nanoprintf is an implementation of snprintf and vsnprintf for embedded systems that, when fully enabled, aim for C11 standard compliance. The primary exceptions are scientific notation (though
%f is fully supported), and the conversions that require
wcrtomb to exist. Additionally, C23 binary integer output is optionally supported as per N2630.
nanoprintf makes no memory allocations and uses less than 100 bytes of stack. nanoprintf compiles to somewhere between 1-3KB of code on a Cortex-M architecture.
nanoprintf is written in a minimal dialect of C99 for maximal compiler compatibility, and compiles cleanly at the highest warning levels on clang, gcc, and msvc in both 32- and 64-bit modes.
nanoprintf does include C standard headers but only uses them for C99 types and argument lists; no calls are made into stdlib / libc, with the exception of any internal double-to-float conversion ABI calls your compiler might emit. As usual, some Windows-specific headers are required if you're compiling natively for msvc.
nanoprintf is statically configurable so users can find a balance between size, compiler requirements, and feature set. Floating point conversion, "large" length modifiers, and size write-back are all configurable and are only compiled if explicitly requested, see Configuration for details.
I wanted a single-file public-domain drop-in printf that came in at under 1000 bytes in the minimal configuration (bootloaders etc), and under 3000 bytes with the floating-point bells and whistles enabled.
This code is optimized for size, not readability or structure. Unfortunately modularity and "cleanliness" even in C adds overhead at this small scale, so most of the functionality and logic is pushed together into
npf_vpprintf. This is not what normal embedded systems code should look like; it's
#ifdef soup and hard to make sense of, and I apologize if you have to spelunk around in the implementation. Hopefully the various tests will serve as guide rails if you hack around in it.
Alternately, perhaps you're a significantly better programmer than I! In that case, please help me make this code smaller and cleaner without making the footprint larger, or nudge me in the right direction. :)
Add the following code to one of your source files to compile the nanoprintf implementation:
// define your nanoprintf configuration macros here (see "Configuration" below) #define NANOPRINTF_IMPLEMENTATION #include "path/to/nanoprintf.h"
Then, in any file where you want to use nanoprintf, simply include the header and call the npf_ functions.
nanoprintf has 4 main functions:
npf_snprintf: Use like snprintf.
npf_vsnprintf: Use like vsnprintf (
npf_pprintf: Use like printf with a per-character write callback (semihosting, UART, etc).
npf_vpprintf: Use like
npf_pprintfbut takes a
pprintf variations take a callback that receives the character to print and a user-provided context pointer.
npf_[v]snprintf to write nothing, and only return the length of the formatted string.
nanoprintf does not provide
putchar itself; those are seen as system-level services and nanoprintf is a utility library. nanoprintf is hopefully a good building block for rolling your own
nanoprintf has the following static configuration flags.
NANOPRINTF_USE_FIELD_WIDTH_FORMAT_SPECIFIERS: Set to
1. Enables field width specifiers.
NANOPRINTF_USE_PRECISION_FORMAT_SPECIFIERS: Set to
1. Enables precision specifiers.
NANOPRINTF_USE_FLOAT_FORMAT_SPECIFIERS: Set to
1. Enables floating-point specifiers.
NANOPRINTF_USE_LARGE_FORMAT_SPECIFIERS: Set to
1. Enables oversized modifiers.
NANOPRINTF_USE_BINARY_FORMAT_SPECIFIERS: Set to
1. Enables binary specifiers.
NANOPRINTF_USE_WRITEBACK_FORMAT_SPECIFIERS: Set to
NANOPRINTF_VISIBILITY_STATIC: Optional define. Marks prototypes as
staticto sandbox nanoprintf.
If no configuration flags are specified, nanoprintf will default to "reasonable" embedded values in an attempt to be helpful: floats are enabled, but writeback, binary, and large formatters are disabled. If any configuration flags are explicitly specified, nanoprintf requires that all flags are explicitly specified.
If a disabled format specifier feature is used, no conversion will occur and the format specifier string simply will be printed instead.
nanoprintf expects a conversion specification string of the following form:
[flags][field width][.precision][length modifier][conversion specifier]
None or more of the following:
0: Pad the field with leading zero characters.
-: Left-justify the conversion result in the field.
+: Signed conversions always begin with
#: Writes extra characters (
.for empty floats, '0' for empty octals, etc).
Field width (if enabled)
A number that specifies the total field width for the conversion, adds padding. If field width is
*, the field width is read from the next vararg.
Precision (if enabled)
Prefixed with a
., a number that specifies the precision of the number or string. If precision is
*, the precision is read from the next vararg.
None or more of the following:
shortfor integral and write-back vararg width.
long doublefor float vararg width (note: it will then be casted down to
double, or wide vararg width.
charfor integral and write-back vararg width.
ll: (large specifier) Use
long longfor integral and write-back vararg width.
j: (large specifier) Use the
[u]intmax_ttypes for integral and write-back vararg width.
z: (large specifier) Use the
size_ttypes for integral and write-back vararg width.
t: (large specifier) Use the
ptrdiff_ttypes for integral and write-back vararg width.
Exactly one of the following:
%%: Percent-sign literal
%s: Null-terminated strings
%d: Signed integers
%u: Unsigned integers
%o: Unsigned octal integers
%X: Unsigned hexadecimal integers
%n: Write the number of bytes written to the pointer vararg
%F: Floating-point values
%B: Binary integers
Floating point conversion is performed by extracting the value into 64:64 fixed-point with an extra field that specifies the number of leading zero fractional digits before the first nonzero digit. No rounding is currently performed; values are simply truncated at the specified precision. This is done for simplicity, speed, and code footprint.
nano in the name, there's no way to do away with double entirely, since the C language standard says that floats are promoted to double any time they're passed into variadic argument lists. nanoprintf casts all doubles back down to floats before doing any conversions.
No wide-character support exists: the
%ls fields require that the arg be converted to a char array as if by a call to wcrtomb. When locale and character set conversions get involved, it's hard to keep the name "nano". Accordingly,
%ls behave like
Currently the only supported float conversions are the decimal forms:
%F. Pull requests welcome!
The CI build is set up to use gcc and nm to measure the compiled size of every pull request. See the Presubmit Checks "size reports" job output for recent runs.
The following size measurements are taken against the Cortex-M0 build.
Minimal: arm-none-eabi-gcc -DNANOPRINTF_SIZE_REPORT=0 -mcpu=cortex-m0 -Os -c -o cm0-0.o tests/size_report.c arm-none-eabi-nm --print-size --size-sort cm0-0.o | python tests/size_report.py 00000016 00000002 t npf_bufputc_nop 00000000 00000016 t npf_bufputc 0000036e 00000016 T npf_pprintf 000003b8 00000016 T npf_snprintf 00000384 00000034 T npf_vsnprintf 00000018 00000356 T npf_vpprintf Total size: 0x3ce (974) bytes Binary: arm-none-eabi-gcc -DNANOPRINTF_SIZE_REPORT=1 -mcpu=cortex-m0 -Os -c -o cm0-1.o tests/size_report.c arm-none-eabi-nm --print-size --size-sort cm0-1.o | python tests/size_report.py 00000016 00000002 t npf_bufputc_nop 00000000 00000016 t npf_bufputc 000003c2 00000016 T npf_pprintf 0000040c 00000016 T npf_snprintf 000003d8 00000034 T npf_vsnprintf 00000018 000003aa T npf_vpprintf Total size: 0x422 (1058) bytes Field Width + Precision: arm-none-eabi-gcc -DNANOPRINTF_SIZE_REPORT=2 -mcpu=cortex-m0 -Os -c -o cm0-2.o tests/size_report.c arm-none-eabi-nm --print-size --size-sort cm0-2.o | python tests/size_report.py 00000016 00000002 t npf_bufputc_nop 00000000 00000016 t npf_bufputc 00000656 00000016 T npf_pprintf 000006a0 00000016 T npf_snprintf 0000066c 00000034 T npf_vsnprintf 00000018 0000063e T npf_vpprintf Total size: 0x6b6 (1718) bytes Field Width + Precision + Binary: arm-none-eabi-gcc -DNANOPRINTF_SIZE_REPORT=3 -mcpu=cortex-m0 -Os -c -o cm0-3.o tests/size_report.c arm-none-eabi-nm --print-size --size-sort cm0-3.o | python tests/size_report.py 00000016 00000002 t npf_bufputc_nop 00000000 00000016 t npf_bufputc 000006c6 00000016 T npf_pprintf 00000710 00000016 T npf_snprintf 000006dc 00000034 T npf_vsnprintf 00000018 000006ae T npf_vpprintf Total size: 0x726 (1830) bytes Float: arm-none-eabi-gcc -DNANOPRINTF_SIZE_REPORT=4 -mcpu=cortex-m0 -Os -c -o cm0-4.o tests/size_report.c arm-none-eabi-nm --print-size --size-sort cm0-4.o | python tests/size_report.py 00000016 00000002 t npf_bufputc_nop 00000000 00000016 t npf_bufputc 000006fc 00000016 T npf_pprintf 00000744 00000016 T npf_snprintf 00000712 00000032 T npf_vsnprintf 00000018 000006e4 T npf_vpprintf Total size: 0x75a (1882) bytes Everything: arm-none-eabi-gcc -DNANOPRINTF_SIZE_REPORT=5 -mcpu=cortex-m0 -Os -c -o cm0-5.o tests/size_report.c arm-none-eabi-nm --print-size --size-sort cm0-5.o | python tests/size_report.py 00000016 00000002 t npf_bufputc_nop 00000000 00000016 t npf_bufputc 00000ab0 00000016 T npf_pprintf 00000af8 00000016 T npf_snprintf 00000ac6 00000032 T npf_vsnprintf 00000018 00000a98 T npf_vpprintf Total size: 0xb0e (2830) bytes
To get the environment and run tests (linux / mac only for now):
- Clone or fork this repository.
./bfrom the root.
This will build all of the unit, conformance, and compilation tests for your host environment. Any test failures will return a non-zero exit code.
The matrix builds [Debug, Release] x [32-bit, 64-bit] x [Mac, Windows, Linux] x [gcc, clang, msvc], minus the 32-bit clang Mac configurations.
One test suite is a fork from the printf test suite, which is MIT licensed. It exists as a submodule for licensing purposes- nanoprintf is public domain, so this particular test suite is optional and excluded by default. To build it, retrieve it by updating submodules and add the
--paland flag to your
./b invocation. It is not required to use nanoprintf at all.
I ported the printf test suite to nanoprintf. It was originally from the mpaland printf project codebase but adopted and improved by Eyal Rozenberg and others. (Nanoprintf has many of its own tests, but these are also very thorough and very good!)