A printf / sprintf Implementation for Embedded Systems

This is a tiny but fully loaded printf, sprintf and (v)snprintf implementation. Primarily designed for usage in embedded systems, where printf is not available due to memory issues or in avoidance of linking against libc.
Using the standard libc printf may pull a lot of unwanted library stuff and can bloat code size about 20k or is not 100% thread safe. In this cases the following implementation can be used. Absolutely NO dependencies are required, printf.c brings all necessary routines, even its own fast ftoa (floating point), ntoa (decimal) conversion.

If memory footprint is really a critical issue, floating point, exponential and 'long long' support can be turned off via the PRINTF_DISABLE_SUPPORT_FLOAT, PRINTF_DISABLE_SUPPORT_EXPONENTIAL and PRINTF_DISABLE_SUPPORT_LONG_LONG compiler switches. When using printf (instead of sprintf/snprintf) you have to provide your own _putchar() low level function as console/serial output.

Notice 2026

Due to other projects, work and challenging things in life, this repo unfortunately stalled the last few years. In the meantime Eyal Rozenberg took over maintenance in his fork eyalroz/printf.
Lately my old turnkeyboard project needed to be brought back to life which was a great opportunity to update and reboot printf as well. This repo is now back on track and actively maintained again.
First of all I ported the improved test case suite of Eyal back. I closed a lot of issues, merged some PRs and some IMHO improvements came in carefully - mind that the code stays clean and small.
The updated test suit passes on all three main compilers now and all changes are backward compatible: _putchar(), the printf_() functions with their alias macros, the PRINTF_DISABLE_SUPPORT_* switches.

Highlights and Design Goals

There is a boatload of so called 'tiny' printf implementations around. So why this one? I've tested many implementations, but most of them have very limited flag/specifier support, a lot of other dependencies or are just not standard compliant and failing most of the test suite. Therefore I decided to write an own, final implementation which meets the following items:

• Very small implementation (around 800 code lines)
• NO dependencies, no libs, just one module file
• Support of all important flags, width and precision sub-specifiers (see below)
• Support of decimal/floating number representation (with an own fast itoa/ftoa)
• Reentrant and thread-safe, malloc free, no static vars/buffers
• LINT and compiler L4 warning free, mature, coverity clean, automotive ready
• Compiling on all main compilers: GCC, Clang and MSVC
• Extensive test suite (> 550 checks) passing
• Simply the best printf around the net
• MIT license

Usage

Add/link printf.c to your project and include printf.h. That's it. A C99 compiler is required (stdbool.h and stdint.h are used). With CMake, add this directory via add_subdirectory() and link the printf library target. Implement your low level output function needed for printf():

void _putchar(char character)
{
  // send char to console etc.
}

Usage is 1:1 like the according stdio.h library version:

int printf(const char* format, ...);
int sprintf(char* buffer, const char* format, ...);
int snprintf(char* buffer, size_t count, const char* format, ...);
int vsnprintf(char* buffer, size_t count, const char* format, va_list va);

// use output function (instead of buffer) for a streamlike interface
int fctprintf(void (*out)(char character, void* arg), void* arg, const char* format, ...);
int vfctprintf(void (*out)(char character, void* arg), void* arg, const char* format, va_list va);

Due to general security reasons it is highly recommended to prefer and use snprintf (with the max buffer size as count parameter) instead of sprintf. sprintf has no buffer limitation, so when needed - use it really with care!

Streamlike Usage

Besides the regular standard printf() functions, this module also provides fctprintf() (and its va_list variant vfctprintf()), which takes an output function as first parameter to build a streamlike output like fprintf():

// define the output function
void my_stream_output(char character, void* arg)
{
  // opt. evaluate the argument and send the char somewhere
}

{
  // in your code
  void* arg = (void*)100;  // this argument is passed to the output function
  fctprintf(&my_stream_output, arg, "This is a test: %X", 0xAA);
  fctprintf(&my_stream_output, nullptr, "Send to null dev");
}

Format Specifiers

A format specifier follows this prototype: %[flags][width][.precision][length]type The following format specifiers are supported:

Supported Types

Type	Output
d or i	Signed decimal integer
u	Unsigned decimal integer
b	Unsigned binary
o	Unsigned octal
x	Unsigned hexadecimal integer (lowercase)
X	Unsigned hexadecimal integer (uppercase)
f or F	Decimal floating point
e or E	Scientific-notation (exponential) floating point
g or G	Scientific or decimal floating point
c	Single character
s	String of characters (a NULL pointer is printed as (null))
p	Pointer address (all digits in uppercase hex, without 0x prefix)
%	A % followed by another % character will write a single %

Supported Flags

Flags	Description
-	Left-justify within the given field width; Right justification is the default.
+	Forces to precede the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign.
(space)	If no sign is going to be written, a blank space is inserted before the value.
#	Used with o, b, x or X specifiers the value is preceded with 0, 0b, 0x or 0X respectively for values different than zero. Used with f, F, e, E, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written. With g or G, trailing zeros are not removed.
0	Left-pads the number with zeros (0) instead of spaces when padding is specified (see width sub-specifier).

Supported Width

Width	Description
(number)	Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*	The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.

Supported Precision

Precision	Description
.number	For integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For f, F, e and E specifiers: this is the number of digits to be printed after the decimal point. By default, this is 6, maximum is 9 (further digits are printed as 0). For g and G specifiers: this is the maximum number of significant digits to be printed (trailing zeros are removed). For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. If the period is specified without an explicit value for precision, 0 is assumed.
.*	The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.

Supported Length

The length sub-specifier modifies the length of the data type.

Length	d i	u o x X
(none)	int	unsigned int
hh	signed char	unsigned char
h	short int	unsigned short int
l	long int	unsigned long int
ll	long long int	unsigned long long int (if PRINTF_SUPPORT_LONG_LONG is defined)
j	intmax_t	uintmax_t
z	size_t	size_t
t	ptrdiff_t	ptrdiff_t (if PRINTF_SUPPORT_PTRDIFF_T is defined)

For the floating point types (f F e E g G), the length L denotes a long double argument, which is converted to double.

Return Value

Upon successful return, all functions return the number of characters written, excluding the terminating null character used to end the string. Functions snprintf() and vsnprintf() don't write more than count bytes, including the terminating null byte ('\0'). Anyway, if the output was truncated due to this limit, the return value is the number of characters that could have been written. Notice that a value equal or larger than count indicates a truncation. Only when the returned value is non-negative and less than count, the string has been completely written. If any error is encountered, -1 is returned.

If buffer is set to NULL (nullptr) nothing is written and just the formatted length is returned.

int length = sprintf(NULL, "Hello, world"); // length is set to 12

Compiler Switches/Defines

Name	Default value	Description
PRINTF_INCLUDE_CONFIG_H	undefined	Define this as compiler switch (e.g. gcc -DPRINTF_INCLUDE_CONFIG_H) to include a "printf_config.h" definition file
PRINTF_NTOA_BUFFER_SIZE	32	ntoa (integer) conversion buffer size. This must be big enough to hold one converted numeric number including leading zeros, normally 32 is a sufficient value. Created on the stack
PRINTF_FTOA_BUFFER_SIZE	32	ftoa (float) conversion buffer size. This must be big enough to hold one converted float number including leading zeros, normally 32 is a sufficient value. Created on the stack
PRINTF_DEFAULT_FLOAT_PRECISION	6	Define the default floating point precision
PRINTF_MAX_FLOAT	1e9	Define the largest suitable value to be printed with %f (and %g in decimal notation), before using exponential representation
PRINTF_DISABLE_SUPPORT_FLOAT	undefined	Define this to disable floating point (%f) support
PRINTF_DISABLE_SUPPORT_EXPONENTIAL	undefined	Define this to disable exponential floating point (%e) support
PRINTF_DISABLE_SUPPORT_LONG_LONG	undefined	Define this to disable long long (%ll) support
PRINTF_DISABLE_SUPPORT_PTRDIFF_T	undefined	Define this to disable ptrdiff_t (%t) support

Caveats

The floating point conversion is optimized for size and uses double arithmetic. It is not a correctly rounded conversion like the one of glibc:

• At most 9 digits after the decimal point are computed, further digits are printed as 0.
• The last digit may differ from a correctly rounded result, e.g. %.1f of 0.95 prints 1.0 (the binary value of 0.95 is slightly smaller than 0.95).
• Values larger than PRINTF_MAX_FLOAT are printed in exponential notation by %f (and %g), instead of printing all digits.
• A long double argument (length L) is printed with the precision of double.

The specifiers %a, %A and %n and wide characters/strings (%lc, %ls) are not supported.

Test Suite

The test suite in test/ uses the catch framework for unit-tests.
Build and run it with CMake:

cmake -S . -B build
cmake --build build
ctest --test-dir build --output-on-failure

The compiler switches are available as CMake options, e.g. -DPRINTF_DISABLE_SUPPORT_FLOAT=ON. With -DPRINTF_BUILD_AUTOTEST=ON the autotest tool is built, which compares randomly generated format strings against the standard library printf until it is stopped.

Projects Using printf

• turnkeyboard uses printf as log and generic tty (formatting) output.
• printf is part of embeddedartistry/libc, a libc targeted for embedded systems usage.

(Just create a PR to get your project listed here)

Contributing

0. Give this project a ⭐
1. Create an issue and describe your idea
2. Fork it
3. Create your feature branch (git checkout -b my-new-feature)
4. Commit your changes (git commit -am 'Add some feature')
5. Publish the branch (git push origin my-new-feature)
6. Create a new pull request
7. Profit! ✔️

License

printf is written under the MIT license.