🔧 printf Clone in C — From Scratch Using Only write(2)

This project is a full-featured reimplementation of the standard C printf function — built entirely from scratch using only the low-level write system call.

It includes its own buffering system, format specifier parser, and modular handlers for each format type — without depending on the standard library's printf, sprintf, or related routines.

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📌 Motivation

The standard C library's printf is deceptively complex, involving variadic arguments, formatting rules, padding logic, and internal buffering.

This project was created to:

• Deepen understanding of low-level I/O, format parsing, and variadic functions
• Gain practical experience building reusable, modular code under constraints

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✨ Key Features

✅ Format Specifiers Supported:

Specifier	Meaning
%c	Character
%s	String
%d/%i	Signed decimal integer
%u	Unsigned decimal integer
%x	Lowercase hexadecimal
%X	Uppercase hexadecimal
%o	Octal
%%	Literal % character

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🧾 Supported Flags:

Flag	Meaning
-	Left-align within the field width
+	Always print the sign (+ or -)
(space)	Print space if no sign is used
0	Pad with zeros instead of spaces
#	Use alternate form (0x, 0X, 0 etc)

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📏 Width & Precision:

• Field width supported (e.g. %10d)
• Precision supported for strings and numbers (e.g. %.5s, %.4d)
• Combined width & precision supported (e.g. %08.4d)

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🧱 Length Modifiers:

Modifier	Meaning
hh	char or unsigned char
h	short or unsigned short
l	long or unsigned long
ll	long long or unsigned long long

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⚙️ Core Architecture

• ✅ Manual parsing of format strings
• ✅ Variadic argument handling with stdarg.h
• ✅ Internal output buffer with flushing control
• ✅ Edge-case handling:
  • LLONG_MIN signed overflow
  • Mixed flags (+, , 0, -)
  • Zero precision with zero value (%.0d)
  • Buffer overflow protection

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🛠️ Building the Project

🔧 Requirements

• GCC or Clang
• make
• Unix-like OS (Linux, macOS, WSL, etc.)

🧪 Build Instructions

1. Clone the repository:

git clone https://github.com/Hullaah/printf.git
cd printf

2. Build the static and shared libraries:

make

This produces:

• libprintf.a → static library
• libprintf.so → shared (dynamic) library

3. Optionally, run the included test program

make test
./printf

🧠 What I Learned

• How printf actually works under the hood
• Designing and implementing format parsers
• Manual buffer management and flushing logic
• Respecting field width, precision, flags, and length modifiers
• Debugging tricky issues like signed integer overflow (LLONG_MIN)
• Writing clean, maintainable low-level C code

👨‍💻 Author

Umar Adelowo

Intermediate systems programmer focused on OS, networking, and low-level development. Aiming to contribute to the Linux Kernel and become a security and systems expert.

📧 Contact me

🌐 GitHub: @Hullaah