format

A lightweight, low-overhead printf-style formatting library for embedded systems.

A platform-independent implementation of printf-style formatting designed for constrained environments where the standard C library is too heavy or unavailable. Three profiles are provided to suit different targets:

Profile	Description	Typical code size
format	Full-featured, all conversions	~4 kB (no FP) / ~8 kB (with FP) on ARM GCC 12.2
tinyformat	Feature-reduced for 16-bit MCUs	~1.3 kB on atmega8 with AVR-GCC
microformat	Minimal for the most constrained platforms	smallest possible footprint

A set of standard printf-compatible wrapper functions (sprintf, snprintf, asprintf, etc.) is provided in lib/ for drop-in use — see lib/README.md.

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Releases

• 1.3 (May 2026) — tinyformat and microformat profiles; ll qualifier; a/A hex float specifiers; six new lib/ wrappers (scprintf, asprintf, asnprintf and v-variants); full lib test harness.
• 1.2 (Mar 2015) — Floating point (e/E/f/F/g/G), fixed-point (k), engineering and SI notation, arbitrary numeric bases, denormalized float support, rounding bug fixes.
• 1.1 (Dec 2010) — ROM string support, length modifiers (hh, j, z, t), character repetition, grouping modifiers.
• 1.0 (Jun 2010) — Initial release.

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Features

• Small code size (4 kB without floating point, 8 kB with, compiled for ARM with GCC 12.2)
• Low system overheads — no large internal buffers
• Fully re-entrant — no global state
• Supports most of ANSI C99 printf() format specifications (see exceptions below)
• Output via a caller-supplied consumer function — works with any device (UART, LCD, buffer, …)

ANSI Exceptions

The following C99 printf() features are not currently supported:

• Length modifier L (long double)

Extensions

format adds several useful features beyond standard printf:

• b — binary conversion for unsigned values (base-2)
• ! flag — with # on b/x/X: always emit the prefix even for zero; with e/E: engineering notation (exponent forced to multiple of 3); with f/F: SI prefix formatting (µ, m, k, M, …)
• ^ flag — centre-justify output within the field width
• I / U conversions with :base modifier — arbitrary numeric base (2–36) (FULL profile only)
• k — fixed-point conversion specifier
• %" continuation specifier — intersperse format strings and arguments
• c precision — repetition count for the character
• C — character taken directly from the format string (with optional repetition)
• # with s and continuation — select ROM-based string pointers (AVR PROGMEM)
• [sym count] grouping modifier — digit grouping for numeric output (e.g. %[,3]d → 1,234,567)

For examples see doc/Examples.md and the test cases in test/testharness.c.

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Producing Output

format never writes to any device directly. Instead it calls a consumer function supplied by the caller, passing it chunks of formatted text. A simple example sends characters to a UART; a more complex one might track an (x, y) position on an LCD.

See doc/ManPage.md for the full API and lib/README.md for the ready-made printf-compatible wrappers.