lib42 – A Personal C Foundation Library

Built during the 42 curriculum by aghergut (42 Madrid)

Overview

lib42 is my personal C library that started as a reimplementation of standard C functions (libft) and grew to include ft_printf and get_next_line, all integrated into a single static library.

The goal wasn't just to reproduce existing functions — it was to understand:

• How C works at a low level
• How to design reusable code
• How to grow a project without breaking its structure

This repository reflects my progression from basic memory manipulation to complex architectural decisions involving state, extensibility, and I/O management.

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What This Project Taught Me

Working on lib42 helped me develop:

Low-level reasoning

Understanding memory, pointers, buffers, and system calls instead of relying on abstractions.

Architectural thinking

Designing functions that can evolve (especially in ft_printf) and managing shared logic inside a single library.

Discipline & debugging

Writing defensive code, handling edge cases, and learning to debug segmentation faults, leaks, and undefined behavior.

Code ownership

Every line was written, tested, and fixed by me. I now understand why things work, not just that they work

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Project Structure

lib42/
├── libft.h              # Main header file
├── Makefile             # Compilation rules
│
├── changechar/          # Character case conversion (toupper, tolower)
├── checkchar/           # Character validation (isalpha, isdigit, etc.)
├── strtoint/            # String ↔ Integer conversion (atoi, itoa)
├── strings/             # String manipulation (20+ functions)
├── memory/              # Memory operations (memcpy, memset, etc.)
├── puts/                # Output functions (putchar_fd, putstr_fd, etc.)
├── lists/               # Linked list operations (9 functions)
├── maps/                # Working with 2D arrays
│
├── ft_printf/           # Printf reimplementation
│   ├── ft_printf.c
│   ├── ft_printf.h
│   ├── flags/           # Flag handlers (-, 0, #, +, space)
│   ├── prints/          # Type-specific printers (char, str, int, hex, ptr)
│   └── utils/           # Helper utilities
│
└── get_next_line/       # Line reading from file descriptors
    ├── get_next_line.c
    └── get_next_line.h

For detailed documentation of each module, see the README files in respective folders:

• ft_printf/ - Formatted output implementation
• get_next_line/ - File reading utilities
• lists/ - Linked list data structure
• memory/ - Memory operations
• maps/ - Two dimensional arrays operations

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Compilation & Usage

# Compile the library
make

This creates libft.a — a static library containing all functions.

Using in your project:

#include "libft.h"

cc main.c -L. -lft -o program

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Testing

Each module was tested through:

• Unit tests: Small dedicated test files for each function
• Edge cases: NULL pointers, empty inputs, buffer overlaps, extreme values
• Memory validation: Valgrind checks for leaks and invalid access
• Comparison testing: Output validation against system functions

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What's Inside

Module	Functions	Purpose
Character checks	5	isalpha, isdigit, isalnum, isascii, isprint
String operations	20+	strlen, strchr, strjoin, split, substr, etc.
Memory operations	5	memset, memcpy, memmove, memchr, memcmp
Linked lists	9	Create, add, delete, iterate, map
Conversions	2	atoi, itoa
Output	4	putchar_fd, putstr_fd, putendl_fd, putnbr_fd
ft_printf	1	Full printf with flags (cspdiuxX%)
get_next_line	1	Read lines from any file descriptor

Total: 50+ functions

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Final Note

This library represents my transition from writing C code to thinking like a C programmer.

It is not perfect — but it is mine, fully understood, and designed to grow.

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Status: Completed with bonuses ✅
Standards: Follows 42 Norm, compiles with -Wall -Wextra -Werror
Memory: Zero leaks (Valgrind verified)