printf: %a not supported (but partially implemented)

[jfinkels]

$ printf "%a\n" 255
0xf.fp+4
$ ./target/debug/coreutils printf "%a\n" 255
printf: %a: invalid conversion specification

Here is the description of the floating-point conversions: https://www.gnu.org/software/libc/manual/html_node/Floating_002dPoint-Conversions.html

This seems to be called CninetyNineHexFloatf in the code: https://github.com/uutils/coreutils/blob/ca705e5ad2bc6b0b0c5dde0d0c1df38bcb0f2148/src/uu/printf/src/tokenize/num_format/formatters/cninetyninehexfloatf.rs#L11 but it seems to be only partially implemented and not exposed to the user: https://github.com/uutils/coreutils/blob/ca705e5ad2bc6b0b0c5dde0d0c1df38bcb0f2148/src/uu/printf/src/tokenize/sub.rs#L163

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[tertsdiepraam]

There's been progress on this in https://github.com/uutils/coreutils/pull/5128, but it's not quite correct yet. The current output of you example is

0x1.fe00000000000p+7

Looks like my implementation isn't quite right.