Use printf("%s", str); to avoid glibc assertion

runtime/vm/log_test.cc

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 
 #include "include/dart_debugger_api.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
 #include "vm/debugger.h"
 #include "vm/globals.h"
 #include "vm/isolate.h"
 #include "vm/log.h"
 #include "vm/message_handler.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 static const char* test_output_;
-static void TestPrinter(const char* print, ...) {
-  test_output_ = print;
+static void TestPrinter(const char* format, ...) {
+  // Measure.
+  va_list args;
+  va_start(args, format);
+  intptr_t len = OS::VSNPrint(NULL, 0, format, args);
+  va_end(args);
+
+  // Print string to buffer.
+  char* buffer = reinterpret_cast<char*>(malloc(len + 1));
+  va_list args2;
+  va_start(args2, format);
+  OS::VSNPrint(buffer, (len + 1), format, args2);
+  va_end(args2);
+
+  // Leaks buffer.
+  test_output_ = buffer;
 }
 
 class LogTestHelper : public AllStatic {
  public:
   static void SetPrinter(Log* log, LogPrinter printer) {
     ASSERT(log != NULL);
     ASSERT(printer != NULL);
     log->printer_ = printer;
   }
 };
@@ skipping 37 matching lines @@
       LogBlock ba(isolate, log);
       log->Print("BANANA");
       EXPECT_EQ(reinterpret_cast<const char*>(NULL), test_output_);
     }
     EXPECT_STREQ("BANANA", test_output_);
   }
   EXPECT_STREQ("APPLE", test_output_);
 }
 
 }  // namespace dart