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unable to use unsafe.Pointer as pointer to C struct in Go 1.8 #19181

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ghost opened this issue Feb 19, 2017 · 2 comments
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unable to use unsafe.Pointer as pointer to C struct in Go 1.8 #19181

ghost opened this issue Feb 19, 2017 · 2 comments
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@ghost
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ghost commented Feb 19, 2017 •
edited by ghost

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

go version go1.8 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/brandonhsiao/gocode"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.8/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.8/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/_v/1ll1sr8d3ml0j1gxhbkfmg580000gp/T/go-build439503373=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

package main

/*
#include <stdio.h>

typedef struct {
  int foo;
} CStruct;

void test(CStruct* p) {
  printf("%d", p->foo);
}
*/
import "C"
import "unsafe"

type CStruct _Ctype_CStruct

func main() {
  var x CStruct
  x.foo = 42

  C.test(unsafe.Pointer(&x))
}
$ go build main.go && ./main

What did you expect to see?

A working compilation, and 42 printed to the screen. This worked in Go 1.7, and the Go docs state

- A Pointer can be converted to a pointer value of any type.

What did you see instead?

$ go build main.go 
# command-line-arguments
./main.go:23: cannot use unsafe.Pointer(&x) (type unsafe.Pointer) as type *C.struct___0 in argument to _Cfunc_test
@ghost ghost changed the title Unable to use unsafe.Pointer as pointer to C struct in Go 1.8 unable to use unsafe.Pointer as pointer to C struct in Go 1.8 Feb 19, 2017
@minux
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minux commented Feb 19, 2017 via email

@ghost
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ghost commented Feb 19, 2017 •
edited by ghost

Was the fact that it worked in 1.7 a bug? Also, this doesn't work either:

C.test((*CStruct)(unsafe.Pointer(&x)))

as it prints out

$ go build main.go 
# command-line-arguments
./main.go:23: cannot use (*CStruct)(unsafe.Pointer(&x)) (type *CStruct) as type *C.struct___0 in argument to _Cfunc_test

Is there a way to do this?

Edit: It looks like using C.struct___0 and *C.struct___0 works. Does this mean we're forced to refer to C structs by index? How are they indexed?

Edit 2: Wow, it looks like you can just directly use C.CStruct! Apparently this is outdated.

@ghost ghost closed this as completed Feb 19, 2017
@golang golang locked and limited conversation to collaborators Feb 19, 2018
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