libc: Default to picolibc where supported

[keith-packard]

This switches the default C library to picolibc for all targets which support it.

Closes #49922

[carlescufi]

@keith-packard labeling this as TSC since this requires a bit more discussion.

[zephyrbot]

The following west manifest projects have been modified in this Pull Request:

Name	Old Revision	New Revision	Diff
mcuboot	zephyrproject-rtos/mcuboot@1558e7a (main)	zephyrproject-rtos/mcuboot@2878eb4 (upstream-sync)	zephyrproject-rtos/mcuboot@1558e7ab..2878eb4e
picolibc	zephyrproject-rtos/picolibc@93b5d5f (main)	zephyrproject-rtos/picolibc@zephyr-source	zephyrproject-rtos/picolibc@93b5d5f2..zephyr-source
sof	zephyrproject-rtos/sof@ffbf9c2 (zephyr)	zephyrproject-rtos/sof#28	zephyrproject-rtos/sof#28/files

Note: This message is automatically posted and updated by the Manifest GitHub Action.

[marc-hb]

One of the commit messages is:

modules/audio/sof: Pull fix for main return type

Upstream PR #28 switches return type of 'main' to 'int'.

Signed-off-by: Keith Packard keithp@keithp.com

Pull request 28 does not seem related. It's from 2017 :-)

EDIT: got it, you meant zephyrproject-rtos/sof/pull/28, not #28

Could you please share a link with more information about this change of the main prototype? It seems related to picolibc but so are many other changes so it does not narrow it down.

---

[keith-packard]

EDIT: got it, you meant zephyrproject-rtos/sof/pull/28, not #28

Sorry for the mis-leading link.

Could you please share a link with more information about this change of the main prototype? It seems related to picolibc but so are many other changes so it does not narrow it down.

Yeah, this is only vaguely related to picolibc itself -- as a complete C library, it's best to compile applications using picolibc without the -ffreestanding compiler flag as that improves code generation and error detection. But, with that option not present, clang makes mis-matching main type a fatal error. So, that means we needed to switch the return type to int. That series, #54628 was merged a while ago.

[stephanosio]

As per #49922 (comment), this needs more documentation and process-related work before it can be approved by the TSC.

I will look into that and create a new PR based on this with the necessary changes.

[tejlmand]

lgtm 🎉

[keith-packard]

sorry for the update -- I rebased on top of an unrelated fix which was blocking CI. Now that it's passing, it would be a good time to get this merged :-)

Can you please re-review?

[nashif]

dont we end up with both picolibc and minimal libc set as default? or does order here play a role?

[keith-packard]

Kconfig appears to pick the first which matches and which can be enabled -- the goal is to use picolibc where supported otherwise use minimal C library.

[carlescufi]

@keith-packard, now that we have a migration guide, would you mind adding a note there? I assume under "Required changes", given that users may see their applications break depending on what they are using?

[keith-packard]

@keith-packard, now that we have a migration guide, would you mind adding a note there? I assume under "Required changes", given that users may see their applications break depending on what they are using?

#62261 has some possible things to check. I'm not sure how these should be integrated into the rest of the migration notes though.