ft_printf

The printf function serves as a pedagogical tool for understanding variadic functions in C. As one of the most familiar functions to C programmers, it provides a concrete, practical example that students can immediately relate to while exploring advanced function concepts.

printf: Definition

printf is a variadic function that formats and prints data to standard output. It's one of the most commonly used functions in C, thus making it a good way to learn about variadic functions.

Note

A variadic function is a function that can accept a variable number of arguments, meaning its arity is not fixed.

printf: Behaviour

Prototype

int printf(const char *format, ...);

Return Type

Returns the number of characters printed.

Parameters

1. A const char (conventionally called format) describing what to print.
2. A variable number of additional arguments based on that format string

General Behaviour

1. Processes the format string character by character.
2. When it finds a % specifier, it extracts the next argument and formats it.
3. Writes the result to standard output (file descriptor 1).

Variadic Type & Macros: va_start, va_list, va_arg, va_end

To handle the variable number of arguments that printf accepts, C provides a set of macros defined in <stdarg.h>. These macros allow functions to access arguments when the number and types aren't known at compile time.

va_list

va_list holds the information needed to retrieve additional arguments with va_arg.

Warning

va_list can be a macro or a type alias depending on your OS and compiler.

va_start

va_start initialises a va_list variable to point to the first variadic argument. It takes the va_list object and the last-named parameter before the ellipsis (...).

va_arg

va_arg retrieves the next argument from the list. It takes the va_list and the type of the argument to retrieve. Each call advances the internal pointer to the next argument.

Important

You must know the correct type of each argument. Passing the wrong type leads to undefined behavior.

va_end

va_end performs cleanup after you're done processing variadic arguments. It must be called before the function returns.

Format Specifiers

Format specifiers tell printf how to interpret and display the variadic arguments. Each specifier begins with a % character followed by a conversion character.

Supported Format Specifiers

Specifier	Description
%c	Prints a single character
%s	Prints a null-terminated string
%p	Prints a pointer address in hexadecimal format
%d	Prints a signed decimal integer (base 10)
%i	Prints a signed integer in base 10
%u	Prints an unsigned decimal integer (base 10)
%x	Prints a number in lowercase hexadecimal (base 16)
%X	Prints a number in uppercase hexadecimal (base 16)
%%	Prints a literal percent sign

Note

%d vs %i: Both print signed integers in base 10. In printf, they behave identically, but they differ in scanf (where %i can parse octal and hexadecimal).