cmd/cgo: access errno from void C function

Fixes #3729.

R=rsc
CC=golang-dev
https://golang.org/cl/6938052

misc/cgo/test/cgo_test.go

@@ -33,5 +33,6 @@ func Test1635(t *testing.T)                { test1635(t) }
 func TestPrintf(t *testing.T)              { testPrintf(t) }
 func Test4029(t *testing.T)                { test4029(t) }
 func TestBoolAlign(t *testing.T)           { testBoolAlign(t) }
+func Test3729(t *testing.T)                { test3729(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue3729.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 3729:	cmd/cgo: access errno from void C function
+// void f(void) returns [0]byte, error in Go world.
+
+// +build !windows
+
+package cgotest
+
+/*
+#include <errno.h>
+
+void g(void) {
+	errno = E2BIG;
+}
+
+// try to pass some non-trivial arguments to function g2
+const char _expA = 0x42;
+const float _expB = 3.14159;
+const short _expC = 0x55aa;
+const int _expD = 0xdeadbeef;
+void g2(int x, char a, float b, short c, int d) {
+	if (a == _expA && b == _expB && c == _expC && d == _expD)
+		errno = x;
+	else
+		errno = -1;
+}
+*/
+import "C"
+
+import (
+	"syscall"
+	"testing"
+)
+
+func test3729(t *testing.T) {
+	_, e := C.g()
+	if e != syscall.E2BIG {
+		t.Errorf("got %q, expect %q", e, syscall.E2BIG)
+	}
+	_, e = C.g2(C.EINVAL, C._expA, C._expB, C._expC, C._expD)
+	if e != syscall.EINVAL {
+		t.Errorf("got %q, expect %q", e, syscall.EINVAL)
+	}
+}

misc/cgo/test/issue3729w.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 3729:	cmd/cgo: access errno from void C function
+// void f(void) returns [0]byte, error in Go world.
+
+// +build windows
+
+package cgotest
+
+import "testing"
+
+func test3729(t *testing.T) {
+	t.Log("skip errno test on Windows")
+}

misc/cgo/test/issue4417.go

-// run
-
 // Copyright 2012 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

src/cmd/cgo/gcc.go

@@ -676,9 +676,6 @@ func (p *Package) rewriteRef(f *File) {
 				break
 			}
 			if r.Context == "call2" {
-				if r.Name.FuncType.Result == nil {
-					error_(r.Pos(), "assignment count mismatch: 2 = 0")
-				}
 				// Invent new Name for the two-result function.
 				n := f.Name["2"+r.Name.Go]
 				if n == nil {
@@ -933,6 +930,7 @@ type typeConv struct {
 	void                                   ast.Expr
 	unsafePointer                          ast.Expr
 	string                                 ast.Expr
+	goVoid                                 ast.Expr // _Ctype_void, denotes C's void
 
 	ptrSize int64
 	intSize int64
@@ -964,6 +962,7 @@ func (c *typeConv) Init(ptrSize, intSize int64) {
 	c.unsafePointer = c.Ident("unsafe.Pointer")
 	c.void = c.Ident("void")
 	c.string = c.Ident("string")
+	c.goVoid = c.Ident("_Ctype_void")
 }
 
 // base strips away qualifiers and typedefs to get the underlying type
@@ -1292,8 +1291,9 @@ func (c *typeConv) Type(dtype dwarf.Type, pos token.Pos) *Type {
 		}
 
 	case *dwarf.VoidType:
-		t.Go = c.void
+		t.Go = c.goVoid
 		t.C.Set("void")
+		t.Align = 1
 	}
 
 	switch dtype.(type) {
@@ -1381,7 +1381,9 @@ func (c *typeConv) FuncType(dtype *dwarf.FuncType, pos token.Pos) *FuncType {
 	}
 	var r *Type
 	var gr []*ast.Field
-	if _, ok := dtype.ReturnType.(*dwarf.VoidType); !ok && dtype.ReturnType != nil {
+	if _, ok := dtype.ReturnType.(*dwarf.VoidType); ok {
+		gr = []*ast.Field{{Type: c.goVoid}}
+	} else if dtype.ReturnType != nil {
 		r = c.Type(dtype.ReturnType, pos)
 		gr = []*ast.Field{{Type: r.Go}}
 	}