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

ft_printf

Description

ft_printf is a re-implementation of the C standard printf, built as a static library (libftprintf.a). It reads a format string, replaces each conversion with the matching variadic argument, writes the result with write, and returns the number of characters printed (or -1 on error).

Result: this project was validated with a score of 100/100.

Only the mandatory part is implemented. Supported conversions:

Conversion	Prints
%c	a single character
%s	a string ((null) for a NULL pointer)
%p	a pointer in hexadecimal ((nil) for NULL)
%d / %i	a signed decimal integer
%u	an unsigned decimal integer
%x / %X	an unsigned hexadecimal number, lowercase / uppercase
%%	a literal percent sign

Behaviour worth knowing:

• An unknown conversion such as %k is printed as it appears and consumes no argument, like glibc's printf.
• A NULL format string returns -1.
• If write fails, ft_printf stops and returns -1.

How it works

ft_printf walks the format string once. For every %, ft_parse returns the handler function matching the next character, and that handler reads exactly one argument with va_arg and prints it. Each handler returns the number of bytes it wrote, and ft_printf adds them up. The handlers share one prototype, int handler(va_list *args), so the dispatcher stays small.

File	Role
ft_printf.c	main loop, running count, error handling
ft_parse.c	maps a specifier to its handler function
ft_print_char.c, ft_print_str.c, ft_print_ptr.c, ft_print_nbr.c, ft_print_unsigned.c, ft_print_hex_low.c, ft_print_hex_up.c, ft_print_percent.c	one handler per conversion
ft_printchar_count.c, ft_utoa.c, ft_itoa.c, ft_strlen.c	small helpers
ft_printf.h	prototypes and types

Instructions

Build the library:

make          # builds libftprintf.a
make clean    # removes object files
make fclean   # removes object files and the library
make re       # fclean + all

Use it in your own program:

#include "ft_printf.h"

int main(void)
{
	int n;

	n = ft_printf("Hello %s, you are %d years old (%x)\n", "42", 42, 42);
	ft_printf("printed %d characters\n", n);
	return (0);
}

cc -Wall -Wextra -Werror main.c libftprintf.a -I.

Resources

• man 3 printf, man 3 stdarg, man 2 write
• The System V AMD64 ABI document, for how variadic arguments are passed (register save area, va_list layout)
• The C standard's section on <stdarg.h>
• YouTube videos on how registers work, how the CPU communicates with RAM, and how the CPU works.
• AI was used as a study and deep to explain how variadic arguments, va_list and the x86-64 calling convention work.