sys/fmt: use fwrite with picolibc

[benpicco]

Contribution description

Use fwrite() instead of write() to fix linking with picolibc.

(Why even keep the separate write codepath?)

Testing procedure

make -C examples/gnrc_networking BOARD=samr21-xpro PICOLIBC=1 -j

should be working now.

Issues/PRs references

follow-up to #14827
#14843 offers a more comprehensive solution.

[maribu]

I assume the reason to use fwrite() is that you prefer buffered I/O, e.g. especially when using USB ACM for stdio?

Wouldn't using fwrite() over write() fix the issues with mixing puts() and printf() with fmt functions for the same reason? As the RIOT code base is full of calls to puts() and printf() anyway, is there any advantage in terms of ROM size in using write() over fwrite()? Otherwise, we might want to consider to just always using fwrite().

[benpicco]

With newlib:

fwrite

  97604	    184	  19236	 117024	  1c920	/home/benpicco/dev/RIOT/examples/gnrc_networking/bin/samr21-xpro/gnrc_networking.elf

write

   text	   data	    bss	    dec	    hex	filename
  97404	    184	  19236	 116824	  1c858	/home/benpicco/dev/RIOT/examples/gnrc_networking/bin/samr21-xpro/gnrc_networking.elf

But I keep wondering how fmt keeps working if the vfs module is used.
If 'our' write function is used, this will crash & burn (or corrupt a file) - #14827 (comment)

[maribu]

With newlib:

fwrite

  97604	    184	  19236	 117024	  1c920	/home/benpicco/dev/RIOT/examples/gnrc_networking/bin/samr21-xpro/gnrc_networking.elf

write

   text	   data	    bss	    dec	    hex	filename
  97404	    184	  19236	 116824	  1c858	/home/benpicco/dev/RIOT/examples/gnrc_networking/bin/samr21-xpro/gnrc_networking.elf

But I keep wondering how fmt keeps working if the vfs module is used.
If 'our' write function is used, this will crash & burn (or corrupt a file) - #14827 (comment)

Hmm, 200 B ROM seems to be worth the pain of using either fwrite() or write(). But we certainly should go for fwrite() if vfs is used - at least until vfs takes special care for fds 0-2.

[benpicco]

Ok if we do this as an intermediate solution I’m fine.

[benpicco]

I was wrong, VFS already does the right thing - no need to avoid write here.
But of course it is preferable to always provide the write function.

[benpicco]

#14843 was merged instead.