ft_printf: Custom Variadic Formatter

Overview

ft_printf is a custom implementation of the standard C library function printf.

From a Systems Engineering perspective, this project is an exploration of Variadic Functions in C. The core challenge lies in interacting with the Application Binary Interface (ABI) conventions to retrieve a variable number of arguments from the stack (or registers) based on a format string parser.

Unlike high-level string formatting, this implementation requires manual handling of data types, memory addressing for pointers, and efficient buffer management to write to stdout with minimal syscall overhead.

Technical Concepts

• Variadic Arguments (stdarg.h): Using macros (va_start, va_arg, va_end) to traverse the stack frame and retrieve undefined arguments.
• Format String Parsing: Implementing a state machine to detect flags and dispatch execution to the correct type handler.
• Base Conversion: Algorithms to convert numerical values (integers, memory addresses) into ASCII representations in varying bases (decimal, hexadecimal).
• Pointer Arithmetic: Handling void * memory addresses for the %p conversion.

Supported Conversions

The function handles the following standard format specifiers:

Specifier	Description	Argument Type
%c	Prints a single character.	int (promoted)
%s	Prints a string (as defined by the common C convention).	char *
%p	The void * pointer argument has to be printed in hexadecimal format.	void *
%d	Prints a decimal (base 10) number.	int
%i	Prints an integer in base 10.	int
%u	Prints an unsigned decimal (base 10) number.	unsigned int
%x	Prints a number in hexadecimal (base 16) lowercase format.	unsigned int
%X	Prints a number in hexadecimal (base 16) uppercase format.	unsigned int
%%	Prints a percent sign.	None

Usage

Prototype

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

1. Compilation

The project compiles into a static library libftprintf.a

make

2. Integration example

#include "ft_printf.h"

int main(void)
{
    int count;

    // Example 1: String and Char
    ft_printf("Hello %s, your grade is %c.\n", "User", 'A');

    // Example 2: Numbers and Hex
    count = ft_printf("Memory Address: %p | Hex: %x\n", &count, 255);
    
    // Return value check (bytes written)
    ft_printf("Bytes written: %d\n", count);

    return (0);
}

To use it in your project, link the previously compiled static library libftprintf.a during compilation:

gcc main.c libftprintf.a -o my_program

Compile with bonus:

gcc -Wall -Wextra -Werror -D BUFFER_SIZE=1024 \
    main.c get_next_line_bonus.c get_next_line_utils_bonus.c -o multi_fd_reader

Author

Sergi Juarez

• GitHub: imserez
• LinkedIn: sergijuarez

---

This project is part of the 42 Barcelona curriculum.