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

ft_printf

Description

ft_printf builds a static library, libftprintf.a, that exposes:

int ft_printf(const char *format, ...);

The goal of the project is to recreate a subset of the standard C printf function without using its internal implementation. It focuses on variadic functions, format parsing, output control, number conversion, memory management and integration with a custom libft.

Features

The implementation supports the core conversions:

• %c for characters
• %s for strings
• %p for pointers
• %d and %i for signed decimal integers
• %u for unsigned decimal integers
• %x and %X for hexadecimal integers
• %% for a literal percent sign

The repository also includes bonus handling for flags, width and precision:

• -
• 0
• .
• field width
• #
• space
• +

Instructions

Build the mandatory library:

make

Build with bonus objects:

make bonus

Clean generated files:

make clean
make fclean
make re

Project Structure

.
├── Makefile
├── ft_printf.h
├── ft_printf.c
├── convert_*.c
├── handle_*.c
├── parse_format.c
├── bonus/
│   ├── ft_printf_bonus.h
│   └── *_bonus.c
└── libft/
    ├── Makefile
    ├── libft.h
    └── ft_*.c

Usage Example

#include "ft_printf.h"

int	main(void)
{
	ft_printf("Number: %d\n", 42);
	ft_printf("Hex: %#x\n", 42);
	return (0);
}

Compile with the generated static library:

cc main.c libftprintf.a -I. -Ilibft -o test_printf

Algorithm and Data Structure Choices

The implementation uses a small t_format structure to keep parsing state for each conversion. This structure stores width, precision and formatting flags in one place, which keeps the parser and the converter functions separated.

The main ft_printf loop scans the format string from left to right. Regular characters are written directly, while % starts a parsing step. The parser fills a t_format value and then delegates the final output to the matching converter function.

Each converter is responsible for one output family: characters, strings, pointers, signed integers, unsigned integers, hexadecimal values and percent signs. Numeric conversions use base-conversion helpers instead of hardcoded printing logic, which makes decimal and hexadecimal formatting easier to reuse.

The project writes output with write and returns the number of printed characters, following the behavior expected from printf. Dynamically allocated strings are freed after use.

Resources

• man 3 printf
• man 3 stdarg
• 42 ft_printf subject
• 42 Norm and peer evaluation practices

What I Learned

• Using variadic arguments with stdarg.h
• Designing a parser for format specifiers
• Handling integer conversion across decimal and hexadecimal bases
• Managing field width, precision and formatting flags
• Returning accurate output counts like the original printf
• Building and linking a static C library with make

Notes

This project follows the constraints and style expected in the 42 curriculum.
The implementation is intentionally written in C and avoids relying on the standard printf internals.

Author

Christian Gómez
Junior Software Developer in training at 42 Barcelona
GitHub: github.com/chgomez04