Picolibc: C Libraries for Smaller Embedded Systems

Picolibc is a set of standard C libraries, both libc and libm, designed for smaller embedded systems with limited ROM and RAM. Picolibc includes code from Newlib and AVR Libc. Picolibc has been tested on RISC-V and ARM processors.

Picolibc was previously named newlib-nano, but that name has another definition within newlib and so the Picolibc name was adopted instead.

Releases

Picolibc packages are available in the dist directory here

Presentations

I've written a presentation for Linaro Connect and the Embedded Linux Conference. I wrote complete notes to go with the slides, so I've uploaded the slides and notes here.

Pre-built GNU Arm Embedded Binaries

Pre-built binaries are available in the gnu-arm-embedded directory here

These packages are designed to be used with specific versions of the GNU Arm Embedded Toolchain. Get the zip file for the version matching the toolchain you have installed.

First, download and install the GNU Arm Embedded Toolchain. Next, download the matching Picolibc zip file. Then, unpack the Picolibc zip file “on top” of the GNU Arm Embedded Toolchain. This is done most easily by opening up a command prompt window and using a zip file extractor to overlay the picolibc bits on top of the GNU Arm Embedded Toolchain bits.

On Windows, you can use the 7-Zip command line tool, 7z. If the Picolibc zip file is in c:\Users\keithp\Downloads\picolibc-1.6-10-2020-q4-major.zip, and the GNU Arm Embedded Toolchain is installed in c:\Program Files (x86)\GNU Arm Embedded Toolchain\10 2020-q4-major, then you'd type:

C:\Users\keithp> cd "c:\Program Files (x86)\GNU Arm Embedded Toolchain\10 2020-q4-major"
C:\Program Files (x86)\GNU Arm Embedded Toolchain\10 2020-q4-major> 7z x C:\Users\keithp\picolibc-1.6-10-2020-q4-major.zip

To verify that you have the right directory, you can check the contents which should look like this:

c:\Program Files (x86)\GNU Arm Embedded Toolchain\10 2020-q4-major>dir
 Volume in drive C has no label.
 Volume Serial Number is 4047-33CE

 Directory of c:\Program Files (x86)\GNU Arm Embedded Toolchain\10 2020-q4-major

06/02/2021  05:01 PM    <DIR>          .
06/02/2021  05:01 PM    <DIR>          ..
06/03/2021  04:25 PM    <DIR>          arm-none-eabi
06/02/2021  05:01 PM    <DIR>          bin
06/02/2021  05:01 PM    <DIR>          lib
06/02/2021  05:01 PM    <DIR>          share
06/02/2021  05:01 PM           162,555 uninstall.exe

On Linux, you can use the unzip tool. If the Picolibc zip file is in ~/Downloads/picolibc-1.6-10-2020-q4-major.zip and the GNU Arm Embedded Toolchain is installed in /opt/arm/gcc-arm-none-eabi-10-2020-q4-major, then you'd type:

$ cd /opt/arm/gcc-arm-none-eabi-10-2020-q4-major
$ unzip ~/Downloads/picolibc-1.6-10-2020-q4-major.zip

To verify that you have the right directory, you can check the contents which should look like this:

$ ls -l
total 16
drwxr-xr-x 7 keithp keithp 4096 Jun  4 18:21 arm-none-eabi
drwxr-xr-x 2 keithp keithp 4096 Nov 23  2020 bin
drwxr-xr-x 3 keithp keithp 4096 Nov 23  2020 lib
drwxr-xr-x 4 keithp keithp 4096 Nov 23  2020 share

Picolibc release 1.8.8

Fixed 3 bugs in the powf computation. Thanks to Fabian Schriever.
Fixed a bunch of build issues found by Zephyr.
Improve C++ testing and compatibility.

Picolibc release 1.8.7

Support ARM v8.1-m BTI and PAC features
Fix stdio buffered backend automatic flushing of stdout when reading stdin.
Support _FORTIFY_SOURCE=3
Fix several fesetround implementations to return an error when passed an invalid argument. Thanks to Abdallah Abdelhafeez.
Document headers which the compiler must provide. Thanks to Alexey Brodkin.
Generate mktemp/tmpnam filenames using random() so they don't repeat even if they aren't used before another name is generated.
Set error flag when fgetc is called on an file without read mode. Thanks to Mohamed Moawad.
Add type casting to CMPLX, CMPLXF and CMPLXL macros (as glibc does). Thanks to Mostafa Salman.
Add mips64 support and build the library during CI.
Make fgets return any accumulated string on EOF instead of always returning NULL. Thanks to Hana Ashour.
Use C99 minimum array size in asctime_r and ctime_r API declarations ('[static 26]'). Bounds check the generated value and return NULL/EOVERFLOW on overflow.
Make Zephyr's -Oz cmake option enable PREFER_SIZE_OVER_SPEED. Thanks to Jonathon Penix.
Add funopen to tinystdio.
Validate all public headers with a C++ compiler to make sure they at least compile successfully. Fix time.h.
Stop using -include picolibc.h during library build.
Add -Wmissing-declarations and -Wmissing-prototypes to library build flags. Fix a rather large pile of missing prototypes caused by source files failing to add GNU_SOURCE or DEFAULT_SOURCE definitions.
Add POSIX "unlocked" I/O functions to tinystdio. These don't actually do anything because tinystdio doesn't do any locking. However, flockfile/funlockfile grab the global C library lock so applications synchronizing with that API will "work".
Fix wide orientation handling in tinystdio. Thanks to Ahmed Shehab.
Add aarch64 soft float support for armv8. Clang allows this with -march=armv8-a+nofp -mabi=aapcs-soft. This required building a custom toolchain that included a compiler-rt library built with the right options.
Add fgetpos and fsetpos to tinystdio. Thanks to Hana Ashour.
Restore missing members of 'struct sigevent'. Over eager removal of _POSIX_THREADS support caused these to be accidentally deleted some time ago.
Test on i386 native target.
Fix hex float scanning and printing. Thanks to Hana Ashour and Ahmed Shehab.
Fix double rounding in %f printf. Thanks to Ahmed Shehab for constructing a test case that identified the issue.
Add mem_align to the "big" malloc version. Thanks to Simon Tatham.
Adjust POSIX and C headers to limit symbol exposure to that specified in the standards.
Fix rounding in float scanf. This does round twice for input longer than the required number of digits, but that's permitted by the C specification.
Support %a/%A in scanf. Support arbitrary precision in %a/%A printf. Fix NaN/INF formatting in %a/%A printf. Thanks to Ahmed Shehab.
Provide a build-time option to enable %n in printf. This is disabled by default for security concerns, but supported in case someone needs strict C conformance. Thanks to Ahmed Shehab.
Make freopen clear the unget buffer. Thanks to Mostafa Salman.
Fix wide and multi-byte character support in printf and scanf. For strict standards conformance, there's now an option that enables %lc/%ls in printf even if multi-byte support is not enabled.
Enable MMU in picocrt on A profile ARM and AARCH64 targets when present. This is required by the latest qemu which now more accurately emulates this hardware. Thanks to Alex Richardson.
Fix AARCH64 asm code in ILP32 mode.
Parse NaN() in sscanf. This is required by the standard, although picolibc doesn't do anything with . Thanks to Mohamed Moawad.
Clean up header files. Picolibc tries to limit symbol definitions to those specified in the C and POSIX specs.
Add support for C's Annex K functions. These are bounds-checking versions of various memory and string functions. Thanks to Mostafa Salman.
Perform locale string validation in newlocale even when _MB_CAPABLE isn't defined. Thanks to Mostafa Salman.
Place compiler-rt library after C library when linking tests. Thanks to Oliver Stannard.

Picolibc version 1.8.6

Fix some FORTITY_SOURCE issues with tinystdio
Add _eh* symbols to picolibc.ld for LLVM libunwind. Thanks Alex Richardson.
Merge in newlib annual release (4.4.0). Some minor updates to aarch64 assembly code formatting (thanks to Sebastian Huber) and a few other fixes.
Enable 32-bit SPARC for testing.
Fix a bunch of fmemopen bugs and add some tests. Thanks to Alex Richardson.
Finish support for targets with unusual float types, mapping target types to 32-, 64-, 80- and 128- bit picolibc code.
Add SuperH support, including testing infrastructure. Thanks to Adrian Siekierka for help with this.
Improve debugger stack trace in risc-v exception code. Thanks to Alex Richardson.
Add an option (-Dfast-bufio=true) for more efficient fread/fwrite implementations when layered atop bufio. Thanks for the suggestion from Zachary Yedidia.
Fix cmake usage of FORMAT_ variables (note the lack of a leading underscore).
Remove explicit _POSIX_C_SOURCE definition in zephyr/zephr.cmake.
Clean up public inline functions to share a common mechanism for using gnu_inline semantics. Fix isblank. This ensures that no static inline declarations exist in public API headers which are required to be external linkage ("real") symbols.
Create an alternate ctype implementation that avoids using the ctype array and just does direct value comparisons. This only works when picolibc is limited to ASCII. Applications can select whether they want this behavior at application compilation time without needing to rebuild the C library. Thanks to P. Frost for the suggestion.
Unify most fenv implementations to use gnu_inline instead of regular functions to improve performance. x86 was left out because those fenv functions are complicated by the mix of 8087 and modern FPU support.
Add a separate FILE for stderr when using POSIX I/O. Split stdin/stdout/stderr into three files to avoid pulling in those which aren't used. Thanks to Zachary Yedidia.

Picolibc version 1.8.5

Detect clang multi-lib support correctly by passing compiler flags. Thanks to xbjfk for identifying the problem.
Create a new 'long-long' printf variant. This provides enough variety to satisfy the Zephyr cbprintf options without needing to build the library from scratch.
Adjust use of custom binary to decimal conversion code so that it is only enabled for types beyond the register size of the target. This avoids the cost of this code when the application is already likely to be using the soft division routines.

Picolibc version 1.8.4

Make math overflow and underflow handlers respect rounding modes.
Add full precision fma/fmaf fallbacks by adapting the long-double code which uses two floats/doubles and some careful exponent management to ensure that only a single rounding operation occurs.
Fix more m68k 80-bit float bugs
Fix m68k asm ABI by returning pointers in %a0 and %d0
Use an m68k-unknown-elf toolchain for m68k testing, including multi-lib to check various FPU configurations on older and more modern 68k targets.
Improve CI speed by using ccache on zephyr and mac tests, compressing the docker images and automatically canceling jobs when the related PR is updated. Thanks to Peter Jonsson.
Move a bunch of read-only data out of RAM and into flash by adding 'const' attributes in various places.
Add a new linker symbol, __heap_size_min, which specifies a minimum heap size. The linker will emit an error if this much space is not available between the end of static data and the stack.
Fix a bunch of bugs on targets with 16-bit int type. Thanks to Peter Jonsson for many of these.
Work around a handful of platform bugs on MSP430. I think these are compiler bugs as they occur using both the binutils simulator and mspsim.
Run tests on MSP430 using the simulator that comes with gdb. Thanks to Peter Jonsson for spliting tests apart to make them small enough to link in under 1MB. This requires a patch adding primitive semihosting to the simulator.
Provide a division-free binary to decimal conversion option for printf at friends. This is useful on targets without hardware divide as it avoids pulling in a (usually large) software implementation. This is controlled with the 'printf-small-ultoa' meson option and is 'false' by default.
Add 'minimal' printf and scanf variants. These reduce functionality by removing code that acts on most data modifers including width and precision fields and alternate presentation modes. A new config variable, minimal-io-long-long, controls whether that code supports long long types.
Add a 'assert-verbose' option which controls whether the assert macro is chatty by default. It is 'true' by default, which preserves the existing code, but when set to 'false', then a failing assert calls __assert_no_msg with no arguments, saving the memory usually occupied by the filename, function name and expression.
Fix arm asm syntax for mrc/mcr instructions to make clang happy. Thanks to Radovan Blažek for this patch.

Picolibc version 1.8.3

Fix bugs in floor and ceil implementations.
Use -fanalyzer to find and fix a range of issues.
Add __ubsan_handle_out_of_bounds implementation. This enables building applications with -fsanitize=bounds and -fno-sanitize-undefined-trap-on-error.
Validate exception configuration on targets with mixed exception support where some types have exceptions and others don't. Right now, that's only arm platforms where any soft float implementations don't build with exception support.
Fix bugs in nexttowards/nextafter on clang caused by the compiler re-ordering code and causing incorrect exception generation.
Use the small/slow string code when -fsanitize=address is used while building the library. This avoids reading beyond the end of strings and triggering faults.
Handle soft float on x86 and sparc targets. That mostly required disabling the hardware exception API, along with a few other minor bug fixes.
Add runtime support for arc, mips, nios2 and m68k. This enables CI testing on these architectures using qemu.
Fix 80-bit floating math library support for m68k targets.
Fix arm testing infra to use various qemu models that expand testing to all standard multi-lib configurations.
Adjust floating exception stubs to return success when appropriate, instead of always returning ENOSYS.
Make sure sNaN raises FE_INVALID and is converted to qNaN in truncl, frexpl and roundl
Avoid NaN result from fmal caused by multiply overflow when addend is infinity (-inf + inf results in NaN in that case).

Picolibc version 1.8.2

Support _ZEPHYR_SOURCE macro which, like _POSIX_SOURCE et al, controls whether the library expresses the Zephyr C library API. This is also automatically selected when the __ZEPHYR__ macro is defined and no other _*_SOURCE macro is defined.
Add another cross compile property, 'libgcc', which specifies the library containing soft float and other compiler support routines.
Fix a couple of minor imprecisions in pow and 80-bit powl.
Merge newlib changes that included an update to the ARM assembly code.
Replace inexact float/string conversion code with smaller code that doesn't use floating point operations to save additional space on soft float targets.
More cmake fixes, including making the inexact printf and locale options work.

Picolibc version 1.8.1

Fix cmake build system to auto-detect compiler characteristics instead of assuming the compiler is a recent version of GCC. This allows building using cmake with clang.
Fix cmake build system to leave out TLS support when TLS is disabled on the cmake command line.
Replace inline asm with attributes for __weak_reference macro
Add allocation attributes to malloc and stdio functions. This allows the compiler to detect allocation related mistakes as well as perform some additional optimizations. Bugs found by this change were also addressed.
Add wchar_t support to tinystdio, eliminating the last missing feature compared with the legacy stdio bits from newlib. With this, libstdc++ can be built with wide char I/O support, eliminating the last missing feature there as well.
Eliminate use of command line tools when building with a new enough version of meson. Thanks to Michael Platings.
Add Microblaze support. Thanks to Alp Sayin.
Switch semihosting to use binary mode when opening files. Thanks to Hardy Griech.
Build and install static library versions of the crt0 startup code. These allows developers to reference them as libraries on the command line instead of needing special compiler support to locate the different variants, which is useful when using clang. Thanks to Simon Tatham.
Simplify the signal/raise implementation to use a single global array of signal handlers and to not use getpid and kill, instead raise now directly invokes _exit. This makes using assert and abort simpler and doesn't cause a large TLS block to be allocated. Thanks to Joe Nelson for discovering the use of a TLS variable here.

Picolibc version 1.8

With the addition of nearly complete long double support in the math library, it seems like it's time to declare a larger version increment than usual.

Improve arc and xtensa support, adding TLS helpers and other build fixes
Fix FPSCR state for Arm8.1-M low overhead loops (thanks to David Green)
Add -Werror=double-promotion to default error set and fix related errors. (thanks to Ryan McClelland)
Fix locking bug in malloc out-of-memory path and freeing a locked mutex in the tinystdio bufio code. These were found with lock debugging code in Zephyr.
Add some missing functions in tinystdio, strto*l_l, remove, tmpname/tmpfile which were published in stdio.h but not included in the library.
Switch read/write functions to use POSIX types instead of legacy cygwin types. This makes mapping to existing an POSIX api work right.
Add %b support to tinystdio printf and scanf. These are disabled by default as they aren't yet standardized.
Fix avr math function support. The avr version of gcc has modes where double and long double are 32 or 64 bits, so the math library code now detects all of that at compile time rather than build time and reconfigures the functions to match the compiler types.
Add nearly complete long double support from openlibm for 80-bit Intel and 128-bit IEEE values (in addition to supporting 64-bit long doubles). Still missing are Bessel functions and decimal printf/scanf support.
Add limited long double support for IBM 'double double' form. This is enough to run some simple tests, but doesn't have any significant math functions yet.
Get Power9 code running under qemu with OPAL. This was mostly needed to validate the big-endian and exception code for 128-bit long doubles, but was also used to validate the double double support.
Provide times() and sysconf() implementations in semihosting. You can now build and run the dhrystone benchmark without any further code.
Fix use of TLS variables with stricter alignment requirements in the default linker script and startup code. (thanks to Joakim Nohlgård and Alexander Richardson who found this issue while working on lld support).

Picolibc version 1.7.9

Support all Zephyr SDK targets
Support relocating the toolchain by using GCC_EXEC_PREFIX for sysroot-install when compiler doesn't use sysroot.
Add MIPS, SPARC and ARC support
Deal with RISC-V changes in gcc that don't reliably include zicsr
Support Picolibc as default C library with -Dsystem-libc option. With this, you can use picolibc without any extra compiler options.
Merge current newlib bits to get code that doesn't use struct _reent
Get rid of struct _reent in legacy stdio code
Support 16-bit int targets by fixing a few places assuming sizeof(int) == 4, object sizes not using size_t, wint_t for ucs-4 values
Add MSP430 support
Fix a couple of clang bugs (one on Cortex M0)
Support libc++ by adding non-standard mbstate_t.h
Merge i686 and x86_64 code to allow x86 multilib builds
Merge Xtensa newlib bits
Support Xtensa ESP32 targets
Add Nios II support

Picolibc version 1.7.8

Fix el/ix level 4 code type errors
Fix out-of-source CMake build (thanks Max Behensky)
Improve build.md docs (thanks Kalle Raiskila)
Fix cmake build for various architectures
Initialize lock in fdopen
Remove %M from linker paths in single-arch builds
Shrink tinystdio vfprintf and vfscanf a bit
Use -fno-builtin-malloc -fno-builtin-free (GCC 12 compat)
Use -fno-builtin-copysignl (GCC 12 compat)
Add _zicsr to -march for risc-v picocrt (binutils 2.38 compat)
Add -no-warn-rwx-segments to link spec (binutils 2.38 compat)

Picolibc version 1.7.7

Fix semihost gettimeofday, add a test.
Fix config option documentation. (Thanks to rdiez)
Document how re-entrant locking APIs are used. (Thanks to rdiez)
Fix some 16-bit int issues in tinystdio. (Thanks to Ayke van Laethem)
Make header files a bit more POSIX compliant, installing rpc headers, moving byte swapping macros to arpa/inet.h
Fix some stdio bugs found by Zephyr test suite: snprintf return value on buffer overflow, add ftello/fseeko, fputc return value, %0a formatting, clear EOF status after ungetc/fseek.
Re-do buffered I/O support to handle mixed read/write files correctly. This adds setbuf, setbuffer, setlinebuf, setvbuf.
Add fmemopen and freopen.
Add enough cmake support to allow Zephyr to build picolibc as a module using that, rather than meson.
Merge current newlib bits
Fix %p printf/scanf on ILP64 targets.

Picolibc version 1.7.6

Fix use with C++ applications caused by a syntax error in picolibc.specs
Automatically include '-nostdlib' to options used while evaluating build rules to ensure tests work as expected.
Publish aarch64 inline math functions, ensure that inline fma functions work in installed applications for arm and risc-v.

Picolibc version 1.7.5

Fix build on big-endian systems (thanks to Thomas Daede)
Add m68k support (thanks to Thomas Daede).
Fix build issues with ARM Cortex-a9 target (thanks to Ilia Sergachev).
Fix fwrite(x,0,y,z) in both tinystdio and legacy stdio. tinystdio returned the wrong value and legacy stdio caused a divide-by-zero fault.
Update Ryu code to match upstream (minor fixes)
Fix various NEWLIB and PICOLIBC macros; they were using a single leading underscore instead of two (thanks to Vincent Palatin).
Fix tinystdio error-handling bugs
Merge recent newlib changes (fixed ltdoa in legacy stdio)
Speed improvements for github CI system
Big-endian PowerPC support
Fail builds if most 'run_command' uses fail (thanks to Johan de Claville Christiansen)
Positional parameters in tinystdio. With this, I think tinystdio is feature complete.
Support for multiple build-styles of picolibc (minsize/release) in one binary package. This still requires separate meson runs.
Testing with glibc test code. This uncovered numerous bugs, mostly math errno/exception mistakes, but also a few serious bugs, including a couple of places where the nano-malloc failed to check for out-of-memory. Picolibc now passes all of the glibc math tests except for jn, yn, lgamma and tgamma. The picolibc versions of those functions are too inaccurate. Picolibc also passes most other relevant glibc tests, including stdio, string and stdlib areas.
Tinystdio version of fcvt now has a static buffer large enough to hold the maximum return size.
Tinystdio versions of ecvtbuf and fcvtbuf have been replaced with ecvt_r and fcvt_r equivalents, which take a 'len' parameter to prevent buffer overruns.
Add the GeneratePicolibcCrossFile.sh script which provides a way to isolate picolibc build scripts from the vagaries of meson version shifts (thanks to R. Diez).
Add 'semihost' version of crt0 that calls 'exit' after main returns. The ARM and RISC-V versions of this also include trap handlers for exceptions that print out information and exit when an exception occurs.

Picolibc version 1.7.4

Clean up meson build bits, including use of 'fs module (thanks to Yasushi Shoji).
Speed up github actions by sharing Debian docker image (thanks to Yasushi Shoji).
Reduce use of intermediate static libraries during build
Use standard Meson architecture names everywhere (thanks to Yasushi Shoji).
Support building with -D_FORTIFY_SOURCE enabled.
Clean up 32-bit arm assembly code, eliminating __aeabi wrappers where possible.
Add basename, dirname and fnmatch back.
Fix all old-style (K&R) function definitions.
Enable lots more compiler warning flags.
Remove last uses of alloca in legacy stdio code.
Add tests from musl libc-testsuite. There aren't many tests, but these identified a few bugs.
Add lots more exception and errno tests for the math functions.
Restructure math library to always use the math_err functions to raise exceptions and set errno. This removes the w_*.c wrapper functions and eliminates the ieee names. This centralizes compiler work-arounds to ensure run-time evaluation of expressions intended to raise exceptions. In the process, all of the libm/math files were reformatted with clang-format.
Make tinystdio '%a' compatible with glibc, including supporting rounding and trimming trailing zeros when possible.
Remove floating point exception generation code on targets without floating point exception support. This reduces code size on soft float machines without affecting results.

Picolibc version 1.7.3

Add -Wall -Wextra to default builds. Fixed warnings this raised.
Add htonl and friends (based on __htonl). Thanks to Johan de Claville Christiansen
Set errno in scalbn and scalbnf (patch forwarded to newlib).
Merge newlib recent changes which includes a couple of libm fixes.

Picolibc version 1.7.2

Fix picolibc.ld to split C++ exceptions back apart (thanks to Khalil Estell)
Add vsscanf to tinystdio (required for libstdc++).
Also stick -isystem in C++ compile command to try and get picolibc headers to be used instead of newlib.

Picolibc version 1.7.1

Add __cxa_atexit implementation to 'picoexit' path as required by C++
Fix lack of 'hh' support in integer-only tinystdio printf path.
Fix tinystdio __file flag initialization for C++ apps

Picolibc version 1.7

Merge libc and libm into a single library. Having them split doesn't offer any advantages while requiring that applications add '-lm' to link successfully. Having them merged allows use of libm calls from libc code.
Add hex float format to printf, scanf and strto{d,f,ld}. This is required for C99 support.
Unify strto{d,f,ld} and *scanf floating point parsing code. This ensures that the library is consistent in how floats are parsed.
Make strto{d,f,ld} set errno to ERANGE on overflow/underflow, including when the result is a subnormal number.

Picolibc version 1.6.2

Change restrict keyword in published headers to __restrict to restore compatibility with applications building with --std=c18.
Additional cleanups in time conversion funcs (Thanks to R. Riez)

Picolibc version 1.6.1

Code cleanups for time conversion funcs (Thanks to R. Diez)
Add '-fno-stack-protector' when supported by the C compiler to avoid trouble building with native Ubuntu GCC.
Bug fix for converting denorms with sscanf and strto{d,f,ld}.
Use asm for inline asm code to allow building applications with --std=c18
Fix exit code for semihosting 'abort' call to make it visible to the hosting system.
Add strfromf and strfromd implementations. These are simple wrappers around sscanf, but strfromf handles float conversions without requiring a pass through 'double' or special linker hacks.

Picolibc version 1.6

Bugfix for snprintf(buf, 0) and vsnprintf(buf, 0) to avoid smashing memory
Support building libstdc++ on top of picolibc
Add 'hosted' crt0 variant that calls exit when main returns. This makes testing easier without burdening embedded apps with unused exit processing code.
Add 'minimal' crt0 variant that skips constructors to save space on systems known to not use any.
Fix HW floating point initialization on 32-bit ARM processors to perform 'dsb' and 'isb' instructions to ensure the FPU enabling write is complete before executing any FPU instructions.
Create a new '--picolibc-prefix' GCC command line parameter that sets the base of all picolibc file names.
Add bare-metal i386 and x86_64 initializatiton code (thanks to Mike Haertel). These initalize the processor from power up to running code without requiring any BIOS.
Merge newlib as of late April, 2021
Add 'timegm' function (thanks to R. Diez).
Fix a number of tinystdio bugs: handle fread with size==0, parse 'NAN' and 'INF' in fscanf in a case-insensitive manner, fix negative precision to '*' arguments in printf, fix handling of 'j', 'z' and 't' argument size specifiers (thanks to Sebastian Meyer).
Make the fenv API more consistent and more conformant with the spec. All architectures now fall back to the default code for soft float versions, which avoids having the various exception and rounding modes get defined when not supported.

Picolibc version 1.5.1

Make riscv crt0 '_exit' symbol 'weak' to allow linking without this function.

Picolibc version 1.5

Make picolibc more compatible with C++ compilers.
Add GCC specs file and linker script for building C++ applications with G++ that enable exception handling by linking in call stack information.
A few clang build fixes, including libm exception generation
Nano malloc fixes, especially for 'unusual' arguments
Merge in newlib 4.1.0 code
More libm exception/errno/infinity fixes, mostly in the gamma funcs.
Add tests for all semihost v2.0 functions.
A few RISC-V assembly fixes and new libm code.
Build fixes to reliably replace generic code with architecture-specific implementations.

With a patch which is pending for GCC 11, we'll be able to build C++ applications that use picolibc with exceptions and iostream.

Picolibc version 1.4.7

Fix numerous libm exception and errno bugs. The math functions are all now verified to match the C19 and Posix standards in this area.
Change behavior of 'gamma' function to match glibc which returns lgamma for this function. Applications should not use this function, they should pick either lgamma or tgamma as appropriate.
Fix fma/fmaf on arm and RISC-V so that the machine-specific versions are used when the hardware has support. Also fix the math library to only use fma/fmaf when it is supported by the hardware.
Fix numerous nano-malloc bugs, especially with unusual parameters.
Change nano-malloc to always clear returned memory.
Improve nano-realloc to perform better in various ways, including merging adjacent free blocks and expanding the heap.
Add malloc tests, both a basic functional test and a stress test.
Improve build portability to Windows. Picolibc should now build using mingw.
Use hardware TLS register on ARM when available.
Support clang compiler. Thanks to Denis Feklushkin denis.feklushkin@gmail.com and Joakim Nohlgård joakim@nohlgard.se.
Avoid implicit float/double conversions. Check this by having clang builds use -Wdouble-promotion -Werror=double-promotion flags
Have portable code check for machine-specific overrides by matching filenames. This avoids building libraries with duplicate symbols and retains compatibility with newlib (which uses a different mechanism for this effect).
Patches to support building with CompCert, a formally verified compiler. Thanks to Sebastian Meyer meyer@absint.com.

Documentation

There's a bunch of documentation written in common mark, which this ikiwiki instance doesn't handle, so for now here are links to the pages rendered at github:

Debian Packages

Pre-built Debian packages for Debian unstable are available upstream

There are four Picolibc packages in the archive:

picolibc-riscv64-unknown-elf. RISC-V embedded libraries, built using gcc-riscv64-unknown-elf.
picolibc-arm-none-eabi. ARM embedded libraries, built using gcc-arm-none-eabi
picolibc-xtensa-lx106-elf. ESP8266 Xtensa lx106 embedded libraries, built using gcc-xtensa-lx106-elf.
picolibc-aarch64-linux-gnu. 64-bit ARM embedded libraries, built using gcc-aarch64-linux-gnu (that's the only compiler for this architecture).

Source Code

You can find Picolibc in either my git repository or github

As Picolibc is a combination of code from both Newlib and AVR-libc, the licensing of the source code is fairly complicated, but the general result is similar to the BSD 3-clause or MIT license.

Contributing

Changes to picolibc code, or to the picolibc build environment, can be submitted in the github project above, or to the picolibc mailing list. Changes to code shared with newlib should also be submitted to newlib.