pkg/mpaland-printf: Add alternative stdio as package

[maribu]

Contribution description

This packs the stdio implementation from https://github.com/mpaland/printf as alternative to what the used standard C lib provides with the intent to provide a thread-safe, smaller, and more feature-complete alternative on newlib targets.

Compared to newlib_nano this reduces .text by a bit more than 200 B while adding support for standard format specifiers such as RPIu8, PRIu16, PRIu64, %z, and %t.

Note that newlib_nano's stdio can be thread-safe in reentrant mode at the cost of RAM (per thread) and latency. Especially the increase in latency can be prohibitive when real time requirements need to be met.

Testing procedure

A test application has been added in tests/sys/snprintf. This will use the package if newlib is used. It should pass, but removing Makefile.board.dep (which pulls in the mpaland-printf package) should a failing test.

Issues/PRs references

#20361

[riot-ci]

Murdock results

✔️ PASSED

42dacd7 tests/sys/snprintf: Test format specifiers

Success	Failures	Total	Runtime
10320	0	10320	13m:26s

Artifacts

• Documentation preview

[Enoch247]

I'll try and take a look at this later since I am familiar with the pkg already.

[Enoch247]

Started looking this over, but ran out of time. I'll pick it up another night.

[Enoch247]

For me, the new test would not build when doing make BOARD=stm32f429i-disc1 until I added the following hack:

diff --git a/tests/sys/snprintf/main.c b/tests/sys/snprintf/main.c
index 151966c0d5..0d27dca3cb 100644
--- a/tests/sys/snprintf/main.c
+++ b/tests/sys/snprintf/main.c
@@ -19,6 +19,7 @@
  * @}
  */
 
+#define __int64_t_defined 1
 #include <inttypes.h>
 #include <stdbool.h>
 #include <stddef.h>

[maribu]

Thx for reporting. I can reproduce the issue on my Ubuntu machine.

Can you give it another try with the fixup commit?

[Enoch247]

Indeed make -C tests/sys/snprintf BOARD=stm32f429i-disc1 test passes if I use this branch unmodified, and fails if I disable mpaland-printf. Is newlib's snprintf implementation really broken? A quick internet search did not turn up anything about this. The test also passes on native where neither newlib nor mpaland-printf are enabled.

[maribu]

The newlib's stdio comes in two flavors: "super bloated" (stdio-nano), and "supermassive black hole of bloat" (regular stdio). The latter would pass the stido unit tests (try disabled newlib_nano), if it would even fit your MCU's flash and RAM. The former is a stripped down version of the latter, trading features (in other words compatibility with format specifiers) for size.

Both are way too large to be a sane option on MCUs. That is why the picolibs fork of newlib just trashed the stdio and replaces it with something that does make sense on MCUs.

[mcr]

Having patches in git is giving me Quilt-PTSD, but I get why. I just hate it.

[maribu]

Thx :-)