printf / sprintf 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 (float), ntoa (decimal) conversion.

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

Highligths 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 500 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
• LINT and compiler L4 warning free, mature, coverity clean, automotive ready
• Extensive test suite (> 300 test cases) 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.
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);

Due to genaral security reasons it is highly recommended to 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!

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
c	Single character
s	String of characters
p	Pointer address
%	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 preceed 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, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero. Used with f, F 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.
0	Left-pads the number with zeroes (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 and F specifiers: this is the number of digits to be printed after the decimal point. By default, this is 6, maximum is 9. 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
l	long int	unsigned long int
ll	long long int	unsigned long long int (if PRINTF_LONG_LONG_SUPPORT is defined)
z	size_t int	unsigned size_t int

Return value

Upon successful return, all functions return the number of characters written, excluding the null byte used to end the string.
Functions snprintf() and vsnprintf() don't write more than count bytes, including the terminating null byte ('\0'). If the output was truncated due to this limit, the return value is count, which indicates any truncation.
If an output error (invalid buffer etc.) is encountered, -1 is returned.

Caveats

Currently snprintf() and vsnprintf() don't support (v)snprintf(nullptr, 0, "Some text") to get the length of the formatted string only. An error value of -1 is returned.

Compiler switches/defines

Name	Default value	Description
PRINTF_BUFFER_SIZE	128	The buffer size used for the printf() function (not for sprintf/snprintf). Set to 0 if the printf() function is unused (just using sprintf/snprintf)
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
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
PRINTF_FLOAT_SUPPORT	undefined	Define this to enable floating point (%f) support
PRINTF_LONG_LONG_SUPPORT	undefined	Define this to enable long long (%ll) support

Test suite

For testing just compile, build and run the test suite located in test/test_suite.cpp. This uses the catch framework for unit-tests, which is auto-adding main().
Running with the --wait-for-keypress exit option waits for the enter key after test end.

Projects using printf

• turnkey-board uses printf as log and generic display formatting/output.
  (Just send me a mail/issue 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.