop interval and current position, then combine the wait and the read into one call:

sleep 5 && tail -n 20 trace.log

Inspecting state (interrupt + query + resume)

Batch all three steps into a single shell call:

kill -INT $(cat .gdb_pid) && sleep 1 \
  && ./scripts/gdb_send.sh "info breakpoints" \
  && sleep 1 && tail -n 15 trace.log \
  && ./scripts/gdb_send.sh "continue"

Shutdown and cleanup

Always use the dedicated stop script — one call handles everything:

./scripts/gdb_stop.sh

Best Practices

• Use the helper scripts instead of raw echo commands. They handle quoting, timing, and PID tracking.
• Always use gdb-multiarch if available (auto-detected by gdb_start.sh).
• Trace lightly: High-frequency tracepoints (e.g., inside tight loops) will slow down execution significantly.
• Avoid blocking commands: Never issue a GDB command that waits for user input (like command without end) unless you are sure you can provide it.
• Check trace.log frequently: Since stdout isn't visible in real-time, the log is your only window into the process.
• Clean up with gdb_stop.sh: Always use the stop script instead of manually removing files. It kills processes first, avoiding orphaned GDB/tail processes.

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