cgo: add support for returning errno with gccgo.

Also eliminate some warnings in builds.

R=golang-dev, fullung, iant, rsc
CC=golang-dev, remy
https://golang.org/cl/5650066

src/cmd/cgo/out.go

@@ -257,23 +257,46 @@ func (p *Package) writeDefsFunc(fc, fgo2 *os.File, n *Name) {
 		Name: ast.NewIdent(n.Mangle),
 		Type: gtype,
 	}
-	conf.Fprint(fgo2, fset, d)
+
 	if *gccgo {
-		fmt.Fprintf(fgo2, " __asm__(\"%s\")\n", n.C)
-	} else {
-		fmt.Fprintf(fgo2, "\n")
+		// Gccgo style hooks.
+		// we hook directly into C. gccgo goes not support cgocall yet.
+		if !n.AddError {
+			fmt.Fprintf(fgo2, "//extern %s\n", n.C)
+			conf.Fprint(fgo2, fset, d)
+			fmt.Fprint(fgo2, "\n")
+		} else {
+			// write a small wrapper to retrieve errno.
+			cname := fmt.Sprintf("_cgo%s%s", cPrefix, n.Mangle)
+			paramnames := []string(nil)
+			for i, param := range d.Type.Params.List {
+				paramName := fmt.Sprintf("p%d", i)
+				param.Names = []*ast.Ident{ast.NewIdent(paramName)}
+				paramnames = append(paramnames, paramName)
+			}
+			conf.Fprint(fgo2, fset, d)
+			fmt.Fprintf(fgo2, "{\n")
+			fmt.Fprintf(fgo2, "\tr := %s(%s)\n", cname, strings.Join(paramnames, ", "))
+			fmt.Fprintf(fgo2, "\treturn r, syscall.GetErrno()\n")
+			fmt.Fprintf(fgo2, "}\n")
+			// declare the C function.
+			fmt.Fprintf(fgo2, "//extern %s\n", n.C)
+			d.Name = ast.NewIdent(cname)
+			l := d.Type.Results.List
+			d.Type.Results.List = l[:len(l)-1]
+			conf.Fprint(fgo2, fset, d)
+			fmt.Fprint(fgo2, "\n")
+		}
+		return
 	}
+	conf.Fprint(fgo2, fset, d)
+	fmt.Fprint(fgo2, "\n")
 
 	if name == "CString" || name == "GoString" || name == "GoStringN" || name == "GoBytes" {
 		// The builtins are already defined in the C prolog.
 		return
 	}
 
-	// gccgo does not require a wrapper unless an error must be returned.
-	if *gccgo && !n.AddError {
-		return
-	}
-
 	var argSize int64
 	_, argSize = p.structType(n)
 
@@ -355,6 +378,11 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 	}
 	p.Written[name] = true
 
+	if *gccgo {
+		// we don't use wrappers with gccgo.
+		return
+	}
+
 	ctype, _ := p.structType(n)
 
 	// Gcc wrapper unpacks the C argument struct
@@ -783,7 +811,7 @@ struct __go_string __go_byte_array_to_string(const void* p, int len);
 struct __go_open_array __go_string_to_byte_array (struct __go_string str);
 
 const char *CString(struct __go_string s) {
-	return strndup(s.__data, s.__length);
+	return strndup((const char*)s.__data, s.__length);
 }
 
 struct __go_string GoString(char *p) {
@@ -796,7 +824,7 @@ struct __go_string GoStringN(char *p, int n) {
 }
 
 Slice GoBytes(char *p, int n) {
-	struct __go_string s = { p, n };
+	struct __go_string s = { (const unsigned char *)p, n };
 	return __go_string_to_byte_array(s);
 }
 `

src/cmd/go/build.go

@@ -1152,8 +1152,6 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 		return nil, nil, errors.New("cannot use cgo when compiling for a different operating system")
 	}
 
-	outObj = append(outObj, "") // for importObj, at end of function
-
 	cgoCFLAGS := stringList(envList("CGO_CFLAGS"), p.info.CgoCFLAGS)
 	cgoLDFLAGS := stringList(envList("CGO_LDFLAGS"), p.info.CgoLDFLAGS)
 
@@ -1238,6 +1236,11 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 		return nil, nil, err
 	}
 
+	if _, ok := buildToolchain.(gccgoToolchain); ok {
+		// we don't use dynimport when using gccgo.
+		return outGo, outObj, nil
+	}
+
 	// cgo -dynimport
 	importC := obj + "_cgo_import.c"
 	if err := b.run(p.Dir, p.ImportPath, cgoExe, "-objdir", obj, "-dynimport", dynobj, "-dynout", importC); err != nil {
@@ -1252,8 +1255,8 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 
 	// NOTE(rsc): The importObj is a 5c/6c/8c object and on Windows
 	// must be processed before the gcc-generated objects.
-	// Put it first.  We left room above.  http://golang.org/issue/2601
-	outObj[0] = importObj
+	// Put it first.  http://golang.org/issue/2601
+	outObj = append([]string{importObj}, outObj...)
 
 	return outGo, outObj, nil
 }