0Pricing
Production Debugging & Incident Response Playbook · Lesson

Post-mortem Debugging with Core Dumps

Learn to analyze core dumps and crash reports to debug issues that occurred in the past, without live access.

Post-mortem Debugging with Core Dumps is a free Production Debugging & Incident Response Playbook lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Production Debugging & Incident Response Playbook learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Debugging After the Fact

Welcome! In this lesson, we'll explore post-mortem debugging. This powerful technique lets you investigate software failures after they've occurred, without needing to reproduce the issue live.

It's incredibly useful when you can't attach a debugger directly to a crashing application, especially in production environments.

What's a Core Dump?

The cornerstone of post-mortem debugging is the core dump. Think of it as a snapshot of a program's entire memory space and CPU state at the exact moment it crashed.

  • It's a file generated by the operating system.
  • It contains critical information about the program's execution.
  • It helps you understand why a crash happened.

When Do Core Dumps Happen?

Core dumps are typically generated when a program encounters a severe, unhandled error that causes it to terminate unexpectedly. Common scenarios include:

  • Segmentation Faults (Segfaults): Accessing invalid memory.
  • Unhandled Exceptions: Language-specific errors not caught by the program.
  • Assertion Failures: When a program's internal assumptions are violated.
  • Program Crashes: Any abrupt, abnormal termination.

Enabling Core Dumps (Linux)

On Linux systems, core dump generation might be disabled by default or limited in size. You can enable it:

  • Temporarily: Use ulimit -c unlimited in your shell session.
  • System-wide: Modify /etc/sysctl.conf (e.g., kernel.core_pattern to specify output path and filename format).

Without proper configuration, your system might not save core dumps when crashes occur.

Core Dump Contents

A core dump is packed with forensic data. It typically includes:

  • Memory Image: A copy of the program's entire virtual memory.
  • CPU Register Values: The state of the CPU registers at the crash time.
  • Stack Trace: The sequence of function calls leading up to the crash.
  • Process Information: Process ID, signal that caused the crash, executable path.
  • Loaded Libraries: Information about shared libraries linked to the program.

Key Analysis Tools

To make sense of a core dump, you need specialized tools. Some popular ones include:

  • GDB (GNU Debugger): Widely used for C/C++ programs on Linux/Unix.
  • WinDbg: Microsoft's powerful debugger for Windows applications.
  • jstack/jmap: For Java applications, these tools can extract thread dumps and memory maps that act as a form of 'core dump'.
  • Delve: A debugger for Go programs.

We'll focus on GDB as a common example.

Crash Program Demo

Let's look at a simple C program that will intentionally cause a segmentation fault. This will generate a core dump file if your system is configured to do so.

Try compiling and running this code:

#include <stdio.h>
#include <stdlib.h>

int main() {
  int *ptr = NULL; // Declare a null pointer
  printf("Attempting to dereference a null pointer...\n");
  *ptr = 10;       // This line will cause a segmentation fault
  printf("This line will not be reached.\n");
  return 0;
}

Basic GDB Usage: Backtrace

After the program crashes and creates a core dump (e.g., core or core.PID), you can load it into GDB. Assuming your executable is a.out:

gdb ./a.out core
bt

The bt (backtrace) command is essential. It shows the call stack leading to the crash, helping you pinpoint the exact function and line number where the error occurred.

Inspecting Variables with GDB

Once you have the backtrace, you can navigate the stack frames (e.g., using frame N where N is the frame number). Then, you can inspect variable values at that point in time:

  • print variable_name: Shows the value of a specific variable.
  • info locals: Lists all local variables in the current stack frame and their values.

This helps you understand the state of the program's data when it crashed.

Core Dump Quiz

Let's check your understanding of core dumps.

Recap: Post-mortem Power

Great work! You've learned about the power of post-mortem debugging using core dumps.

  • Core dumps are memory snapshots of crashed programs.
  • They contain vital info like stack traces and variable states.
  • Tools like GDB help analyze them without live access.

This technique is indispensable for debugging hard-to-reproduce or production-only issues, enabling you to fix problems even after the event.

Frequently asked questions

Is the “Post-mortem Debugging with Core Dumps” lesson free?

Yes — the full text of “Post-mortem Debugging with Core Dumps” is free to read here on the web, and the Production Debugging & Incident Response Playbook course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Production Debugging & Incident Response Playbook course, upgrade to CoddyKit PRO.

What will I learn in “Post-mortem Debugging with Core Dumps”?

Learn to analyze core dumps and crash reports to debug issues that occurred in the past, without live access. You practise Production Debugging & Incident Response Playbook with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Production Debugging & Incident Response Playbook?

No prior experience is required. Production Debugging & Incident Response Playbook on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Post-mortem Debugging with Core Dumps” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Production Debugging & Incident Response Playbook lesson?

Yes. Every Production Debugging & Incident Response Playbook lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Remote Debugging Live Applications
  2. Post-mortem Debugging with Core Dumps
  3. Memory and CPU Profiling Techniques
  4. Distributed Tracing for Latency Hotspots
← Back to Production Debugging & Incident Response Playbook