libft

A hand-written C standard library, built from scratch as part of the curriculum at 42. Every function in this project is a faithful reimplementation of its libc counterpart — no calls to the real thing, no shortcuts.

No AI was used during the development or research of this project. AI was used solely to write this README.

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What is it?

libft is a static C library (libft.a) that reimplements a broad subset of the standard C library (libc), plus a handful of extra utilities and a full singly-linked list module. It serves as the personal toolbox that carries forward into every subsequent 42 project — instead of relying on libc, you rely on code you understand completely because you wrote it.

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Why?

The point of the exercise is not to reinvent the wheel for its own sake. It is to force a deep understanding of how foundational tools actually work:

• How does memcpy handle overlapping memory regions (it doesn't — that's what memmove is for)?
• What does strlcpy return when the destination is too small?
• How do you split a string without knowing its length in advance?

Writing these functions by hand makes the answers obvious. It also means you can read, audit, and trust every line of code in your toolbox.

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Contents

Character classification & conversion

Function	Description
ft_isalpha	Is the character alphabetic?
ft_isdigit	Is it a decimal digit?
ft_isalnum	Is it alphanumeric?
ft_isascii	Is it a valid ASCII character (0–127)?
ft_isprint	Is it a printable character?
ft_toupper	Convert to uppercase
ft_tolower	Convert to lowercase

Memory

Function	Description
ft_memset	Fill a block of memory with a byte value
ft_bzero	Zero out a block of memory
ft_memcpy	Copy memory (no overlap handling)
ft_memmove	Copy memory safely, even with overlapping regions
ft_memchr	Find the first occurrence of a byte in memory
ft_memcmp	Compare two blocks of memory
ft_calloc	Allocate zero-initialised memory

Strings

Function	Description
ft_strlen	Length of a null-terminated string
ft_strlcpy	Safe string copy (always null-terminates)
ft_strlcat	Safe string concatenation
ft_strchr	First occurrence of a character in a string
ft_strrchr	Last occurrence of a character in a string
ft_strncmp	Compare up to n characters of two strings
ft_strnstr	Find a substring within a length-bounded string
ft_strdup	Duplicate a string (heap-allocated)
ft_substr	Extract a substring
ft_strjoin	Concatenate two strings into a new allocation
ft_strtrim	Trim a set of characters from both ends of a string
ft_split	Split a string by a delimiter into a null-terminated array
ft_strmapi	Apply a function to each character, returning a new string
ft_striteri	Apply a function to each character in place

Conversion

Function	Description
ft_atoi	Parse an integer from a string
ft_itoa	Convert an integer to a heap-allocated string

Output (file-descriptor based)

Function	Description
ft_putchar_fd	Write a character to a file descriptor
ft_putstr_fd	Write a string to a file descriptor
ft_putendl_fd	Write a string followed by a newline to a file descriptor
ft_putnbr_fd	Write an integer to a file descriptor

Bonus — Linked lists

A generic singly-linked list built around t_list:

typedef struct s_list
{
    void          *content;
    struct s_list *next;
}   t_list;

Function	Description
ft_lstnew	Allocate and initialise a new node
ft_lstadd_front	Prepend a node to a list
ft_lstadd_back	Append a node to a list
ft_lstlast	Return the last node
ft_lstsize	Count the nodes in a list
ft_lstdelone	Delete a single node using a provided del function
ft_lstclear	Delete all nodes and free the list
ft_lstiter	Apply a function to the content of every node
ft_lstmap	Build a new list by applying a function to every node

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Building

# Compile the library (mandatory functions only)
make

# Compile with the bonus linked-list functions included
make bonus

# Remove object files
make clean

# Remove object files and the archive
make fclean

# Full rebuild
make re

The Makefile produces libft.a in the project root. It compiles with cc -Wall -Wextra -Werror.

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Using it in your own project

Copy libft.a and libft.h into your project, then link against the archive:

cc -o my_program my_program.c -L. -lft

Or, if you keep libft as a subdirectory, adjust the paths:

cc -o my_program my_program.c -Ilibft -Llibft -lft

Include the header wherever you need it:

#include "libft.h"

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Project structure

libft/
├── libft.h                  # All function prototypes and the t_list type
├── Makefile
├── ft_*.c                   # One file per function (mandatory)
└── ft_*_bonus.c             # Linked-list functions (bonus)

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Author

Akhmed Dovletov — [email protected]
42 — September 2023