Context, we have a 13 character fixed width field whose last two characters need to be "00". We can easily work around this, but it was unclear if this behaviour should be expected or not.

Padding a string with zeros doesn't seem right, but the documentation doesn't say it shouldn't work.

Just for reference, as the fmt package was written to broadly follow C's stdio, I tried the equivalent C program, and got these warnings from clang:

x.c:6:11: warning: flag '0' results in undefined behavior with 's' conversion specifier [-Wformat]
printf("%013.13s\n", s);
~^~~~~~~
x.c:8:12: warning: flag '0' results in undefined behavior with 's' conversion specifier [-Wformat]
printf("%-013.13s\n", s);
~~^~~~~~~
x.c:8:12: warning: flag '0' is ignored when flag '-' is present [-Wformat]
printf("%-013.13s\n", s);
~~^~~~~~~
x.c:9:11: warning: flag '0' results in undefined behavior with 's' conversion specifier [-Wformat]
printf("%0-13.13s\n", s);
~^~~~~~~~
x.c:9:11: warning: flag '0' is ignored when flag '-' is present [-Wformat]
printf("%0-13.13s\n", s);

Meanwhile its output is:

123456789
0000123456789
1234567890000
1234567890000

which implies the warnings are largely irrelevant. Yay warnings.

So although you're on thin ice here, I'd be willing to accept this as a bug, as long as fixing it doesn't break too many things, which seems plausible.

I didn't see this existing test which is explicit about rejecting using zeros as right-padding.

https://github.com/golang/go/blob/go1.17.7/src/fmt/fmt_test.go#L990-L992

It looks like this was changed as part of a performance improvement, but I'm unsure why it was changed back then.

abcad1e

It is very odd to right-pad a string with zeros. It's easy to argue it's wrong to expect the library to do it for you.

So I'm back on the 'not a bug' side of the fence, perhaps with a line in the documentation about it.

Agreed. There's been a test rejecting this behavior since 2016, so I'm going to file a change updating the docs. Changing this would break people's code.

Change https://go.dev/cl/389434 mentions this issue: fmt: clarify right-padded strings use spaces

/cc @martisch

Change https://go.dev/cl/390674 mentions this issue: fmt: use tabs for indentation