core-dumps

name	core-dumps
description	Core dump analysis skill for production crash triage. Use when loading core files in GDB or LLDB, enabling core dump generation on Linux/macOS, mapping symbols with debuginfo or debuginfod, or extracting backtraces from crashes without re-running the program. Activates on queries about core files, ulimit, coredumpctl, debuginfod, crash triage, or analyzing segfaults from production binaries.

Core Dumps

Purpose

Guide agents through enabling, collecting, and analysing core dumps for post-mortem crash investigation without rerunning the buggy program.

Triggers

• "My program crashed in production — how do I analyse the core?"
• "How do I enable core dumps on Linux?"
• "I have a core file but no symbols / source"
• "How do I use debuginfod to get symbols for a core?"
• "coredumpctl show me the crash"

Workflow

1. Enable core dumps (Linux)

# Per-session (lost on logout)
ulimit -c unlimited

# Persistent (add to /etc/security/limits.conf)
*   soft   core   unlimited
*   hard   core   unlimited

# Check current limit
ulimit -c

# Set core pattern (where and how cores are named)
# Default: 'core' in CWD — often not useful
sudo sysctl -w kernel.core_pattern=/tmp/core-%e-%p-%t
# %e = executable, %p = PID, %t = timestamp

# Persistent (add to /etc/sysctl.d/99-core.conf)
kernel.core_pattern=/tmp/core-%e-%p-%t
kernel.core_uses_pid=1

2. systemd/coredumpctl (modern Linux)

If systemd manages core dumps (common on Ubuntu 20+, Fedora, Arch):

# List recent crashes
coredumpctl list

# Show details of the latest crash
coredumpctl info

# Load latest crash in GDB
coredumpctl gdb

# Load specific PID crash
coredumpctl gdb 12345

# Export core file
coredumpctl dump -o myapp.core PID

Core storage location: /var/lib/systemd/coredump/.

3. Enable core dumps (macOS)

# macOS uses /cores by default (must be root-writable)
ulimit -c unlimited

# Check
ls /cores/

# launchd-launched services: set in plist
# <key>HardResourceLimits</key>
# <dict><key>Core</key><integer>9223372036854775807</integer></dict>

4. Analyse a core with GDB

# Load binary and core
gdb ./prog core.12345

# If the binary was stripped, provide the unstripped copy
gdb ./prog-with-symbols core.12345

# Essential first commands
(gdb) bt                    # call stack
(gdb) bt full               # stack + locals
(gdb) info registers        # CPU state at crash
(gdb) frame 2               # jump to interesting frame
(gdb) info locals           # local variables in frame
(gdb) print ptr             # inspect a pointer

# All threads (multi-threaded crash)
(gdb) thread apply all bt full

5. Analyse a core with LLDB

lldb ./prog -c core.12345

# Or
lldb
(lldb) target create ./prog --core core.12345

# Commands
(lldb) bt
(lldb) thread backtrace all
(lldb) frame select 2
(lldb) frame variable

6. Missing symbols: debuginfod

debuginfod serves debug symbols from a central server, mapping build IDs to DWARF data.

# Install client (Debian/Ubuntu)
sudo apt install debuginfod

# Enable (add to ~/.bashrc or /etc/environment)
export DEBUGINFOD_URLS="https://debuginfod.ubuntu.com https://debuginfod.elfutils.org"

# GDB auto-fetches symbols when DEBUGINFOD_URLS is set
gdb ./prog core

# Manually query
debuginfod-find debuginfo <build-id>
debuginfod-find source <build-id> /path/to/file.c

7. Missing symbols: manual approach

# Check if binary has a build ID
readelf -n ./prog | grep Build

# Find the correct debug package
# Debian: apt install prog-dbg or prog-dbgsym
# RPM: dnf install prog-debuginfo

# Point GDB to debug symbols directory
(gdb) set debug-file-directory /usr/lib/debug

# Or use eu-readelf to dump build ID, then find .debug file
eu-readelf -n ./prog
find /usr/lib/debug -name "*.debug" | xargs eu-readelf -n 2>/dev/null | grep <build-id>

8. Strip binaries and keep symbols

Best practice: build with symbols, strip for distribution, keep an unstripped copy.

# Build
gcc -g -O2 -o prog main.c

# Separate debug info
objcopy --only-keep-debug prog prog.debug
objcopy --strip-debug prog prog.stripped

# Add a debuglink so GDB finds the debug file automatically
objcopy --add-gnu-debuglink=prog.debug prog.stripped

# Deploy prog.stripped; keep prog.debug in a symbols store indexed by build-id

9. Quick triage from core without full debug session

# Print backtrace non-interactively
gdb -batch -ex 'bt full' -ex 'thread apply all bt full' ./prog core 2>&1 | tee crash.txt

# Print registers
gdb -batch -ex 'info registers' ./prog core

# Check signal that caused crash
gdb -batch -ex 'info signal' ./prog core

For a full cheatsheet covering core pattern tokens, coredumpctl, GDB/LLDB commands, debuginfod servers, and strip/symbol workflows, see references/cheatsheet.md.

Related skills

• Use skills/debuggers/gdb for full GDB session details
• Use skills/debuggers/lldb for LLDB-based analysis
• Use skills/runtimes/sanitizers to catch the bug before it reaches production
• Use skills/binaries/elf-inspection for readelf, build IDs, and binary inspection