runtime/cgo: explicitly link msvcrt on windows

It's because runtime links to ntdll, and ntdll exports a couple
incompatible libc functions. We must link to msvcrt first and
then try ntdll.

Fixes #12030.

Change-Id: I0105417bada108da55f5ae4482c2423ac7a92957
Reviewed-on: https://go-review.googlesource.com/14472Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/cgo_test.go

@@ -65,5 +65,6 @@ func Test9026(t *testing.T)                  { test9026(t) }
 func Test9557(t *testing.T)                  { test9557(t) }
 func Test10303(t *testing.T)                 { test10303(t, 10) }
 func Test11925(t *testing.T)                 { test11925(t) }
+func Test12030(t *testing.T)                 { test12030(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue12030.go

+// Copyright 2015 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 12030. sprintf is defined in both ntdll and msvcrt,
+// Normally we want the one in the msvcrt.
+
+package cgotest
+
+/*
+#include <stdio.h>
+#include <stdlib.h>
+void issue12030conv(char *buf, double x) {
+	sprintf(buf, "d=%g", x);
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"testing"
+	"unsafe"
+)
+
+func test12030(t *testing.T) {
+	buf := (*C.char)(C.malloc(256))
+	defer C.free(unsafe.Pointer(buf))
+	for _, f := range []float64{1.0, 2.0, 3.14} {
+		C.issue12030conv(buf, C.double(f))
+		got := C.GoString(buf)
+		if want := fmt.Sprintf("d=%g", f); got != want {
+			t.Fatalf("C.sprintf failed for %g: %q != %q", f, got, want)
+		}
+	}
+}

src/runtime/cgo/cgo.go

@@ -20,7 +20,9 @@ package cgo
 #cgo !android,linux LDFLAGS: -lpthread
 #cgo netbsd LDFLAGS: -lpthread
 #cgo openbsd LDFLAGS: -lpthread
-#cgo windows LDFLAGS: -lm -mthreads
+// we must explicitly link msvcrt, because runtime needs ntdll, and ntdll
+// exports some incompatible libc functions. See golang.org/issue/12030.
+#cgo windows LDFLAGS: -lmsvcrt -lm -mthreads
 
 #cgo CFLAGS: -Wall -Werror