_printf

Simple reimplementation of a subset of the C library function printf(3).

This project provides a minimal _printf function that supports a small set of conversion specifiers and is implemented using a dispatch table and a custom _putchar helper. The documentation below describes how to build and use the program, what is implemented, and how to view the bundled man page.

Build

Compile with GCC (the project uses C89/GNU extensions by default):

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

Requirements

• POSIX-compatible system (Linux/macOS)
• GCC (or compatible C compiler)

Supported conversion specifiers

This implementation supports the following conversion specifiers:

• %c — print a single character
• %s — print a NUL-terminated string (prints (null) when a NULL pointer is given)
• %d, %i — print a signed decimal integer (handles INT_MIN)
• %% — print a literal percent sign

Unknown specifiers are printed as a percent sign followed by the unknown character (for example, "%q" prints %q). If the format string is NULL, _printf returns -1. A trailing single % at the end of the format string is considered an error and causes -1 to be returned.

Behaviour and return value

On success _printf returns the number of characters written to standard output. On error (for example, when format is NULL, or the format string ends with an unmatched %) the function returns -1.

String printing: when a NULL pointer is passed for %s, this implementation writes the literal (null) (6 characters).

Integer printing: signed integers are printed correctly. INT_MIN is handled specially to avoid overflow when negating.

Examples

Example 1 — character:

_printf("I'm the letter: %c\n", 'A');
/* Output: I'm the letter: A */

Example 2 — string:

_printf("I'm a %s!\n", "string");
/* Output: I'm a string! */

Example 3 — percent sign:

_printf("Progress: 100%%\n");
/* Output: Progress: 100% */

Example 4 — integer:

_printf("Number: %d\n", 123);
/* Output: Number: 123 */

Example 5 — NULL string handling:

_printf("Null: %s\n", NULL);
/* Output: Null: (null) */

Example 6 — unknown specifier:

_printf("Unknown: %q\n");
/* Output: Unknown: %q */

Project structure

_printf.c
_putchar.c
main.h
print_types.c
man_3_printf
image/

Man page

A man page is included at man_3_printf (troff format). You can view it locally without installing it system-wide in several ways:

• Directly with man if your man accepts local files:

man ./man_3_printf

• Use groff to format and display it:

groff -Tutf8 -man man_3_printf | less

• Install locally to a man tree (example creates ./man/man3):

mkdir -p ./man/man3
gzip -c man_3_printf > ./man/man3/_printf.3.gz
man -M ./man _printf

Testing & Valgrind

Compile and run simple examples to exercise the implementation. To check for memory leaks using Valgrind run:

valgrind --leak-check=full --show-leak-kinds=all ./_printf "Hello %s" "world"

Note: _printf is a small educational implementation; memory allocation is minimal but it's good practice to run Valgrind on your examples.

Man page (screenshots)

Screenshots of the bundled manual page are included below (ascending order):

Flowchart

High-level flowcharts describing the _printf parsing and dispatch flow are included below (ascending order):

Implementation notes

• A dispatch table (printer array) maps format characters to handler functions declared in main.h.
• _putchar is a tiny wrapper around write(1, &c, 1) to perform unbuffered output from handlers.
• Variadic arguments are handled with stdarg.h macros.

Authors

• Joan FAROUX
• Joshua BURLE