Bare metal printf – C standard library without OS

I was feeling a bit like the Petunia and thought "Oh no, not again." :-) One of the annoyances of embedded programming can be having the wheel re-invented a zillion times. I was pleased to see that the author was just describing good software architecture that creates portable code on top of an environment specific library.

For doing 'bare metal' embedded work in C you need the crt0 which is the weirdly named C startup code that satisfies the assumption the C compiler made when it compiled your code. And a set of primitives to do what the i/o drivers of an operating system would have been doing for you. And voila, your C program runs on 'bare metal.'

Another good topic associated with this is setting up hooks to make STDIN and STDOUT work for your particular setup, so that when you type printf() it just automagically works.

This will also then introduce you to the concept of a basic input/output system or BIOS which exports those primitives. Then you can take that code in flash/eprom and load a binary compilation into memory and start it and now you've got a monitor or a primitive one application at a time OS like CP/M or DOS.

Its a fun road for students who really want to understand computer systems to go down.

honestly love reading about this stuff - always makes me realize how much gets glossed over in school. you think modern cpus and all the abstraction layers help or just make things messier for folks trying to learn the real basics?

Newlib is huge and complex (even including old K&R syntax) and adapting the build process to a new system is not trivial. I spent a lot of time with it when I re-targeted chibicc and cparser to EiGen, and finally switched to PDCLib for libc and a part of uClibc for libm; see https://github.com/rochus-keller/EiGen/tree/master/ecc/lib. The result is platform independent besides esentially one file.

220k just to include studio? That's insane. I have 12k and still do IO. Just without the overblown stdio and sbrk, uart_puts is enough. And only in DEBUG mode.

In school we were taught that the OS does the printf. I think the professors were just trying to generalize to not go on tangents. But, once I learned that no embedded libc variants had printf just no output path, it got a lot easier to figure out how to get it working. I wish I knew about SWO and the magic of semihosting back then. I don't think those would be hard to explain and interestingly it's one of the few things students asked about that in the field I'm also asked how to do by coworkers (the setting up _write).

Has anybody played with newlib, but grown the complexity as the system came together?

It seems like one thing to get a bare-bones printf() working to get you started on a bit of hardware, but as the complexity of the system grows you might want to move on from (say) pushing characters out of a serial interface onto pushing them onto a bitmapped display.

Does newlib allow you to put different hooks in there as the complexity of the system increases?

  char buffer[100];
  printf("Type something: ");
  scanf("%s", buffer);

I always felt with these kinds of things you strip out `stdio.h` and your new API/ABI/blackbox becomes `syscall` for `write()`, etc.

I am coding RISC-V assembly (which I run on x86_64 with a mini-interpreter) but I am careful to avoid the usage of the pseudo-instructions and the registers aliases (no compressed instruction ofc). I have a little tool to generate constant loading code, one-liner (semi-colon separated instructions).

And as a pre-processor I use a simple C preprocessor (I don't want to tie the code to the pre-processor of a specific assembler): I did that for x86_64 assembly, and I could assemble with gas, nasm and fasmng(fasm2) transparently.

I was very confused by the title, expected someone writing their own printf — i.e. the part that parses the format string, grabs varargs, converts numbers, lines up strings, etc.

I'd have called it "Bare metal puts()" or "Bare metal write()" or something along those lines instead.

(FWIW, FreeBSD's printf() is quite easy to pluck out of its surrounding libc infrastructure and adapt/customize.)

[flagged]