Project 19: File Utility Suite

Create basic file utilities like cat, head, tail, and grep.

Quick Reference

Attribute Value
Difficulty Level 3 (Advanced)
Time Estimate 1-2 weeks
Language C
Prerequisites Basic C syntax, Functions and control flow, File I/O basics, Strings and parsing
Key Topics file I/O, text processing, argv parsing, error handling, testing

1. Learning Objectives

By completing this project, you will:

  1. Apply file I/O in a real program
  2. Apply text processing in a real program
  3. Apply argv parsing in a real program
  4. Apply error handling in a real program

2. Theoretical Foundation

2.1 Core Concepts

  • file I/O: Core concept needed for this project.
  • text processing: How it shapes correctness and design trade-offs.
  • argv parsing: Practical rules that impact implementation.

2.2 Why This Matters

These topics appear in production C code constantly and are easiest to learn by building a full end-to-end tool.

2.3 Historical Context / Background

C practices around file I/O evolved to keep programs portable across compilers and platforms.

2.4 Common Misconceptions

  • Assuming input is always well-formed
  • Forgetting that C does not manage memory for you
  • Ignoring edge cases until late in development

3. Project Specification

3.1 What You Will Build

A set of small utilities with shared I/O helpers.

3.2 Functional Requirements

  1. Implement the core features described above
  2. Validate inputs and handle error cases
  3. Provide a CLI demo and tests

3.3 Non-Functional Requirements

  • Performance: Must handle typical inputs without noticeable delay
  • Reliability: Must reject invalid inputs and fail safely
  • Usability: Output should be readable and consistent

3.4 Example Usage / Output

$ ./mycat file.txt
(contents)
$ ./myhead -n 3 file.txt
(first 3 lines)

3.5 Real World Outcome

$ ./mygrep error log.txt
3:error: failed to connect

4. Solution Architecture

4.1 High-Level Design

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│  Input      │────▶│  Core Logic │────▶│  Output     │
└─────────────┘     └─────────────┘     └─────────────┘

Program Flow

4.2 Key Components

Component Responsibility Key Decisions
Parser Turn text into structured input Use simple rules first
Reader Stream input safely Buffered reads
Core Logic Apply main rules Keep functions small

4.3 Data Structures

struct LineBuf { char *buf; size_t len; size_t cap; };

4.4 Algorithm Overview

Key Algorithm: Streaming scan

  1. Read chunks
  2. Update counters
  3. Report results

Complexity Analysis:

  • Time: O(n)
  • Space: O(1)

5. Implementation Guide

5.1 Development Environment Setup

# Build
cc -std=c99 -Wall -Wextra -o demo *.c

5.2 Project Structure

project-root/
├── src/
│   ├── main.c
│   ├── core.c
│   └── core.h
├── tests/
│   └── test_core.c
├── Makefile
└── README.md

Project Structure

5.3 The Core Question You’re Answering

“How do core CLI file tools handle streaming input?”

5.4 Concepts You Must Understand First

Stop and research these before coding:

  1. File I/O
    • What is it and why does it matter here?
    • How will you validate inputs around it?
    • Book Reference: Ch. 21-22
  2. Text Processing
    • What common mistakes happen with this concept?
    • How do you test it?
    • Book Reference: Ch. 21-22
  3. Argv Parsing
    • What edge cases show up in real programs?
    • How will you observe failures?
    • Book Reference: Ch. 21-22

5.5 Questions to Guide Your Design

Before implementing, think through these:

  1. What is the smallest input that should work?
  2. What is the riskiest edge case?
  3. Where should errors be reported to the user?

5.6 Thinking Exercise

Sketch a sample input and output by hand, then trace the steps your program must perform to transform one into the other.

5.7 The Interview Questions They’ll Ask

Prepare to answer these:

  1. Explain how file I/O works in C.
  2. What are common mistakes with text processing?
  3. How do you test edge cases for argv parsing?
  4. How do you handle errors without exceptions in C?
  5. What would you refactor first in your solution?

5.8 Hints in Layers

Hint 1: Start with the smallest possible input and prove the output. Hint 2: Add validation before adding features. Hint 3: Write tests for edge cases early. Hint 4: Refactor once the behavior is correct.

5.9 Books That Will Help

Topic Book Chapter
Core project concepts “C Programming: A Modern Approach” Ch. 21-22
C idioms “The C Programming Language” Ch. 1-3
Defensive C “Effective C” Ch. 2-5

5.10 Implementation Phases

Phase 1: Foundation (2-4 hours)

Goals:

  • Set up project structure
  • Implement minimal working path

Tasks:

  1. Create headers and source files
  2. Build a minimal demo

Checkpoint: You can compile and run a basic demo

Phase 2: Core Functionality (4-8 hours)

Goals:

  • Implement main features
  • Handle errors

Tasks:

  1. Add main functions
  2. Add validation and error codes

Checkpoint: All main requirements work with sample inputs

Phase 3: Polish & Edge Cases (2-4 hours)

Goals:

  • Handle edge cases
  • Improve usability

Tasks:

  1. Add tests for edge cases
  2. Clean output and docs

Checkpoint: All tests pass and output is stable

5.11 Key Implementation Decisions

Decision Options Recommendation Rationale
Data representation Array vs struct Struct Clear invariants and interfaces
Error handling Return codes vs global Return codes Easier to test and reason

6. Testing Strategy

6.1 Test Categories

Category Purpose Examples
Unit Tests Test core functions Single input -> expected output
Integration Tests End-to-end CLI Sample input files
Edge Case Tests Boundaries and invalid input Empty input, max values

6.2 Critical Test Cases

  1. Valid input for the happy path
  2. Invalid input that should be rejected
  3. Boundary values and empty input

6.3 Test Data

$ ./mycat file.txt

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

Pitfall Symptom Solution
Ignoring partial reads Missing data Loop until EOF
Off-by-one errors Boundary failures Test edges explicitly
Memory misuse Leaks or crashes Free resources on all paths

7.2 Debugging Strategies

  • Use -Wall -Wextra -fsanitize=address during development
  • Add small debug prints around the failing step

7.3 Performance Traps

Avoid unnecessary copies and repeated scans of the same data.

8. Extensions & Challenges

8.1 Beginner Extensions

  • Add basic configuration flags
  • Improve output formatting

8.2 Intermediate Extensions

  • Support file input and output
  • Add richer error messages

8.3 Advanced Extensions

  • Optimize performance for large inputs
  • Add extra features beyond the original scope

9. Real-World Connections

9.1 Industry Applications

  • Developer tooling: Uses similar parsing and reporting patterns
  • Systems utilities: Requires careful input validation and performance
  • coreutils: Small utilities that mirror these patterns
  • musl: Clean, minimal C implementations

9.3 Interview Relevance

  • file I/O: Common in systems interviews
  • text processing: Used to test fundamentals

10. Resources

10.1 Essential Reading

  • “C Programming: A Modern Approach” by K.N. King - Ch. 21-22
  • “The Linux Programming Interface” by Michael Kerrisk - relevant I/O chapters

10.2 Video Resources

  • “YouTube: C programming deep dives”
  • “YouTube: Debugging C with gdb”

10.3 Tools & Documentation

  • GCC/Clang: Compiler and warnings
  • GDB/LLDB: Debugging runtime behavior
  • Previous Project: Bit Manipulation Toolkit
  • Next Project: Math and String Library Implementation

11. Self-Assessment Checklist

Before considering this project complete, verify:

11.1 Understanding

  • I can explain the main concepts without notes
  • I can describe the data flow and key structures
  • I understand why key design decisions were made

11.2 Implementation

  • All functional requirements are met
  • All test cases pass
  • Code is clean and documented
  • Edge cases are handled

11.3 Growth

  • I can identify one improvement for next time
  • I’ve documented lessons learned
  • I can explain this project in an interview

12. Submission / Completion Criteria

Minimum Viable Completion:

  • Functional CLI or library output with sample inputs
  • Clear error handling for invalid inputs
  • Tests for at least 3 edge cases

Full Completion:

  • All minimum criteria plus:
  • Clean project structure with docs
  • Expanded tests for invalid inputs

Excellence (Going Above & Beyond):

  • Performance or UX improvements
  • Extra extensions implemented

This guide was generated from LEARN_C_MODERN_APPROACH_KING.md. For the complete learning path, see the parent directory.