syscall: mksyscall_windows.go "Too many return values..."

[jeffreydwalter]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 freebsd/amd64

Does this issue reproduce with the latest release?

yes.

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="freebsd"
GOOS="freebsd"
GOPATH="/home/jeff_walter/src/agent"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/freebsd_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build944315930=/tmp/go-build -gno-record-gcc-switches"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

I ran go generate to generate syscall wrappers for some windows functions I need.

Here's my windows.go file:

package windows

//go:generate go run mksyscall_windows.go -output zwindows.go windows.go

import (
    errorspkg "errors"
    "sync"
    "syscall"
    "unicode/utf16"
    "unsafe"
)

type Handle uintptr

//sys   WTSQuerySessionInformation(hServer Handle, sessionId uint32, WTSInfoClass WTS_INFO_CLASS) (buffer *uintptr, bytesReturned *uint32, err error) [failretval==0] = wtsapi32.WTSQuerySessionInformationW

What did you expect to see?

I expected to have a zwindows.go with my window syscall wrappers.

func WTSQuerySessionInformation(hServer syscall.Handle, sessionId uint32, WTSInfoClass WTS_INFO_CLASS) (buffer uintptr, bytesReturned uint32, err error) {
    r1, _, e1 := syscall.Syscall6(procWTSQuerySessionInformationW.Addr(), 5, uintptr(hServer), uintptr(sessionId), uintptr(WTSInfoClass), uintptr(unsafe.Pointer(buffer)), uintptr(unsafe.Pointer(bytesReturned)), 0)
    if r1 == 0 { 
        if e1 != 0 {
            err = errnoErr(e1)
        } else { 
            err = syscall.EINVAL
        } 
    }       
    return
}   

What did you see instead?

go generate
2018/02/12 12:03:12 Too many return values in "WTSQuerySessionInformation(hServer syscall.Handle, sessionId uint32, WTSInfoClass WTS_INFO_CLASS) (buffer uintptr, bytesReturned uint32, err error) [failretval==0] = wtsapi32.WTSQuerySessionInformationW"
exit status 1

If I change the comment to this:

//sys   WTSQuerySessionInformation(hServer Handle, sessionId uint32, WTSInfoClass WTS_INFO_CLASS, buffer *uintptr, bytesReturned *uint32) (err error) [failretval==0] = wtsapi32.WTSQuerySessionInformationW

The code generation succeeds, but I get this signature:

func WTSQuerySessionInformation(hServer syscall.Handle, sessionId uint32, WTSInfoClass WTS_INFO_CLASS, buffer *uintptr, bytesReturned *uint32) (err error) {
    r1, _, e1 := syscall.Syscall6(procWTSQuerySessionInformationW.Addr(), 5, uintptr(hServer), uintptr(sessionId), uintptr(WTSInfoClass), uintptr(unsafe.Pointer(buffer)), uintptr(unsafe.Pointer(bytesReturned)), 0)
    if r1 == 0 { 
        if e1 != 0 {
            err = errnoErr(e1)
        } else { 
            err = syscall.EINVAL
        } 
    }       
    return
}   

BUT, I don't really want that as my func signature...

[jeffreydwalter]

After looking at the output further, I understand why it is this way. It would be nice if there was a better mechanism for specifying the r0 return value, instead of mapping the go function signature directly to the windows function.