Language FoundationsC Programming Deep DiveHard

Variadic Function - Mini Printf

Master the fundamental concepts of c programming deep dive through this focused micro-challenge.

~90 minutes C Programming Deep Dive Ready to learn

Topic Overview

How printf Reaches Into the Stack

Variadic functions in C rely on stdarg.h. The caller places extra arguments according to the ABI; va_start, va_arg, and va_end let the callee walk them. Every printf, syslog, and kernel printk uses this machinery. GCC's -Wformat exists because a %s with an int argument is UB and a classic CVE category.

Building a Minimal Formatter

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%d: pull an int, print digits (handle negatives)
%s: pull a char*, print until \0; print (null) if the pointer is NULL
%x: pull an unsigned, print hex nibbles
%%: emit a literal percent, no va_arg

Walk the format string character by character. Ordinary characters go to putchar; a percent starts specifier handling.

For this exercise, you will implement my_printf with those four specifiers and return a character count. This task asks you to see format strings as a protocol between caller and callee, because mismatched specifiers break security checks and logging pipelines in real systems.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB.

Implementation Task

Your Task

Write int my_printf(const char *format, ...) from scratch using <stdarg.h> (va_list, va_start, va_arg, va_end), printing character by character with putchar. Support:

%d: a signed int in decimal;
%s: a string, or (null) for a NULL pointer;
%x: an unsigned int in lowercase hex, without a prefix;
%%: a literal percent sign.

Any other character after % is printed as-is, together with the %.

Input

One word selecting a call: int, str, hex or pct. Anything else (or empty input) makes all four calls in that order.

Output

mode	call	output
`int`	`my_printf("Integer: %d\n", 42)`	`Integer: 42`
`str`	`my_printf("String: %s\n", "Hello")`	`String: Hello`
`hex`	`my_printf("Hex: %x\n", 255)`	`Hex: ff`
`pct`	`my_printf("Literal %%: %%\n")`	`Literal %: %`

Example

Input:

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Output:

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Requirements

Don't call printf for the conversions: build digits yourself (recursion or a small buffer).
Read each argument with va_arg using the type its conversion expects.

Hidden tests cover %s, %x and %%.