fmt: differing output of printf %#010x with int and string types

[Sebi2020]

What did you do?

I wanted print a padded (zeros) and prepended (0x) string character as a hex number.

Link to example on play.golang.org

What did you expect to see?

The same result for a string character and an integer.
The string printed as an hexadecimal number should be prepended with '0x'.

What did you see instead?

> A hex Number (a int) with padding:    0x0000000030
> A hex Number (a string) with padding: 0000000x30
> 
> Program exited.

System details

go version go1.11.2 linux/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/sebi/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/sebi/go/"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.11.2 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.11.2
uname -sr: Linux 4.15.0-38-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.5 LTS
Release:	16.04
Codename:	xenial
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.23-0ubuntu10) stable release version 2.23, by Roland McGrath et al.
gdb --version: GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1

[martisch]

This seems to be consistent with the documentation (https://godoc.org/fmt):

• int: # for 0x for hex (%#x) and 0 for pad with leading zeros rather than spaces (padding after the sign).
• string: %x base 16, lower-case, two characters per byte then # for 0x for hex (%#x) for each byte and pad the string with leading zeros rather than spaces.

The difference is that the verb %x applies to each byte in strings and # is an alternate format for the verb so also per byte but the padding is not described to be per byte but applies to the output as a whole.

An improvement for the documentation that could be made is for '0':
instead of 'for numbers, this moves the padding after the sign' it could say 'for numbers, this moves the padding after the sign and leading 0x'

[Sebi2020]

But it does not add an '0x' for each byte:
Example on Playground

> 00000x3031

As far as I understood you it should print something like:

> 000x300x31

[martisch]

you are right it i was confusing this with ' ' where it applies per byte and is mentioned explicitly "% x" in documentation.

Referring back to the original issue of 0 padding I think its still in line with documentation that padding of 0 applies to the whole string after it has been formatted? Could maybe made explicit.

To format the string like the number in the first example you could do:
fmt.Printf("0x%08x\n", "0");

If its not an obvious bug or contradicting documentation its likely not to change how fmt formats this as its established behavior by now that other programs will rely on.

[Sebi2020]

Referring back to the original issue of 0 padding I think its still in line with documentation that padding of 0 > applies to the whole string after it has been formatted? Could maybe made explicit.

I cannot find this in the documentation for printf.

This is what I've found:

 ' '	(space) leave a space for elided sign in numbers (% d);
	put spaces between bytes printing strings or slices in hex (% x, % X)
0	pad with leading zeros rather than spaces;
	for numbers, this moves the padding after the sign

It does only say what happens if you pad it with spaces, so I agree it should be more explicitly stated what happens if you use zeros instead of spaces.

[bcmills]

CC @robpike

[robpike]

Padding applies to the whole result.