ft_printf

A reimplementation of the C standard library's printf, written for the 42 curriculum.

The project compiles to a static library, libftprintf.a, which exposes one public function, ft_printf. It parses a format string, writes the result to standard output and returns the number of characters written. Nothing from stdio is used: the only external pieces are write from unistd.h and the variadic macros from stdarg.h.

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

Supported conversions

Specifier	Output
%c	A single character.
%s	A NUL-terminated string. Prints (null) when the pointer is NULL.
%d, %i	A signed decimal integer.
%u	An unsigned decimal integer.
%x	An unsigned integer in lowercase hexadecimal.
%X	An unsigned integer in uppercase hexadecimal.
%p	A pointer as 0x followed by lowercase hexadecimal. Prints (nil) when the pointer is NULL.
%%	A literal percent sign.

Building

make

That leaves libftprintf.a in the project root. The usual housekeeping rules are available too:

Rule	Effect
all	Builds the library (default).
clean	Removes the object files.
fclean	Removes the object files and the library.
re	fclean followed by all.

Everything is compiled with -Wall -Wextra -Werror.

Usage

Include the header and link against the archive:

#include "ft_printf.h"

int	main(void)
{
	int	len;

	len = ft_printf("%s scored %d points, 100%% of them at %p\n", "Ada", 42, &len);
	ft_printf("that line was %d bytes long\n", len);
	return (0);
}

cc -Wall -Wextra -Werror example.c libftprintf.a -o example
./example

Testing

main.c is a small test driver that is deliberately kept out of the library sources. It runs one dense format string through ft_printf, then the identical string through the real printf, and finally prints both return values. Any difference in the output or in the character count shows up immediately on screen.

make
cc -Wall -Wextra -Werror main.c libftprintf.a -o test_printf
./test_printf

The string it exercises covers consecutive conversions, INT_MIN, negative values passed to %u, a NULL pointer and escaped percent signs.

Layout

File	Contents
ft_printf.c	The parsing loop, and the routine that sends each specifier to its handler.
ft_printf.h	Public prototype plus the internal helper prototypes.
ft_putchar.c	Writes one character.
ft_putstr.c	Writes a string, or (null).
ft_putnbr.c	Signed decimal conversion.
ft_putdecun.c	Unsigned decimal conversion.
ft_putnbr_hexa.c	Hexadecimal conversion in either case.
ft_putptr.c	Pointer conversion.
ft_strlen.c	String length.
main.c	Test driver, not part of the archive.

Implementation notes

Every helper takes a pointer to the running character counter and increments it with whatever write actually returned, rather than with the number of bytes it was asked to send. A short or failed write therefore cannot inflate the value ft_printf hands back.

INT_MIN is the one signed value that cannot be negated inside an int, so ft_putnbr emits its leading digit first and negates the remainder, which fits. %u reaches the same result from the other direction by adding 2^32 to a negative argument before converting it.

The hexadecimal and pointer conversions are recursive. Each call divides by 16 and recurses before printing, so the digits come out most significant first without needing a buffer to reverse.

The source follows the 42 Norm: no for loops, at most 25 lines per function and 5 functions per file, and declarations grouped at the top of each block.

Limitations

The scope is the mandatory part of the assignment, so the format grammar stops at the conversion character. Not implemented:

• Flags (-, 0, +, , #)
• Field width and precision
• Length modifiers (l, ll, h, hh)
• Floating point conversions

Output always goes to file descriptor 1. A specifier that is not in the table above produces no output at all.

Author

cvizcain, 42 Madrid.

This is coursework. If you are working through the same project, read it for ideas but write your own, since 42 runs plagiarism checks on submissions.