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

ft_printf

Description

ft_printf is a re-implementation of the C standard library function printf(). The goal of the project is to learn how variadic functions work in C (<stdarg.h>).

The project produces a library, libftprintf.a, which exposes one function:

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

Like the original, it writes to the standard output and returns the number of characters printed. The buffer management of the original printf() is not reimplemented, as required by the subject.

Supported conversions

Conversion	Description
%c	Prints a single character
%s	Prints a string
%p	Prints a void * pointer in hexadecimal
%d	Prints a signed decimal (base 10) number
%i	Prints a signed integer (base 10)
%u	Prints an unsigned decimal (base 10) number
%x	Prints a number in hexadecimal (lowercase)
%X	Prints a number in hexadecimal (uppercase)
%%	Prints a percent sign

Instructions

Compilation

The project is compiled with cc -Wall -Wextra -Werror.

make          # builds libftprintf.a at the root of the repository
make clean    # removes the object files
make fclean   # removes the object files and libftprintf.a
make re       # fclean + all

The library is created with ar.

Usage

#include "ft_printf.h"

int main(void)
{
	int	len;

	len = ft_printf("Hello %s! %d + %d = %i (0x%x)\n", "42", 1, 2, 3, 255);
	ft_printf("Printed %d characters\n", len);
	return (0);
}

cc -Wall -Wextra -Werror main.c -L. -lftprintf -o test
./test

Algorithm and data structure

Algorithm

ft_printf reads the format string in a single pass, from left to right:

1. A va_list is initialised with va_start.
2. Each character of the format string is inspected:
   • if it is not a %, it is written as is with write;
   • if it is a %, the next character is the conversion specifier and the matching handler is called (c, s, p, d/i, u, x, X, %).
3. Each handler gets its argument with va_arg, writes it directly with write and returns the number of bytes written.
4. The counts are added up and the total is returned after va_end.

Numbers are printed with a recursive base conversion: the number is divided by the base until a single digit remains, then digits are written on the way back. The same function serves base 10 (d, i, u) and base 16 (x, X, p); only the base string changes.

Special cases follow the behaviour of the original printf() on Linux:

• %s with a NULL pointer prints (null);
• %p with a NULL pointer prints (nil), otherwise 0x followed by the address;
• INT_MIN is handled by converting to a wider type before negating, to avoid overflow.

Resources

References

• man 3 printf
• man 3 stdarg
• man 2 write
• The 42 Norm

Use of AI

AI was used for: README.