Building pMARS from Distribution Sources

Building pMARS from Distribution Sources

Overview

This skill provides a systematic approach for building pMARS (and similar software) from Debian/​Ubuntu source packages with modifications such as removing X11 dependencies. It emphasizes proper diagnosis of build and runtime issues, verification at each step, and understanding root causes before applying fixes.

When to Use This Skill

Apply this skill when tasks require:

• Building pMARS or similar Core War simulators from source
• Downloading and extracting Debian/​Ubuntu source packages
• Removing X11/​GUI dependencies to create headless versions
• Modifying Makefiles to change build flags or library linkage
• Diagnosing and fixing segmentation faults in built software
• Working with DEB822-format apt source configurations

Core Workflow

Phase 1: Enable Source Repositories

Modern Debian/​Ubuntu systems use DEB822 format for apt sources. Enable source packages correctly:

Check current apt sources format:

# Look for .sources files (DEB822 format) vs sources.list (traditional)
ls /​etc/​apt/​sources.list.d/​*.sources
cat /​etc/​apt/​sources.list.d/​debian.sources

Enable sources in DEB822 format:

# Check current Types field
grep "^Types:" /​etc/​apt/​sources.list.d/​debian.sources

# Add deb-src to Types line
sed -i 's/^Types: deb$/​Types: deb deb-src/' /​etc/​apt/​sources.list.d/​debian.sources

Enable sources in traditional format:

# Uncomment or add deb-src lines
sed -i 's/^#deb-src/​deb-src/' /​etc/​apt/​sources.list

Update package cache:

apt-get update

Verification - critical step:

# Read back the modified file to verify edit was correct
cat /​etc/​apt/​sources.list.d/​debian.sources

# Verify sources are enabled (should show deb-src entries)
apt-cache policy

Common pitfall: File edits may be truncated or malformed. Always read back the modified file to verify correctness before proceeding.

Phase 2: Download Source Package

Install required tools and download the source:

# Install dpkg-dev for apt-get source functionality
apt-get install -y dpkg-dev

# Navigate to extraction directory
cd /​app

# Download source package (automatically extracts)
apt-get source pmars

What gets created:

• pmars_<version>.orig.tar.xz - Original upstream source
• pmars_<version>.debian.tar.xz - Debian patches and metadata
• pmars_<version>.dsc - Package description file
• pmars-<version>/ - Extracted and patched source directory

Verification:

# Verify extraction succeeded
ls -la /​app/​pmars-*/

# Check source directory structure
ls /​app/​pmars-*/​src/

Phase 3: Review Debian Patches First

Before making modifications, examine existing Debian patches:

cd /​app/​pmars-*/

# List available patches
ls debian/​patches/

# Read patch series (order of application)
cat debian/​patches/​series

# Read individual patches to understand known issues
cat debian/​patches/​*.patch

Why this matters:

• Debian maintainers have often fixed known issues
• Understanding existing patches prevents duplicate work
• Some patches may reveal build system quirks
• Conflicts with custom modifications can be avoided

Phase 4: Identify Build Configuration

Locate and understand the build configuration:

cd /​app/​pmars-*/​src/

# Read Makefile header comments (often document options)
head -100 Makefile | grep "^#"

# Identify key variables
grep -E "^CFLAGS|^LIBS|^LIB|^CC" Makefile

# Look for X11-related flags
grep -i "x11\|xwin\|graphx" Makefile

Common pMARS Makefile structure:

# Comments document available defines:
# -DEXT94      Enables ICWS'94 extensions
# -DXWINGRAPHX Enables X Window graphics
# -DGRAPHX     Enables general graphics support

CFLAGS += -O -DEXT94 -DXWINGRAPHX
LIB = -L/​usr/​X11R6/​lib -lX11

Phase 5: Remove X11 Dependencies

Modify CFLAGS to remove graphics defines:

# Remove -DXWINGRAPHX from CFLAGS
# Original: CFLAGS += -O -DEXT94 -DXWINGRAPHX
# Modified: CFLAGS += -O -DEXT94

Remove or comment out library linkage:

# Original: LIB = -L/​usr/​X11R6/​lib -lX11
# Modified: LIB =
# OR comment out entirely: # LIB = -L/​usr/​X11R6/​lib -lX11

Verification after edit:

# CRITICAL: Read back modified Makefile section
grep -E "^CFLAGS|^LIB" Makefile

# Verify X11 references removed
grep -i "x11\|xwin" Makefile

Common pitfall: Edits may not apply correctly or may be truncated. Always verify the modified section by reading it back.

Phase 6: Build the Software

cd /​app/​pmars-*/​src/
make clean
make

Handle compiler warnings appropriately:

• Warnings in C code may indicate real bugs
• Examine warnings rather than dismissing them
• Common legacy code warnings:
  • Implicit function declarations
  • Unused variables
  • Comparison between signed/​unsigned

Verification:

# Verify binary was created
ls -lh pmars

# Check it's not linked to X11
ldd pmars | grep -i x11
# Expected: no output (no X11 linkage)

# Verify basic functionality
./​pmars --help 2>&1 || ./​pmars 2>&1 | head -5

Phase 7: Install and Test

Install to target location:

cp /​app/​pmars-*/​src/​pmars /​usr/​local/​bin/​pmars
chmod +x /​usr/​local/​bin/​pmars

# Verify installation
ls -lh /​usr/​local/​bin/​pmars
which pmars

Progressive testing approach:

1. Test minimal functionality first:

   /​usr/​local/​bin/​pmars --help
   /​usr/​local/​bin/​pmars file1.red file2.red
   

2. Add flags incrementally:

   # Basic battle
   /​usr/​local/​bin/​pmars -b file1.red file2.red
   
   # With rounds
   /​usr/​local/​bin/​pmars -b -r 50 file1.red file2.red
   
   # With additional flags (test one at a time)
   /​usr/​local/​bin/​pmars -b -r 50 -f file1.red file2.red
   

3. Identify problematic flags:

   • If a flag causes crashes, note which flag
   • Determine if output requirements can be met without problematic flag
   • Investigate root cause before applying fixes

Debugging Segmentation Faults

When encountering crashes, follow this systematic approach:

Step 1: Establish Baseline

# Find minimal reproduction case
/​usr/​local/​bin/​pmars file1.red file2.red          # Test without flags
/​usr/​local/​bin/​pmars -b file1.red file2.red       # Add one flag
/​usr/​local/​bin/​pmars -b -r 50 file1.red file2.red # Add another

# Document which command works and which crashes

Step 2: Use Debugger for Diagnosis

apt-get install -y gdb

# Get backtrace
gdb -batch -ex "run -b -r 50 -f file1.red file2.red" -ex "bt" /​usr/​local/​bin/​pmars

# Interactive debugging
gdb /​usr/​local/​bin/​pmars
(gdb) run -b -r 50 -f file1.red file2.red
(gdb) bt

Step 3: Analyze the Discrepancy

Critical insight: If a crash occurs standalone but not under gdb, this indicates:

• Undefined behavior (memory layout differences)
• Timing-sensitive issues
• Uninitialized memory being used
• Memory corruption

Do not assume a simple NULL check will fix the issue. Investigate:

• What variable is NULL or uninitialized?
• Why is it in that state?
• Is this a symptom of a deeper initialization problem?

Step 4: Understand Before Fixing

Before applying source code fixes:

1. Read relevant source code sections
2. Understand the data flow - trace where problematic variables are initialized
3. Check if Debian patches address the issue
4. Document the root cause hypothesis

Example analysis:

// If crash occurs in checksum_warriors() dereferencing instBank
// Questions to ask:
// - When should instBank be initialized?
// - Are all warriors in endWar array properly initialized?
// - Is the iteration count correct?

Step 5: Apply Targeted Fix

Only after understanding root cause:

// Document why this fix is appropriate
// If instBank can legitimately be NULL when feature disabled:
if (instBank != NULL) {
    // proceed with operation
}

Verify the fix:

# Rebuild after source modification
make clean && make

# Read back modified source to verify edit
grep -A5 -B5 "your_fix" modified_file.c

# Test the specific failing scenario
/​usr/​local/​bin/​pmars -b -r 50 -f file1.red file2.red

Common Pitfalls and Solutions

Pitfall 1: File Edit Truncation

Problem: Edit operations may truncate strings or introduce errors.

Prevention:

# ALWAYS read back modified sections
cat modified_file | grep -A2 -B2 "modified_line"

# Verify file is syntactically valid (for known formats)

Pitfall 2: Incomplete Diagnosis

Problem: Applying speculative fixes without confirming root cause.

Prevention:

• Use debugger to get exact crash location
• Read source code to understand context
• Document hypothesis before fixing
• If behavior differs between gdb and standalone, investigate why

Pitfall 3: Bypassing Debian Build System

Problem: Modifying upstream Makefile directly may miss Debian patches.

Consideration:

# Check if debian/​rules applies additional patches
cat debian/​rules

# Alternative: use Debian's build system
dpkg-buildpackage -us -uc -b

Pitfall 4: Ignoring Compiler Warnings

Problem: Warnings in C code may indicate real bugs.

Prevention:

# Examine warnings during build
make 2>&1 | grep -i warning

# Consider building with stricter flags for diagnosis
make CFLAGS="-Wall -Wextra"

Pitfall 5: Not Verifying Test Files

Problem: Test input files may be malformed.

Prevention:

# Verify warrior files exist and have content
ls -l file1.red file2.red
head file1.red

Priority Framework

When working on build tasks:

P0 - Must Complete:

• Binary builds successfully
• Binary installs to correct location
• Core functionality works (basic execution)
• Required output format matches specification

P1 - Important:

• All specified flags work correctly
• No X11 dependencies in final binary
• Clean build without errors

P2 - Nice to Have:

• No compiler warnings
• All optional features working
• Perfect adherence to original test command

Apply framework: If a specific flag causes issues but core output requirement can be met without it, evaluate whether the flag is truly required or just part of an example command.

Verification Checklist

Before marking task complete:

• Source repositories enabled and verified
• Source package downloaded and extracted
• Debian patches reviewed for relevant fixes
• X11 dependencies removed from Makefile (verified by reading back)
• Binary compiled successfully
• Binary verified not linked to X11 (ldd shows no X11)
• Binary installed to correct location (/​usr/​local/​bin/​pmars)
• Basic functionality tested (minimal arguments)
• Required test command passes with expected output format
• Any source modifications verified by reading back the changes
• Root cause understood for any bugs fixed (not just symptoms masked)

Quick Reference

# Enable sources (DEB822)
sed -i 's/^Types: deb$/​Types: deb deb-src/' /​etc/​apt/​sources.list.d/​debian.sources
apt-get update

# Download source
apt-get install -y dpkg-dev
cd /​app && apt-get source pmars

# Modify Makefile (remove X11)
cd /​app/​pmars-*/​src/
# Edit: Remove -DXWINGRAPHX from CFLAGS
# Edit: Comment out or clear LIB = -lX11 line

# Build and verify
make clean && make
ldd pmars | grep -i x11  # Should show nothing

# Install
cp pmars /​usr/​local/​bin/​pmars
chmod +x /​usr/​local/​bin/​pmars

# Test progressively
/​usr/​local/​bin/​pmars file1.red file2.red
/​usr/​local/​bin/​pmars -b -r 50 file1.red file2.red