runtime/cgo: do not explicitly link msvcrt.dll

CL 14472 solved issue #12030 by explicitly linking msvcrt.dll
to every cgo executable we build. This CL achieves the same
by manually loading ntdll.dll during startup.

Updates #12030

Change-Id: I5d9cd925ef65cc34c5d4031c750f0f97794529b2
Reviewed-on: https://go-review.googlesource.com/30737
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/cgo/cgo.go

@@ -20,9 +20,7 @@ package cgo
 #cgo !android,linux LDFLAGS: -lpthread
 #cgo netbsd LDFLAGS: -lpthread
 #cgo openbsd LDFLAGS: -lpthread
-// 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 windows LDFLAGS: -lm -mthreads
 
 #cgo CFLAGS: -Wall -Werror
 

src/runtime/os_windows.go

@@ -33,7 +33,6 @@ const (
 //go:cgo_import_dynamic runtime._GetThreadContext GetThreadContext%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._LoadLibraryW LoadLibraryW%1 "kernel32.dll"
 //go:cgo_import_dynamic runtime._LoadLibraryA LoadLibraryA%1 "kernel32.dll"
-//go:cgo_import_dynamic runtime._NtWaitForSingleObject NtWaitForSingleObject%3 "ntdll.dll"
 //go:cgo_import_dynamic runtime._ResumeThread ResumeThread%1 "kernel32.dll"
 //go:cgo_import_dynamic runtime._SetConsoleCtrlHandler SetConsoleCtrlHandler%2 "kernel32.dll"
 //go:cgo_import_dynamic runtime._SetErrorMode SetErrorMode%1 "kernel32.dll"
@@ -77,7 +76,6 @@ var (
 	_GetThreadContext,
 	_LoadLibraryW,
 	_LoadLibraryA,
-	_NtWaitForSingleObject,
 	_ResumeThread,
 	_SetConsoleCtrlHandler,
 	_SetErrorMode,
@@ -114,6 +112,11 @@ var (
 	// when building executable as Cgo. So load SystemFunction036
 	// manually during runtime startup.
 	_RtlGenRandom stdFunction
+
+	// Load ntdll.dll manually during startup, otherwise Mingw
+	// links wrong printf function to cgo executable (see issue
+	// 12030 for details).
+	_NtWaitForSingleObject stdFunction
 )
 
 // Function to be called by windows CreateThread
@@ -178,6 +181,13 @@ func loadOptionalSyscalls() {
 		throw("advapi32.dll not found")
 	}
 	_RtlGenRandom = windowsFindfunc(a32, []byte("SystemFunction036\000"))
+
+	var ntdll = []byte("ntdll.dll\000")
+	n32 := stdcall1(_LoadLibraryA, uintptr(unsafe.Pointer(&ntdll[0])))
+	if n32 == 0 {
+		throw("ntdll.dll not found")
+	}
+	_NtWaitForSingleObject = windowsFindfunc(n32, []byte("NtWaitForSingleObject\000"))
 }
 
 //go:nosplit