_printf – Custom C Library Implementation of printf

A feature-rich reimplementation of the standard C printf function from scratch. This project showcases advanced formatting, custom flags, size specifiers, and modular print handlers — making it an ideal learning and interview preparation tool for low-level systems programming in C.

Overview

This project redefines the standard printf functionality under the name _printf, replicating most of its capabilities while extending the logic with:

• Buffer-based character writing
• Flag and width parsing
• Length/size modifiers
• Format-specific print handlers
• Graceful handling of unknown specifiers

It supports nearly all classic printf features such as %d, %s, %x, %p, etc., and extends it to include:

• ROT13 encoding via %R
• String reversal via %r
• Hex representation for non-printable characters via %S

Features

• Format specifiers: %c, %s, %d, %i, %u, %x, %X, %o, %p, %b, %r, %R, %S
• Flags: +, -, 0, #, and space
• Width and precision support
• Length modifiers: l, h
• Buffer writing for performance
• Print handlers implemented modularly
• Extended support: ROT13, string reversal, non-printable ASCII

Key Files

File	Purpose
_printf.c	Core loop for parsing and dispatching format specifiers
main.h	Declarations, macros, structs for handlers and flags
handle_print.c	Dispatcher that routes each specifier to its handler
function.c & function1.c	Handlers for char, string, int, binary, unsigned, octal, hex
function2.c	Handles pointer printing, ROT13, and reverse
write_handlers.c	Internal low-level formatting and padding logic
get_flags.c, get_width.c, get_precision.c, get_size.c	Format parser helpers
utils.c	Helper functions like size casting, print checks
main_test1.c, Test_Files/main_test.c	Sample tests comparing _printf with printf

Example Usage

_printf("Character: [%c]\n", 'H');
_printf("String: [%s]\n", "Custom printf");
_printf("Integer: [%d]\n", 1024);
_printf("Binary: [%b]\n", 5);
_printf("Pointer: [%p]\n", (void *)main);
_printf("Reverse: [%r]\n", "Hello");
_printf("ROT13: [%R]\n", "Hello");

Learning Objectives

• Deepen knowledge of variadic functions and argument handling in C
• Explore buffer management and custom string formatting
• Implement modular code and handler-based logic for extensibility
• Gain practical experience with parsing, memory, and error management

Build & Test

Build:

gcc -Wall -Werror -Wextra -pedantic -std=gnu89 *.c -o test_printf

Run sample:

./test_printf

Or compile and test against provided test files.

Limitations

• Not 100% compatible with all edge cases of the standard printf
• Floating-point formats (%f, %e, %g) are not implemented
• Locale and multi-byte/wide character support not included
• Error reporting is minimal — invalid formats may cause undefined behavior