shell/cmds: GNRC: replace puts() with printf()

[benpicco]

Contribution description

Strings output with printf() will automatically be placed in the .text section on AVR now.
This is not true for strings output with puts().

Replace puts() with puts() by running

sed -Ei 's/puts\(\"(.*)\"\)/printf\(\"\1\\n\"\)/g'

Testing procedure

Observe the reduced .data RAM usage on AVR:

master

   text	   data	    bss	    dec	    hex	filename
 123950	   6710	  13123	 143783	  231a7	/home/benpicco/dev/RIOT/examples/gnrc_networking/bin/avr-rss2/gnrc_networking.elf

this PR

   text	   data	    bss	    dec	    hex	filename
 129840	   2042	  13123	 145005	  2366d	/home/benpicco/dev/RIOT/examples/gnrc_networking/bin/avr-rss2/gnrc_networking.elf

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

b1e327d examples/gnrc_networking: remove microduino-corerf from Makefile.ci

Success	Failures	Total	Runtime
6864	0	6864	10m:45s

Artifacts

• Documentation preview

[maribu]

Since we can rely on the compiler to emit calls to puts("Foo"); when it sees printf("Foo\n");, this should not impact non-AVR platforms. And it still is idiomatic C code.

[benpicco]

bors merge

[bors]

Build failed (retrying...):

• Murdock

[bors]

Canceled.

[benpicco]

bors cancel
bors merge

[bors]

Build succeeded:

• Murdock
• static-tests
• tools-build-success

[OlegHahm]

Why do strings in a puts() call don't get into the text section? How do we prevent this from being re-introduced in the future?

[benpicco]

We don't enforce the puts() string to be const a string literal but we do for printf() (originally to prevent format string bugs).

There is a case for outputting non-const literal strings with puts() so we can't automatically place the argument in flash.

[OlegHahm]

Thanks for the explanation. I still wonder if or how we could check for any regressions in this direction.

[maribu]

On ATmega and Xtensa const char * will still land in RAM due to flash memory not being mapped in the data address space. When declaring and initializing a variable we can add the __flash attribute for those to force it to flash (and the compiler will correctly load them from flash on access), but this requires some set up.

For printf() the first argument has to be a string literal to allow the compiler to validate format specifiers to match the remaining arguments. This is enforced in RIOT with -Wformat=2 for ages anyway. We can levarage this to just force the format string to flash on AVR / Xtensa transparently.

For puts() the argument can be a non-literal. We have no reason to force it to be a literal (well, other than AVR / Xtensa profiting), so we cannot do that. Hence, we cannot do the same trick with puts().

[benpicco]

I suppose we could to

#define puts_literal(s) flash_puts(TO_FLASH(s))

but that might be a bit controversial