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cmd/cgo: const initializer C: extern const is not a constant #51044

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zyxkad opened this issue Feb 7, 2022 · 9 comments
Closed

cmd/cgo: const initializer C: extern const is not a constant #51044

zyxkad opened this issue Feb 7, 2022 · 9 comments
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FrozenDueToAge help wanted NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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@zyxkad
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zyxkad commented Feb 7, 2022

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

$ go version
go version go1.17.2 darwin/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
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOVERSION="go1.17.2"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2 -std=c17"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2 -std=c++17"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/fh/sxp5v53x66j5_p6_9rxtx6b80000gp/T/go-build3209812224=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

consttest.go

package testc

// #include "consttest.h"
import "C"

const (
  CONST_VAR int = (int)(C.CONST_VAR)
)

consttest.h

#ifndef __CGO_CONSTTEST_H__
#define __CGO_CONSTTEST_H__

#ifdef __cplusplus
extern "C"{
#endif // __cplusplus

extern const int CONST_VAR;

#ifdef __cplusplus
}
#endif // __cplusplus

#endif // __CGO_CONSTTEST_H__

consttest.cpp

#include "consttest.h"
const int CONST_VAR = 1;

consttest_test.go

package testc_test

import "testing"
import (
  . "localhost/testc"
)

func TestConstVar(t *testing.T){
  t.Log("CONST_VAR:", CONST_VAR)
}

What did you expect to see?

=== RUN   TestConstVar
    consttest_test.go:10: CONST_VAR: 1
--- PASS: TestConstVar (0.00s)
PASS
ok  	localhost/testc

What did you see instead?

# localhost/testc
./consttest.go:8:3: const initializer int(*_Cvar_CONST_VAR) is not a constant
FAIL	localhost/testc [build failed]
FAIL
@zyxkad
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zyxkad commented Feb 7, 2022

If I change extern const int CONST_VAR; to const int CONST_VAR = 1; it will work.

But in fact, I was mapping c++ enum to golang, I can't use namespace in the header file

@beoran
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beoran commented Feb 7, 2022

Afaik, CGO does not support C++. You will have to make a pure C wrapper first

@zyxkad
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zyxkad commented Feb 7, 2022 •
edited
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Afaik, CGO does not support C++. You will have to make a pure C wrapper first

Yes, I'm making c wrapper, but how can I warp c++ enum to c (constantly)?
If I use extern const, as you see, that not actually const in golang

@ianlancetaylor ianlancetaylor added help wanted NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Feb 7, 2022
@ianlancetaylor ianlancetaylor added this to the Backlog milestone Feb 7, 2022
@beoran
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beoran commented Feb 7, 2022

An extern int in C is much like a global variable, so I think that is why it maps to a variable in Go. The concept of a const in Go, where they are compile time constants), and in C(++) where const are in fact run time immutable variable, is fundamentally different. The one cannot always be mapped to the other.

@ianlancetaylor
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A C extern const has to be a Go variable, not a Go constant, but beyond that it ought to work. I haven't tried the test case above, but I don't see why it should fail. But evidently it does, so this seems like a bug.

@zyxkad
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zyxkad commented Feb 7, 2022 •
edited
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A C extern const has to be a Go variable, not a Go constant, but beyond that it ought to work. I haven't tried the test case above, but I don't see why it should fail. But evidently it does, so this seems like a bug.

I cannot understand why extern const is a variable, not a constant.
Do you worry about how to get the value from C?
Or because you think It can change when runtime?

And it cannot work, if I use

var (
  CONST_VAR int = (int)(C.CONST_VAR)
)

It will work. but how about want to use const, not var

@ianlancetaylor
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Go does not have the concept of an "external constant." In Go, all constant values are known at the point where the constant is declared. When the cgo tool see a C extern const, it can only see the header file, not the C file. It can't see the actual value of the constant. So there is no way to represent a C extern const as a Go const. It can only be represented as a Go variable.

@ianlancetaylor
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Oh, now I see that I misunderstood the example. The code

const (
  CONST_VAR int = (int)(C.CONST_VAR)
)

can't work in Go for a C extern const.

@zyxkad
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zyxkad commented Feb 7, 2022

Ok, I think I can understand now, but It's hard to change my habit, I'll close it now

@zyxkad zyxkad closed this as completed Feb 7, 2022
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