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spe printf

A printf for small micro controllers, mainly Cortex M3.

Supported conversion tags

It supports:

  • minimal width and optional precision for all numerical types.
  • long modifier for all numerical conversion tags.
  • plain percent (%), character (c), string (s), signed integer, unsigned integer(u) and hex (x).
  • double if compiled in, see below.
  • Includes variadic versions of all printf() functions.
  • Includes snprintf()/vsnprintf() versions to print to strings.

It does not support:

  • minimal width and optional precision for strings.
  • negative minimal width.
  • minimal width as a parameter to the conversion string (*)

double()

It can print doubles too. Then the variable USE_DOUBLE needs to be set when compiling, by for instance adding CFLAGS+=-DUSE_DOUBLE in the Makefile.

Documentation

This library is documented using the Doxygen format.

In the src directory, execute make doxygen. This will generate a subdirectory in the src directory called docs. By default only html is generated. Start of by pointing your browser at docs/html/index.html.

Unit tests

This library has a set of unit tests using the CppUTest framework in the tests directory.

The tests are supposed to run on a platform with a working printf implementation. The generated string is compared to a string generated by the test systems ordinary printf.

cpputest is supposed to be installed at $(HOME)/tools/cpputest, update in tests/Makefile to fit your installation.

Motivation

For Cortex M3 using stdio.h available in newlib makes the binary about 24k bytes bigger. This library adds less than 2k. This library has no input functions though.

What is so special about this library?

This printf library does not use any buffer to convert numerics to printable strings. Instead it divides down the numeric until there is one digit left and then starts multiplying back to print the numeric value character by character.

Another advantage of not using any internal buffers (except it saves precious RAM) is that it could be considered reentrant. Great news if you intend to use an RTOS, for instance.

License

Released under the very generous Expat License, see LICENSE.txt.

Caveats

You can probably optimize some bytes here and there, since I used int through out the source code. You can probably also use the stdint.h types (uint32_t et al) to make it more portable. The functions print_uil() and print_ui() are almost the same, though one are for long and the other for int. print_ui() can probably be optimized away and only print_uil() be used if necessary. print_ui() is left if a need to remove usage of long completely is necessary.

Usage

  • Include "spe_printf.h" in your file.
  • Write a function that sends out a character on your device.
  • Register that function.
  • Make sure you define stdout and stderr as pointers to the variable that stores the character printing function.

As a simple example, see spe_print-example.c. It is much simpler than it sounds.

Good luck, Stefan Petersen (spe [at] ciellt (dot) se)