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cmd/cgo: const in c translate to go const without obey original type. #64923

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newacorn opened this issue Jan 2, 2024 · 1 comment
Open

cmd/cgo: const in c translate to go const without obey original type. #64923

newacorn opened this issue Jan 2, 2024 · 1 comment
Labels
compiler/runtime Issues related to the Go compiler and/or runtime. NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Backlog

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@newacorn
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newacorn commented Jan 2, 2024 •
edited

Go version

go version go1.21.5 darwin/amd64

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

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/Users/acorn/Library/Caches/go-build'
GOENV='/Users/acorn/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/acorn/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/acorn/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/acorn/workspace/programming/golang/go1.21.5'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/acorn/workspace/programming/golang/go1.21.5/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.21.5'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/n1/750_y87d4cx826dsgx4rrd9r0000gn/T/go-build1840385617=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

reg2expand.go

package main

// size_t var1 = ~(size_t)0;
// const size_t const1 = ~(size_t)0;
import "C"
import "log"


func main() {
	println(C.var1)
	// 18446744073709551615
	println(C.const1)
	// -1
	log.Println(C.var1 == C.const1)
	// (_Ciconst_const1) (untyped int constant -1) overflows uint64
}
❯ go build ./reg2expand.go  
# command-line-arguments
./reg2expand.go:12:20: (_Ciconst_const1) (untyped int constant -1) overflows uint64
1. compile error happen when:

a unsigined type c const with a value bigger than go math.MaxInt.
when this translated const compare with same type c variable or const.

2.why

cgo translate a const in this case to a negative value.
_cgo_gotypes.go

var _Cvar_var1 *_Ctype_size_t = (*_Ctype_size_t)(unsafe.Pointer(&__cgo_var1))
const _Ciconst_const1 = -0x1
3. questiont source

c pcre2 library has a const named PCRE2_UNSET.

#define PCRE2_UNSET           (~(PCRE2_SIZE)0)

when check whether a group is set come with compiler error.

4. temp workaround

define a same type variable whith the const value.

package main

// #cgo pkg-config: libpcre2-8
// #define PCRE2_CODE_UNIT_WIDTH 8
// #include <pcre2.h>
// PCRE2_SIZE GOPCRE2_UNSET = PCRE2_UNSET;
// PCRE2_SIZE mockVectorIndex = PCRE2_UNSET;
import "C"

func main() {
	println(C.mockVectorIndex == C.GOPCRE2_UNSET)
}

What did you expect to see?

go tool cgo reg2expand.go

_cgo_gotypes.go

const _Ciconst_const1 = 18446744073709551615

What did you see instead?

go tool cgo reg2expand.go

_cgo_gotypes.go

const _Ciconst_const1 = -0x1
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jan 2, 2024
@mdempsky mdempsky added the NeedsFix The path to resolution is known, but the work has not been done. label Jan 2, 2024
@mdempsky mdempsky added this to the Backlog milestone Jan 2, 2024
@mdempsky
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mdempsky commented Jan 2, 2024

I agree in principle that

// size_t var1 = ~(size_t)0;
// const size_t const1 = ~(size_t)0;
import "C"

should provide equal values when accessed via cgo, though there might be backwards compatibility concerns with fixing that now.

/cc @golang/compiler

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Labels
compiler/runtime Issues related to the Go compiler and/or runtime. NeedsFix The path to resolution is known, but the work has not been done.
Projects
Go Compiler / Runtime
Status: Todo
Milestone
Backlog
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@mdempsky @gopherbot @newacorn