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runtime/cgo: bitfields not supported #59982

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gucio321 opened this issue May 4, 2023 · 3 comments
Closed

runtime/cgo: bitfields not supported #59982

gucio321 opened this issue May 4, 2023 · 3 comments
Labels
compiler/runtime Issues related to the Go compiler and/or runtime.

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@gucio321
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gucio321 commented May 4, 2023

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

$ go version
go version go1.20.2 linux/amd64

Does this issue reproduce with the latest release?

yep

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

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/me/.cache/go-build"
GOENV="/home/me/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/me/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/me/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2059306280=/tmp/go-build -gno-record-gcc-switches"

What did you do?

consider the following code:

package main

/*
typedef struct Foo Foo;
struct Foo {
	int Bar;
};
*/
import "C"
import "fmt"

func main() {
	f := new(C.Foo)
	f.Bar = C.int(5)
	fmt.Println(f.Bar)
}

and now edit C code in the following way:

typedef struct Foo Foo;
struct Foo {
        int Bar : 1;
};

What did you expect to see?

everything should work

What did you see instead?

# command-line-arguments
./main.go:14:4: f.Bar undefined (type *_Ctype_struct_Foo has no field or method Bar)
./main.go:15:16: f.Bar undefined (type *_Ctype_struct_Foo has no field or method Bar)
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label May 4, 2023
@gucio321 gucio321 mentioned this issue May 4, 2023
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@ianlancetaylor
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Unfortunately, this simply isn't possible, as Go does not support bitfields. There is no way to represent that C struct in Go.

What you can do is write tiny functions in the C prologue that get and set the value of the field

Closing because there is nothing we can to fix this.

@ianlancetaylor ianlancetaylor closed this as not planned Won't fix, can't repro, duplicate, stale May 4, 2023
@gucio321
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gucio321 commented May 4, 2023

@ianlancetaylor well I don't insist on implementing bitfields in go, but these defined in C could be exported to GO anyway, couldn't they? If something exceeds bitfield size, C will throw panic, right?

@ianlancetaylor
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How could they be exported to Go? The Go code that refers to these fields is ordinary Go code. The Go compiler is not a C compiler. The Go compiler does not support bitfields.

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@ianlancetaylor @gopherbot @gucio321