diff --git a/testdata/libc/libc_windows.go b/testdata/libc/libc_windows.go
index 35066c7..0acfd44 100644
--- a/testdata/libc/libc_windows.go
+++ b/testdata/libc/libc_windows.go
@@ -1,6 +1,7 @@
 package libc
 
 import (
+	"fmt"
 	unsafe "unsafe"
 )
 
@@ -32,7 +33,7 @@ type struct_threadmbcinfostruct struct {
 }
 
 func __acrt_iob_func(index uint32) *struct__iobuf {
-	panic("notimpl")
+	return nil
 }
 func X__builtin_llabs(int64) int64 {
 	panic("notimpl")
@@ -49,9 +50,16 @@ func __mingw_strtod(*int8, **int8) float64 {
 func __mingw_strtof(*int8, **int8) float32 {
 	panic("notimpl")
 }
-func __mingw_vfprintf(*struct__iobuf, *int8, []interface {
+func __mingw_vfprintf(f *struct__iobuf, format *int8, args []interface {
 }) int32 {
-	panic("notimpl")
+	goformat := gostring(format)
+	for i, arg := range args {
+		if v, ok := arg.(*int8); ok {
+			args[i] = gostring(v)
+		}
+	}
+	fmt.Printf(goformat, args...)
+	return 0
 }
 func __mingw_vfscanf(fp *struct__iobuf, Format *int8, argp []interface {
 }) int32 {
diff --git a/testdata/libc/src/foo.h b/testdata/libc/src/foo.h
index ba22309..0c3df05 100644
--- a/testdata/libc/src/foo.h
+++ b/testdata/libc/src/foo.h
@@ -20,3 +20,4 @@ typedef size_t foo_t;
 
 foo_t foo();
 void unknown();
+int printf_x(const char *restrict fmt, ...);