fmt: Printf flag '0' is not ignored for strings #56486

yejianfengblue · 2022-10-30T01:39:25Z

What version of Go are you using (`go version`)?

$ go version
go version go1.19.2 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (`go env`)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/k/.cache/go-build"
GOENV="/home/k/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/k/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/k/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19.2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build536070510=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://go.dev/play/p/thI6FNjWrY_G

package main

import "fmt"

func main() {
    fmt.Printf("%05s", "abc")
    // print 00abc
}

What did you expect to see?

abc

According to the doc, the flag '0' is ignored for strings.

'0' pad with leading zeros rather than spaces; for numbers, this moves the padding after the sign; ignored for strings, byte slices and byte arrays

What did you see instead?

00abc

Background

I originally asked this question in Stackoverflow. Pak Uula helps check the source code and this issue seems to be a bug.

The source code resets the zero flag only for - (minus) flag. It is not modified neither for strings nor for any other type.
And the function that outputs a string doesn't reset the zero flag either.

The text was updated successfully, but these errors were encountered:

seankhliao · 2022-10-30T11:57:14Z

seems to have been this way for a long time

cc @robpike

robpike · 2023-05-13T09:11:56Z

It's sometimes ignored. See https://go.dev/play/p/_-73vYP6W4p

It's a bug that it's not always ignored.

gopherbot · 2024-01-14T04:37:57Z

Change https://go.dev/cl/555776 mentions this issue: fmt: don't pad strings with zeros

robpike · 2024-01-23T21:26:15Z

This is not complete yet. Another change to come: must also protect quoted strings and characters, all of which are lovely demonstrated in the golden test as badly broken. Good work, guy.

gopherbot · 2024-01-23T22:56:32Z

Change https://go.dev/cl/558055 mentions this issue: fmt: do not zero-pad strings and runes

dmitshur · 2024-01-29T15:35:06Z

@robpike Note that the release-blocker label applies to the milestone an issue is in, and this issue was in Unplanned, so it had no effect. I've moved it to the Go1.23 milestone since that seems to be the intention.

gopherbot · 2024-01-29T17:17:42Z

Change https://go.dev/cl/559197 mentions this issue: fmt: update docs for %03s

gopherbot · 2024-01-29T17:17:43Z

Change https://go.dev/cl/559196 mentions this issue: fmt: revert "don't pad strings with zeros"

rsc · 2024-01-29T17:17:56Z

CL 555776 broke a bunch of code. Documenting the long-time behavior instead seems like it makes more sense than causing breakage, so I sent CL 559196 (revert) and CL 559197 (document).

This reverts CL 555776 (commit 704401f). Scores of tests break inside Google, and there was a test for the old behavior, so clearly we thought it was correct at one point. An example of code that broke inside Google is: func (pn ProjectNumber) PaddedHexString() string { return fmt.Sprintf("%016s", strconv.FormatInt(int64(pn), 16)) } Here is another example: // IPv4toISO create ISO address base on a given IPv4 address. func IPv4toISO(v4 string) (string, error) { if net.ParseIP(v4).To4() == nil { return "", fmt.Errorf("invalid IPv4 address") } s := strings.Split(v4, ".") var ss string for _, n := range s { ss = ss + fmt.Sprintf("%03s", n) } if len(ss) != 12 { return "", fmt.Errorf("invalid IPv4 address") } return fmt.Sprint("49.0001." + ss[0:4] + "." + ss[4:8] + "." + ss[8:12] + ".00"), nil } This is doing the weird but apparently standard conversion from IPv4 to ISO ISIS Area 1 (see for example [1]). Here is an example from github.com/netbirdio/netbird: func generateNewToken() (string, string, error) { secret, err := b.Random(PATSecretLength) if err != nil { return "", "", err } checksum := crc32.ChecksumIEEE([]byte(secret)) encodedChecksum := base62.Encode(checksum) paddedChecksum := fmt.Sprintf("%06s", encodedChecksum) plainToken := PATPrefix + secret + paddedChecksum hashedToken := sha256.Sum256([]byte(plainToken)) encodedHashedToken := b64.StdEncoding.EncodeToString(hashedToken[:]) return encodedHashedToken, plainToken, nil } base62.Encode returns a string no leading zeros; the %06s adds leading zeros. Are there other ways to write these examples? Yes. Has all this code worked until now? Also yes. The change to this behavior observed that right padding doesn't add zeros, only left padding, but that makes sense: in numbers without decimal points, zeros on the left preserve the value while zeros on the right change it. Since we agree that this case is probably not important either way, preserve the long-time behavior of %0s. Will document it in a followup CL: this is a clean revert. Reopen #56486. [1] https://community.cisco.com/t5/routing/isis-net-address-configuration/m-p/1338984/highlight/true#M127827 Change-Id: Ie7dd35227f46933ccc9bfa1eac5fa8608f6d1918 Reviewed-on: https://go-review.googlesource.com/c/go/+/559196 Reviewed-by: Cherry Mui <[email protected]> Reviewed-by: Rob Pike <[email protected]> LUCI-TryBot-Result: Go LUCI <[email protected]>

It's what the documentation says, and oddly it already behaves correctly for right padding, not left. (We never pad with zeros on the right.) Just don't do it. Fixes golang#56486 Change-Id: I2465edea93c69084e33bee0d945d5a1b85e6cd14 Reviewed-on: https://go-review.googlesource.com/c/go/+/555776 Reviewed-by: Russ Cox <[email protected]> TryBot-Result: Gopher Robot <[email protected]> Run-TryBot: Rob Pike <[email protected]> Reviewed-by: Cherry Mui <[email protected]>

This reverts CL 555776 (commit 704401f). Scores of tests break inside Google, and there was a test for the old behavior, so clearly we thought it was correct at one point. An example of code that broke inside Google is: func (pn ProjectNumber) PaddedHexString() string { return fmt.Sprintf("%016s", strconv.FormatInt(int64(pn), 16)) } Here is another example: // IPv4toISO create ISO address base on a given IPv4 address. func IPv4toISO(v4 string) (string, error) { if net.ParseIP(v4).To4() == nil { return "", fmt.Errorf("invalid IPv4 address") } s := strings.Split(v4, ".") var ss string for _, n := range s { ss = ss + fmt.Sprintf("%03s", n) } if len(ss) != 12 { return "", fmt.Errorf("invalid IPv4 address") } return fmt.Sprint("49.0001." + ss[0:4] + "." + ss[4:8] + "." + ss[8:12] + ".00"), nil } This is doing the weird but apparently standard conversion from IPv4 to ISO ISIS Area 1 (see for example [1]). Here is an example from github.com/netbirdio/netbird: func generateNewToken() (string, string, error) { secret, err := b.Random(PATSecretLength) if err != nil { return "", "", err } checksum := crc32.ChecksumIEEE([]byte(secret)) encodedChecksum := base62.Encode(checksum) paddedChecksum := fmt.Sprintf("%06s", encodedChecksum) plainToken := PATPrefix + secret + paddedChecksum hashedToken := sha256.Sum256([]byte(plainToken)) encodedHashedToken := b64.StdEncoding.EncodeToString(hashedToken[:]) return encodedHashedToken, plainToken, nil } base62.Encode returns a string no leading zeros; the %06s adds leading zeros. Are there other ways to write these examples? Yes. Has all this code worked until now? Also yes. The change to this behavior observed that right padding doesn't add zeros, only left padding, but that makes sense: in numbers without decimal points, zeros on the left preserve the value while zeros on the right change it. Since we agree that this case is probably not important either way, preserve the long-time behavior of %0s. Will document it in a followup CL: this is a clean revert. Reopen golang#56486. [1] https://community.cisco.com/t5/routing/isis-net-address-configuration/m-p/1338984/highlight/true#M127827 Change-Id: Ie7dd35227f46933ccc9bfa1eac5fa8608f6d1918 Reviewed-on: https://go-review.googlesource.com/c/go/+/559196 Reviewed-by: Cherry Mui <[email protected]> Reviewed-by: Rob Pike <[email protected]> LUCI-TryBot-Result: Go LUCI <[email protected]>

%03s zero-pads a string with spaces; always has and now always will. Fixes golang#56486. Change-Id: Ia336581ae7db1c3456699e69e14a3071f50c9f2a Reviewed-on: https://go-review.googlesource.com/c/go/+/559197 Reviewed-by: Rob Pike <[email protected]> Reviewed-by: Michael Knyszek <[email protected]> LUCI-TryBot-Result: Go LUCI <[email protected]>

seankhliao changed the title ~~fmt: Printf() flag '0' is not ignored for strings~~ fmt: Printf flag '0' is not ignored for strings Oct 30, 2022

seankhliao added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Oct 30, 2022

robpike self-assigned this Oct 30, 2022

seankhliao added this to the Unplanned milestone Nov 19, 2022

robpike added release-blocker and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Jan 17, 2024

gopherbot closed this as completed in 704401f Jan 23, 2024

robpike reopened this Jan 23, 2024

dmitshur modified the milestones: Unplanned, Go1.23 Jan 29, 2024

dmitshur added the NeedsFix The path to resolution is known, but the work has not been done. label Jan 29, 2024

gopherbot closed this as completed in b32ec6c Jan 31, 2024

gabyhelp mentioned this issue Jul 29, 2024

fmt.Printf("%02x", ([]byte)(nil)) outputs "00" #68630

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fmt: Printf flag '0' is not ignored for strings #56486

fmt: Printf flag '0' is not ignored for strings #56486

yejianfengblue commented Oct 30, 2022

seankhliao commented Oct 30, 2022

robpike commented May 13, 2023

gopherbot commented Jan 14, 2024

robpike commented Jan 23, 2024

gopherbot commented Jan 23, 2024

dmitshur commented Jan 29, 2024 •

edited

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gopherbot commented Jan 29, 2024

gopherbot commented Jan 29, 2024

rsc commented Jan 29, 2024

fmt: Printf flag '0' is not ignored for strings #56486

fmt: Printf flag '0' is not ignored for strings #56486

Comments

yejianfengblue commented Oct 30, 2022

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

What did you expect to see?

What did you see instead?

Background

seankhliao commented Oct 30, 2022

robpike commented May 13, 2023

gopherbot commented Jan 14, 2024

robpike commented Jan 23, 2024

gopherbot commented Jan 23, 2024

dmitshur commented Jan 29, 2024 • edited Loading

gopherbot commented Jan 29, 2024

gopherbot commented Jan 29, 2024

rsc commented Jan 29, 2024

What version of Go are you using (`go version`)?

What operating system and processor architecture are you using (`go env`)?

dmitshur commented Jan 29, 2024 •

edited

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