Make picolibc the default C library for Zephyr

[stephanosio]

#44096 introduced the picolibc, a full-featured yet light-weight C library and fork of newlib, as a module for Zephyr that can be configured and fine-tuned at the Zephyr build time to minimise its footprint.

This issue describes the tasks required for making the picolibc the default C library support in the Zephyr RTOS.

Tasks

• Adding picolibc as a module #44143
• Add picolibc documentation #49620
  • doc/develop: Add picolibc documentation #54293
• Ensure that picolibc is compatible with and tested on all architectures supported by Zephyr
  • aarch64-zephyr-elf
  • arc-zephyr-elf
  • arc64-zephyr-elf
  • arm-zephyr-eabi
  • mips-zephyr-elf
  • nios2-zephyr-elf
  • riscv64-zephyr-elf
  • sparc-zephyr-elf
  • x86_64-zephyr-elf
  • xtensa-*zephyr-elf
• Bundle picolibc in Zephyr SDK for libstdc++ integration
  (Add picolibc sdk-ng#602)
• Fix any incompatibilities with the picolibc in the Zephyr codebase. These are validated in Complete set of changes to enable picolibc by default #60124.
  • modules/hal/openisa: Support building without -ffreestanding #51908
  • Add -lc -lgcc from picolibc toolchain library after module libraries #51949
  • (modules: Update picolibc to 1.8 #53337)
  • (Allow use of SDK version of picolibc with C++ #53338)
  • (libc: Add remaining picolibc-supported targets #53339)
  • (samples/modules/chre: Support picolibc %p for NULL pointers #53574)
  • (samples/philosophers: Make output atomic with picolibc #53575)
  • (sample/zbus: qemu_nios2 needs faster tick for zbus test with picolibc #53684)
  • (Enabling TLS detects stack overflow in a few tests #53687)
  • (modules/cmsis: Change libc requirement to allow picolibc #53950)
  • subsys/bluetooth: Avoid RX overflow in lt_tx_real_no_encode #54119 (no longer appears to cause test suite failures)
  • qemu_x86_tiny places picolibc text outside of pinned.text #54148
  • lib/libc: Enable picolibc on qemu_x86_tiny #54186
  • modules/picolibc: Include clang+cmake and old GCC changes #54394
  • Switch return type of main from 'void' to 'int' #54628
  • Allow tests to filter on availability of full C library #54637
  • Only use -ffreestanding with minimal C library #54638
  • modules/audio/sof: Pull fix for main return type #57398
  • Fix linker scripts to define _end correctly #57727
  • libc/common: Add memalign #58012
  • libc/common: malloc fixes #58450
  • tests, samples: Fix a bunch of new instances of void main(void) #58458
  • tests: Increase a couple of test stack sizes #58459
  • tests/c_lib: strerror_r is POSIX #58460 (Removes _POSIX_C_SOURCE from CFLAGS: not a compatibility fix)
  • libc/picolibc: Picolibc fixes #58462
  • tests/iterable_sections: Set common stack arena size to zero #58464 (no longer necessary for this test to work)
  • tests/spinlock: Terminate temporary thread after tests #60123 (Fixed via Testcases fixes #60567)
  • samples/boards/google_twinkie_v2_pda: fix main return #61233
• Document the policy to ensure that the Zephyr kernel can keep building with the minimal libc
  • Add coding guidelines on C standard library function usage #57598
• Document the policy to prevent the abuse of non-standard functions available in Picolibc
  • Add coding guidelines on C standard library function usage #57598
• Implement tests and CI checks to enforce the above policies
  • Enforce "Rule A.4: C Standard Library Usage Restrictions in Zephyr Kernel"
    • Build the "Zephyr kernel" tests with the minimal libc to ensure that they are compatible with the minimal libc. tests/kernel/common: Add an explicit test using the minimal C library #58108
  • Enforce "Rule A.5: C Standard Library Usage Restrictions in Zephyr Codebase"
    • Add _ZEPHYR_SOURCE support to Picolibc to limit the visibility of non-standard functions from the Zephyr codebase. modules/lib/picolibc: Use version with _ZEPHYR_SOURCE support #58106
    • Add a CI check to prevent any upstream source files from defining the feature test macros (e.g. _GNU_SOURCE) to pull "disallowed" functions. scripts/checkpatch: Check for patches adding #defines for libc APIs #60487
• Document module and toolchain-bundled Picolibc synchronisation policy
  • Initial proposal: Make picolibc the default C library for Zephyr #49922 (comment)
  • Zephyr doc update: Add module and toolchain-bundled Picolibc synchronisation policy #58006
  • sdk-ng doc update: README: Add submodule update process sdk-ng#673
• Make PICOLIBC the default LIBC_IMPLEMENTATION
  • libc: Default to picolibc where supported #57340

Related discussions

• picolibc footprint comparison
• Rationale and overall requirements
• Configuration requirements
• Adding picolibc as a module

[keith-packard]

I think we should use an SDK version that includes picolibc -- that way we can run both C and C++ tests and use the picolibc module and the SDK version. I think that means getting picolibc into a version of the SDK that we can use from the github CI system?

[stephanosio]

I think we should use an SDK version that includes picolibc -- that way we can run both C and C++ tests and use the picolibc module and the SDK version. I think that means getting picolibc into a version of the SDK that we can use from the github CI system?

Yes, zephyrproject-rtos/sdk-ng#287 will need to be completed as part of this.

[stephanosio]

cc @nashif @carlescufi @tejlmand

[stephanosio]

Toolchain WG on 2022-09-20

• @nashif: This will need to be discussed and reviewed at the project level (TSC).
• @nashif: In general, no strong objection to the general direction of making picolibc the default C library; however, there are some concerns regarding the subsequent effects that this can have:
  • @nashif: How do we prevent the core parts of Zephyr (e.g. kernel) from abusing all the available C library functions that may lead to overall footprint increase?
    • @stephanosio: We can define a minimal set of C standard library functions that are sanctioned to be used in the core parts of Zephyr and guard any extraneous functions by a Kconfig such that they are not available for use by default.
  • @nashif: Using the C standard library functions that are supported by the picolibc but not by the minimal libc will render Zephyr unusable with the minimal libc. How do we prevent that?
    • @stephanosio: We can take a similar approach as the above. But, the minimal libc is TOO minimal and it should not be the "golden standard" to which all Zephyr subsystems are doomed.
    • @stephanosio: For kernel, limiting the availability of the C standard library functions to what is currently provided in the minimal libc seems reasonable. For other subsystems, more flexibility should be allowed even at the cost of breaking the compatibility with the minimal libc.