Mon, 23 Sep 2019 23:18:12 -0700
tags/debian, tags/picolibc, tags/rockets

Picolibc Version 1.0 Released

I wrote a couple of years ago about the troubles I had finding a good libc for embedded systems, and for the last year or so I've been using something I called 'newlib-nano', which was newlib with the stdio from avrlibc bolted on. That library has worked pretty well, and required very little work to ship.

Now that I'm doing RISC-V stuff full-time, and am currently working to improve the development environment on deeply embedded devices, I decided to take another look at libc and see if a bit more work on newlib-nano would make it a good choice for wider usage.

One of the first changes was to switch away from the very confusing "newlib-nano" name. I picked "picolibc" as that seems reasonably distinct from other projects in the space and and doesn't use 'new' or 'nano' in the name.

Major Changes

Let's start off with the big things I've changed from newlib:

1. Replaced stdio. In place of the large and memory-intensive stdio stack found in newlib, picolibc's stdio is derived from avrlibc's code. The ATmel-specific assembly code has been replaced with C, and the printf code has seen significant rework to improve standards conformance. This work was originally done for newlib-nano, but it's a lot cleaner looking in picolibc.

2. Switched from 'struct _reent' to TLS variables for per-thread values. This greatly simplifies the library and reduces memory usage for all applications -- per-thread data from unused portions of the library will not get allocated for any thread. On RISC-V, this also generates smaller and faster code. This also eliminates an extra level of function call for many code paths.

3. Switched to the 'meson' build system. This makes building the library much faster and also improves the maintainability of the build system as it eliminates a maze of twisty autotools configure scripts.

4. Updated the math test suite to use glibc as a reference instead of some ancient Sun machine.

5. Manually verified the test results to see how the library is doing; getting automated testing working will take a lot more effort as many (many) tests still have invalid 'correct' values resulting in thousands of failure.

6. Remove unused code with non-BSD licenses. There's still a pile of unused code hanging around, but all non-BSD licensed bits have been removed to make the licensing situation clear. Picolibc is BSD licensed.

Picocrt

Starting your embedded application requires initializing RAM as appropriate and calling initializers/constructors before invoking main(). Picocrt is designed to do that part for you.

Building Simplified

Using newlib-nano meant specifying the include and library paths very carefully in your build environment, and then creating a full custom linker script. With Picolibc, things are much easier:

Compile with -specs=picolibc.specs. That and the specification of the target processor are enough to configure include and library paths. The Debian package installs this in the gcc directory so you don't need to provide a full path to the file.

Link with picolibc.ld (which is used by default with picolibc.specs). This will set up memory regions and include Picocrt to initialize memory before your application runs.

Debian Packages

I've uploaded Debian packages for this version; they'll get stuck in the new queue for a while, but should eventually make there way into the repository. I'll plan on removing newlib-nano at some point in the future as I don't plan on maintaining both.

More information

You can find the source code on both my own server and over on github:

keithp.com

github.com

You'll find some docs and other information linked off the README file