libft

A hand-crafted C standard library. 40+ functions reimplemented from scratch, covering string manipulation, memory management, character classification, I/O, and linked lists. The foundation of every subsequent 42 project.

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Overview

libft is the first project of the 42 curriculum. The goal is to reimplement a subset of the C standard library functions, understanding them deeply rather than just using them as black boxes and to build a personal utility library that will be reused and extended throughout the entire curriculum.

Every function is written in C, without relying on any standard library functions except where explicitly permitted (malloc, free, write). The library is compiled into a static archive libft.a using ar.

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Function Reference

Part 1 — Libc Reimplementations

These functions replicate their standard libc counterparts exactly, prefixed with ft_.

Character classification (ft_is.c)

Function	Description
ft_isalpha	Returns 1 if the character is alphabetic
ft_isdigit	Returns 1 if the character is a digit
ft_isalnum	Returns 1 if the character is alphanumeric
ft_isascii	Returns 1 if the character is in the ASCII table
ft_isprint	Returns 1 if the character is printable
ft_toupper	Converts a lowercase letter to uppercase
ft_tolower	Converts an uppercase letter to lowercase

String functions (ft_str.c, ft_str2.c, ft_str3.c)

Function	Description
ft_strlen	Returns the length of a string
ft_strlcpy	Copies a string with size-bounded truncation
ft_strlcat	Concatenates a string with size-bounded truncation
ft_strchr	Locates the first occurrence of a character in a string
ft_strrchr	Locates the last occurrence of a character in a string
ft_strncmp	Compares two strings up to n characters
ft_strnstr	Locates a substring in a string, within n characters

Memory functions (ft_mem.c)

Function	Description
ft_memset	Fills a block of memory with a constant byte
ft_bzero	Zeroes a block of memory
ft_memcpy	Copies a block of memory
ft_memmove	Copies a block of memory, handling overlaps safely
ft_memchr	Scans memory for a byte
ft_memcmp	Compares two memory blocks

Conversion & allocation (ft_alloc.c, ft_itoa.c)

Function	Description
ft_atoi	Converts a string to an integer
ft_itoa	Converts an integer to a string
ft_calloc	Allocates and zeroes memory
ft_strdup	Duplicates a string with malloc

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Part 2 — Additional Functions

Functions not present in libc, or reimplemented with a different interface.

Function	Description
ft_substr	Returns a substring from a string
ft_strjoin	Concatenates two strings into a new one
ft_strtrim	Trims characters from both ends of a string (ft_strtrim.c)
ft_split	Splits a string by a delimiter into a NULL-terminated array (ft_split.c)
ft_strmapi	Applies a function to each character, creating a new string
ft_striteri	Applies a function to each character in place
ft_putchar_fd	Outputs a character to a file descriptor
ft_putstr_fd	Outputs a string to a file descriptor
ft_putendl_fd	Outputs a string followed by a newline to a file descriptor
ft_putnbr_fd	Outputs an integer to a file descriptor

All fd-based output functions use only write — no formatted output from the standard library.

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Part 3 — Linked List (Bonus)

A singly linked list implementation using the t_list structure:

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

Function	Description
ft_lstnew	Creates a new list node (ft_lstnew_bonus.c)
ft_lstadd_front	Adds a node at the front of the list
ft_lstsize	Returns the number of nodes in the list
ft_lstlast	Returns the last node
ft_lstadd_back	Adds a node at the end of the list
ft_lstdelone	Frees one node using a custom delete function
ft_lstclear	Frees all nodes and sets the pointer to NULL (ft_lstfree_bonus.c)
ft_lstiter	Iterates through the list and applies a function to each node
ft_lstmap	Creates a new list by applying a function to each node

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File Structure

libft/
├── libft.h             # All prototypes and the t_list struct
├── ft_is.c             # Character classification functions
├── ft_str.c            # Core string functions
├── ft_str2.c           # Additional string functions
├── ft_str3.c           # strmapi, striteri, fd output
├── ft_strtrim.c        # ft_strtrim
├── ft_split.c          # ft_split
├── ft_mem.c            # Memory functions
├── ft_alloc.c          # calloc, strdup, substr, strjoin
├── ft_itoa.c           # ft_itoa
├── ft_fds.c            # putchar_fd, putstr_fd, putendl_fd, putnbr_fd
├── ft_utils.c          # Helper functions
├── ft_lstnew_bonus.c   # Linked list — creation
├── ft_lstfree_bonus.c  # Linked list — deletion / clearing
└── Makefile

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Compilation

make          # Build libft.a (mandatory)
make bonus    # Build libft.a with linked list functions
make clean    # Remove object files
make fclean   # Remove object files and libft.a
make re       # Full rebuild

Usage

# Link libft.a when compiling your project
cc -Wall -Wextra -Werror main.c -L. -lft -o my_program

#include "libft.h"

int main(void)
{
    char    *s = ft_strjoin("Hello, ", "world!");
    ft_putendl_fd(s, 1);
    free(s);
    return 0;
}

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Resources

• man 3 string — Linux man pages
• man 3 mem — memset, memcpy, memmove...
• GNU C Library documentation

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This project was created as part of the 42 curriculum by Kheldin Acherchour.