Picolibc Updates (October 2019)

Picolibc is in pretty good shape, but I've been working on a few updates which I thought I'd share this evening.

Dummy stdio thunk

Tiny stdio in picolibc uses a global variable, __iob, to hold pointers to FILE structs for stdin, stdout, and stderr. For this to point at actual usable functions, applications normally need to create and initialize this themselves.

If all you want to do is make sure the tool chain can compile and link a simple program (as is often required for build configuration tools like autotools), then having a simple 'hello world' program actually build successfully can be really useful.

I added the 'dummyiob.c' module to picolibc which has an iob variable initialized with suitable functions. If your application doesn't define it's own iob, you'll get this one instead.

$ cat hello.c
#include <stdio.h>

int main(void)
{
    printf("hello, world\n");
}
$ riscv64-unknown-elf-gcc -specs=picolibc.specs hello.c
$ riscv64-unknown-elf-size a.out
   text    data     bss     dec     hex filename
    496      32       0     528     210 a.out

POSIX thunks

When building picolibc on Linux for testing, it's useful to be able to use glibc syscalls for input and output. If you configure picolibc with -Dposix-io=true, then tinystdio will use POSIX functions for reading and writing, and also offer fopen and fdopen functions as well.

To make calling glibc syscall APIs work, I had to kludge the stat structure and fcntl bits. I'm not really happy about this, but it's really only for testing picolibc on a Linux host, so I'm not going to worry too much about it.

Remove 'mathfp' code

The newlib configuration docs aren't exactly clear about what the newlib/libm/mathfp directory contains, but if you look at newlib faq entry 10 it turns out this code was essentially a failed experiment in doing a 'more efficient' math library.

I think it's better to leave 'mathfp' in git history and not have it confusing us in the source repository, so I've removed it along with the -Dhw-fp option.

Other contributions

I've gotten quite a few patches from other people now, which is probably the most satisfying feedback of all.

  • powerpc build patches
  • stdio fixes
  • cleanup licensing, removing stale autotools bits
  • header file cleanups from newlib which got missed

Semihosting support

RISC-V and ARM both define a 'semihosting' API, which provides APIs to access the host system from within an embedded application. This is useful in a number of environments:

  • GDB through OpenOCD and JTAG to an embedded device
  • Qemu running bare-metal applications
  • Virtual machines running anything up to and including Linux

I really want to do continuous integration testing for picolibc on as many target architectures as possible, but it's impractical to try and run that on actual embedded hardware. Qemu seems like the right plan, but I need a simple mechanism to get error messages and exit status values out from the application.

Semihosting offers all of the necessary functionality to run test without requiring an emulated serial port in Qemu and a serial port driver in the application.

For now, that's all the functionality I've added; console I/O (via a definition of _iob) and exit(2). If there's interest in adding more semihosting API calls, including file I/O, let me know.

I wanted to make semihosting optional, so that applications wouldn't get surprising results when linking with picolibc. This meant placing the code in a separate library, libsemihost. To get this linked correctly, I had to do a bit of magic in the picolibc.specs file. This means that selecting semihost mode is now done with a gcc option, -semihost', instead of just adding -lsemihost to the linker line.

Semihosting support for RISC-V is already upstream in OpenOCD. I spent a couple of hours last night adapting the ARM semihosting support in Qemu for RISC-V and have pushed that to my riscv-semihost branch in my qemu project on github

A real semi-hosted 'hello world'

I've been trying to make using picolibc as easy as possible. Learning how to build embedded applications is hard, and reducing some of the weird tool chain fussing might make it easier. These pieces work together to simplify things:

  • Built-in crt0.o
  • picolibc.specs
  • picolibc.ld
  • semihost mode

Here's a sample hello-world.c:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    printf("hello, world\n");
    exit(0);
}

On Linux, compiling is easy:

$ cc hello-world.c 
$ ./a.out 
hello, world
$

Here's how close we are to that with picolibc:

$ riscv64-unknown-elf-gcc -march=rv32imac -mabi=ilp32 --specs=picolibc.specs -semihost -Wl,-Tqemu-riscv.ld hello-world.c
$ qemu-system-riscv32 -semihosting -machine spike -cpu rv32imacu-nommu -kernel a.out -nographic
hello, world
$

This requires a pile of options to specify the machine that qemu emulates, both when compiling the program and again when running it. It also requires one extra file to define the memory layout of the target processor, 'qemu-riscv.ld':

__flash = 0x80000000;
__flash_size = 0x00080000;
__ram = 0x80080000;
__ram_size = 0x40000;
__stack_size = 1k;

These are all magic numbers that come from the definition of the 'spike' machine in qemu, which defines 16MB of RAM starting at 0x80000000 that I split into a chunk for read-only data and another chunk for read-write data. I found that definition by looking in the source; presumably there are easier ways?

Larger Examples

I've also got snek running on qemu for both arm and riscv processors; that exercises a lot more of the library. Beyond this, I'm working on freedom-metal and freedom-e-sdk support for picolibc and hope to improve the experience of building embedded RISC-V applications.

Future Plans

I want to get qemu-based testing working on both RISC-V and ARM targets. Once that's running, I want to see the number of test failures reduced to a more reasonable level and then I can feel comfortable releasing version 1.1. Help on these tasks would be greatly appreciated.