runtime: empty struct type pointer comparison varies

[jsageryd]

Go version

go version go1.24.2 darwin/arm64

Output of go env in your module/workspace:

AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/j/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/j/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/0j/7fvjv4jn4qq5_44fpjn9cjzr0000gn/T/go-build138579852=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/Users/j/go/pkg/mod'
GOOS='darwin'
GOPATH='/Users/j/go'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.24.2/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/j/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.24.2/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.2'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Two instances of pointers to an empty struct compare differently based on, as far as I can tell, whether they have been read before.

package main

import (
	"fmt"
)

type Foo struct{}

func main() {
	var a, b Foo

	pA := &a
	pB := &b

	// fmt.Printf("Pointer values: %p %p\n", pA, pB)

	fmt.Printf("pA == pB? %t\n", pA == pB)
}

https://go.dev/play/p/dVDP0x-H5WH

vs.

package main

import (
	"fmt"
)

type Foo struct{}

func main() {
	var a, b Foo

	pA := &a
	pB := &b

	fmt.Printf("Pointer values: %p %p\n", pA, pB)

	fmt.Printf("pA == pB? %t\n", pA == pB)
}

https://go.dev/play/p/T9ZMpBqgad8

What did you see happen?

In the first example the struct pointers compare false. In the second example they compare true.

What did you expect to see?

I would have expected the comparison to have the same result regardless of the print statement.

[jsageryd]

The spec says:

Pointers to distinct zero-size variables may or may not be equal.

So I suppose this could be dismissed as not a bug and working according to spec, but it is still surprising to me that the result of the comparison changes.

[gabyhelp]

Related Issues

• fmt.Printf("%p", &a) changes result of pointer of empty struct comparison #68701 (closed)
• Empty struct comparison behavior is inconsistent #43755 (closed)
• cmd/compile: comparison of pointers to zero sized values is inconsistent #52772 (closed)
• Pointers to empty structs behave inconsistently #47950 (closed)
• spec: Empty struct address comparison returns false even if addresses are equal #23440 (closed)
• gccgo: struct comparison with blank field could be wrong #55142
• new(struct{})==new(struct{}) dissimilarity #36384 (closed)
• cmd/compile: counter-intuitive comparison of zero-sized pointers wrapped in interfaces #65878 (closed)
• cmd/compile: empty struct addresses are not equal #23370 (closed)

Related Discussions

• Equality of interface of an empty struct - why?

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[jsageryd]

Thank you bot; the responses in the linked issues answer my question.