printf: floats and doubles do not round correctly

[64]

See e.g https://github.com/managarm/frigg/blob/51d873bc18d4ca82220a0d59c48b40cad6b87495/include/frg/formatting.hpp#L310-L311 and

mlibc/tests/ansi/sprintf.c

Lines 71 to 81 in bbac41b

	// TODO: We don't yet round properly
	// sprintf(buf, "%5.2f", 1.2);
	// assert(!strcmp(buf, " 1.20"));
	// sprintf(buf, "%5.2f", 1.23);
	// assert(!strcmp(buf, " 1.23"));
	// sprintf(buf, "%5.2f", 1.234);
	// assert(!strcmp(buf, " 1.23"));
	// sprintf(buf, "%5.2f", 12.345);
	// assert(!strcmp(buf, "12.35"));
	// sprintf(buf, "%-5.2f", 1.2);
	// assert(!strcmp(buf, "1.20 "));

To fix this, we could use a C++ float-to-string implementation like dragonbox.

[jk-jeon]

The author of Dragonbox here.

Dragonbox does a different thing from what printf does and I don't think it can help implementing printf. There is a known algorithm for printf-like behavior called Ryu-printf (which is a different algorithm from Ryu, but by the same author, and whose implementation is included in the same repo for Ryu's reference implementation), but it is relying on a gigantic table of size over 100kb. I have an implementation of Ryu-printf using 39kb, but it is still too huge for mlibc, I guess. I also have an algorithm idea which might require a smaller table, but haven't worked it out.

But I believe there are printf-like float-to-string conversion implementations out there which don't require ridiculously big tables, while slower than Ryu-printf. A typical such an implementation probably relies on some form of big integer calculation, so probably with some usage of heap memory. Not sure if that's something mlibc cares about, though.

[64]

Hi there, thanks for the reply.

A 40k table isn't too bad, especially in a DSO. Do you have the code for that available somewhere?

[jk-jeon]

It's here. I'm pretty confident about the correctness of the implementation, but I don't quite recommend using it in a serious project like mlibc, because not like the dragonbox repo, this project is far from being completed, not maintained, and not under active development at this moment. I hope I will work on it at some point, but currently I'm too busy living and I don't expect that moment will come soon.