This project has been created as part of the 42 curriculum by jiylee.

Description

• For?

  Helping me understand how printf() works by implementing it myself, and learn about using a variable number of arguments, variadic functions in C.

• Overview?

  This project involves recoding printf() for my programs in the future as assignments by 42school. Its version is 12.1 without the Bonus part.

• What?

  printf(), variadic functions in C

Instructions

• Installation

git clone <repository-url> working_directory
cd working_directory

• Compilation

  • Type make in the commandline of where this Makefile exists, then you'll get "*.o" files and "libftprintf.a"
  • Type make clean to remove just "*.o" files.
  • Type make fclean which means fully-clean, to remove not only "*.o" files, but also "libftprintf.a".
  • Type make re to fully-clean then get "*.o" files and "libftprintf.a" again.
  • After you get "libftprintf.a", type cc -Wall -Wextra -Werror main.c libftprintf.a -o program to compile together.
  • Type ./program to execute.

• Usage

#include "ft_printf.h"

int main(void)
{
	ft_printf("Hello %s, you are %d years old!\n", "World", 42);
	ft_printf("Hex: %x, Pointer: %p\n", 255, &main);
	return (0);
}

Compile with:

cc -Wall -Wextra -Werror main.c libftprintf.a -o program

Execute the program with:

./program

Supported Format Specifiers

Specifier	Description
%c	Single character
%s	String
%p	Pointer address in hexadecimal
%d	Signed decimal integer
%i	Signed integer
%u	Unsigned decimal integer
%x	Unsigned hexadecimal (lowercase)
%X	Unsigned hexadecimal (uppercase)
%%	Percent sign

Algorithm

Format String Parsing (Batch Write Approach)

1. ft_printf scans the format string looking for %
2. All characters before % are identified as a contiguous segment within the original format string and written in a single write() call
3. When % is found, the next character is read and ft_handle_format is called
4. ft_handle_format dispatches to the appropriate print function based on the specifier
5. This continues until the end of the format string

Why Batch Write Approach?

• Instead of calling write() one character at a time, consecutive plain-text characters up to the next % are written together in a single write() call
• No separate memory buffer is allocated; the pointer to the original format string is used directly
• Minimizes the number of write() system calls, reducing system call overhead

Number Conversion (Recursive Approach)

For %d, %u, %x, %X, %p, I use recursion to print digits in correct order:

1. If n >= base, recursively call with n / base first
2. Then print n % base as the current digit
3. This ensures the most significant digit prints first

Why recursion?

• Simple and elegant solution
• Avoids needing a buffer to reverse digits
• Stack naturally handles digit ordering

Data Structure

ft_printf/
├── ft_printf.c         ← ft_printf, ft_handle_format
├── ft_print_string.c   ← ft_print_char, ft_print_str
├── ft_print_hex.c      ← ft_print_hex, ft_print_hex_upper, ft_print_ptr
├── ft_print_nbr.c      ← ft_print_nbr
├── ft_printf.h         ← All function prototypes
└── Makefile

File Structure Justification

File	Responsibility
ft_printf.c	Main logic and format dispatching
ft_print_string.c	Character and string output
ft_print_hex.c	Hexadecimal and pointer output
ft_print_dec.c	Decimal number output (signed/unsigned)

Why this separation?

• Each file handles related functionality
• Stays within Norm's 5 functions per file limit
• Easy to maintain and extend

Resources

• References

  • ft_printf-version12.1.pdf
  • Web manual of Linux
  • Guide for Makefile in Korean
  • Wikipedia of The Standard of C
  • 378page for 7.23.6.4 The printf function
  • 364page for Formatted input/output functions in ISO/IEC 9899:202y (en) - n3467 working draft

• How AI was used

  • What?

    • I used only Opus 4.5 of Claude.

  • Which tasks?

    • Getting hints of how logics should flow when I got stuck.
    • Getting detailed knowledge of everything like knowing full-name of each title.
    • Getting evaluation of my original source-code and refactored one, to choose which one should survive.
    • Getting to know why this code must be like this, like each intention of some codes, which cases I should use which code, as the right code in the right place.