Go 1.6 - C pointer <-> Go pointer rules change causes crash

[vitaminwater]

The new rules: https://golang.org/cmd/cgo/#hdr-Passing_pointers
causes a crash in convert.go line 150:

_, errno = C.iconv(iconv, &inputAsCCharsPtr, &bytesLeftInCSize, &outputCharsPtr, &bytesLeftOutCSize)

With the trace:

panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 3 [running]:
panic(0x437c780, 0xc82000a1c0)
        /usr/local/Cellar/go/1.6/libexec/src/runtime/panic.go:464 +0x3e6
github.com/GeertJohan/cgo%2ewchar.convertWcharStringToGoString(0xc8200e4100, 0x12, 0x20, 0x0, 0x0, 0x0, 0x0)
        /Users/stant/Documents/hackerloop/rotonde/src/github.com/GeertJohan/cgo.wchar/convert.go:150 +0x521
github.com/GeertJohan/cgo%2ewchar.WcharStringPtrToGoString(0x53059f0, 0x0, 0x0, 0x0, 0x0)
        /Users/stant/Documents/hackerloop/rotonde/src/github.com/GeertJohan/cgo.wchar/wchar.go:141 +0x91
github.com/GeertJohan/go%2ehid.Enumerate(0x42, 0x0, 0x0, 0x0, 0x0, 0x0)
        /Users/stant/Documents/hackerloop/rotonde/src/github.com/GeertJohan/go.hid/hid.go:205 +0x35f
main.StartHID.func2(0xc820010140, 0xc820010160, 0xc820010180, 0xc82006e1e0)

[...]

There must be other places in the code causing this crash.

[vitaminwater]

I'm not so familiar with the new cgo rules, but I could reproduce the crash.
As I understood my tests, the main issue here is that we have to pass **C.char, with the *C.char being created with the make function of go. ie. make([]C.char, 0, len(ws)*4), which obviously creates a pointer to a go pointer, which falls in the restriction of the new cgo pointer rules.

What I think of as a solution is replacing all make([]C.char, 0, len(ws)*4) to C.malloc calls:

vitaminwater@4a627dc

Which is a problem when it comes to putting data in inputAsCChars, because *C.char is not indexable.
So I created a putWcharAt in the C preamble, but it feels bad to make so many cgo calls.

Anyway I can't test it right now, I need a HID device first, this afternoon, or tomorrow.

[vitaminwater]

Still trying to fix it, this fork doesn't crash cgo anymore: https://github.com/vitaminwater/cgo.wchar/

But now the whole iconv thing is broken, output for tests is:

--- FAIL: TestStringConversion (0.00s)
        wchar_test.go:18: Error on conversion. invalid argument
--- FAIL: TestWcharStringConversion (0.00s)
        wchar_test.go:43: Error on conversion. illegal byte sequence
--- FAIL: TestRuneConversion (0.00s)
        wchar_test.go:74: Error on conversion. invalid argument

(last test not displayed hangs forever)

Still lacking knowledge, if you could provide me with some input I might create a good pull request to fix it.

[vitaminwater]

The first test says invalid argument, when I check the desired conversions in convertGoStringToWcharString I see that the tocode parameter for iconv_open is wchar_t, which is not listed when I do a iconv -l, how is that meant to work ?

[vitaminwater]

Looks like the wchar_t type does not exist on OSX: http://www.firstobject.com/wchar_t-string-on-linux-osx-windows.htm

[vitaminwater]

Further readings tend to make me think that cgo.wchar could be avoided thanks to golang's unicode package. Is that right ?

Maybe it would be wise to remove cgo.wchar from go.hid ?