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

ft_printf

Description

ft_printf is a partial reimplementation of the C standard library function printf. The goal is to understand how variadic functions work in C and how formatted output is produced without relying on the original function.

The function parses a format string character by character. When it encounters a % followed by a conversion specifier, it pulls the next argument from the variadic list and dispatches it to the appropriate output function. For all other characters, it writes them directly to stdout. The total number of characters written is tracked and returned.

Prototype:

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

Supported conversion specifiers:

Specifier	Output
%c	Single character
%s	String of characters ((null) if NULL)
%p	Pointer address in hexadecimal (0x..., (nil) if NULL)
%d	Signed decimal integer
%i	Signed integer in base 10
%u	Unsigned decimal integer
%x	Unsigned hexadecimal integer (lowercase)
%X	Unsigned hexadecimal integer (uppercase)
%%	Literal % character

Algorithm and Data Structure

The core logic is a simple linear scan of the format string using an index i. No intermediate buffer is used — each character or formatted argument is written to stdout immediately via write(1, ...). This keeps memory usage minimal and avoids any need for dynamic allocation.

When a % is detected, i is incremented and the next character is passed to ft_dispatch, a static dispatcher function that selects the correct output function based on the conversion type. Each output function returns the number of characters it wrote, which is accumulated in a count variable.

For numeric output, a single recursive function ft_putnbr_base handles all bases (decimal, hexadecimal) by dividing the number recursively until it reaches a single digit, then writing digits on the way back up the call stack. This avoids duplicating logic across %u, %x, and %X. Signed integers are handled separately in ft_putnbr, which prepends a - sign and negates the value before passing it to ft_putnbr_base.

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Instructions

Compilation

make

Produces libftprintf.a in the project root.

Linking in your project

cc -Wall -Wextra -Werror your_file.c libftprintf.a -o your_program

Include the header in your source:

#include "ft_printf.h"

Makefile rules

Rule	Description
make / make all	Compile the static library
make clean	Remove object files
make fclean	Remove object files and the library
make re	Full rebuild

Example

#include "ft_printf.h"

int main(void)
{
    int n = 42;

    ft_printf("String: %s\n", "hello");
    ft_printf("Integer: %d\n", n);
    ft_printf("Hex: %x\n", 255);
    ft_printf("Pointer: %p\n", &n);
    return (0);
}

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Resources

References

• printf(3) — Linux manual page
• stdarg.h — cppreference
• va_start, va_arg, va_end — cppreference
• Variadic functions in C — GNU C Library

AI Usage

Claude was used in the following ways during this project:

• Learning va_* functions: I asked Claude to explain how va_list, va_start, va_arg, va_copy, and va_end work internally, and how variadic argument traversal works at the ABI level. This helped me understand why char and short arguments must be retrieved as int via va_arg.
• Understanding printf behavior: I asked Claude to clarify edge case behavior of the real printf, such as how NULL is handled for %s and %p, and the practical difference between %d and %i.
• README writing assistance: I used Claude to help structure and phrase this README in English.