cmd/cgo: compiler complains about C.free(*C.char) but is happy with defer C.free(*C.char)

[fanyang01]

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

go version go1.7.3 linux/amd64

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

GOARCH="amd64"
GOBIN="/home/fan/go/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/fan/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build668013101=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

What did you do?

package main

// #include <stdlib.h>
// #include <string.h>
//
// const char *s = "hello";
//
// char *cstring(int *len) {
//     *len = strlen(s);
//     char *buf = (char *)malloc(*len);
//     return strncpy(buf, s, *len);
// }
import "C"
import "fmt"

func main() {
    fmt.Println(f())
}

func f() string {
    var (
        buf *C.char
        len C.int
    )

    if buf = C.cstring(&len); buf == nil {
        return ""
    }
    // defer C.free(unsafe.Pointer(buf))
    defer C.free(buf)
    return C.GoStringN(buf, len)
}

func g() string {
    var (
        buf *C.char
        len C.int
    )

    if buf = C.cstring(&len); buf == nil {
        return ""
    }
    s := C.GoStringN(buf, len)

    // C.free(unsafe.Pointer(buf))
    C.free(buf)

    return s
}

What did you expect to see?

Consistent behavior.

What did you see instead?

• Compile error for C.free(*C.char):

./test.go:46: cannot use buf (type *C.char) as type unsafe.Pointer in argument to _cgoCheckPointer0

• Runtime error for defer C.free(*C.char):

panic: interface conversion: interface {} is *main._Ctype_char, not unsafe.Pointer

[quentinmit]

/cc @ianlancetaylor

[ianlancetaylor]

Thanks for providing a standalone test case. This is a dup of #16591, already fixed for 1.8. With 1.8 I get

foo8.go:30: cannot use buf (type *C.char) as type unsafe.Pointer in argument to func literal
foo8.go:46: cannot use buf (type *C.char) as type unsafe.Pointer in argument to func literal