QString asprintf behaved differently with printf

the code is like this:

  setbuf( stdout, NULL );
  qDebug() << QString().asprintf( "msg is \"%s\"", "test" );
  printf( "msg is \"%s\"", "test" ); 

qDebug()<< QString().asprintf( "msg is \"%s\"", "test" );

the expected result is :

msg is "test"

but the actual result is:

"msg is \"test\""

the output string in the console should not contain escape character \.