This project has been created as part of the 42 curriculum by [jkrishna].

ft_printf #

Description #

ft_printf is a re-implementation of the standard C printf function from scratch, without using the original. The goal is to understand variadic functions, format string parsing, and low-level output formatting in C.

The function parses a format string and handles a set of conversion specifiers, writing formatted output to standard output and returning the total number of characters written.

Supported conversions #

Specifier	Description
%c	Single character
%s	String
%p	Pointer address (hex)
%d / %i	Signed decimal integer
%u	Unsigned decimal integer
%x	Unsigned hex (lowercase)
%X	Unsigned hex (uppercase)
%%	Literal percent sign

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Instructions #

Compilation #

The project compiles into a static library libftprintf.a.

make

This runs:

cc -Wall -Wextra -Werror -c <files>.c
ar rcs libftprintf.a <objects>.o

Cleaning #

make clean    # removes object files
make fclean   # removes object files and library
make re       # fclean + rebuild

Usage #

Link the library in your project:

cc -Wall -Wextra -Werror main.c libftprintf.a -o program

Include the header in your code:

#include "ft_printf.h"

int main(void)
{
    ft_printf("Hello, %s! You are %d years old.\n", "world", 42);
    return (0);
}

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Algorithm and Data Structure #

Approach #

ft_printf scans the format string character by character using a while loop and an index i. When it encounters a % followed by a valid specifier, it dispatches to a dedicated handler function via ft_channel. Otherwise it writes the character directly using write.

A running count variable tracks total characters written, which is returned at the end.

Dispatch via ft_channel #

Rather than a large nested if-else inside ft_printf itself, the specifier routing is delegated to ft_channel(char specifier, va_list *args). This keeps ft_printf clean and makes adding new specifiers straightforward — only ft_channel needs to change.

Each specifier maps to its own function:

%c → ft_cprintf      (writes a single char)
%s → ft_sprintf      (iterates and writes each char of a string)
%p → ft_pprintf      (formats pointer as 0x + hex)
%d/%i → ft_diprintf  (handles sign, converts int to ASCII digits)
%u → ft_uprintf      (unsigned version of above)
%x → ft_xprintf      (hex with lowercase a-f)
%X → ft_cxprintf     (hex with uppercase A-F)
%% → ft_percprintf   (writes a literal %)

Variadic Arguments #

va_list from <stdarg.h> is used to access arguments at runtime. va_start initialises it, va_arg retrieves each argument by type, and va_end cleans up. The pointer to va_list is passed to ft_channel so each handler can pull its own argument.

Data Structures #

No heap allocation or complex data structures are used. The function operates entirely on the stack using the format string pointer, a counter, and the va_list.

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Resources #

• man 3 printf — official printf specification
• cppreference: va_list — variadic function documentation
• 42 Docs — community reference for ft_printf

AI Usage #

Claude (claude.ai) was used during this project in the following ways:

• Conceptual guidance: Understanding how va_list, va_start, va_arg, and va_end work together for variadic functions.
• Debugging: Identifying edge cases in specifier handling and return value counting.
• README writing: This README was drafted with Claude's assistance based on the project code and 42 requirements.

Claude did not write the implementation code directly. All logic was reasoned through and written by the student, with Claude acting as a guide asking questions rather than providing answers.