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There's a lot missing from your code (how are you getting messages? is there a subscriber? why do you want a specific timestamp? how are you reading the bag file?) , so the best I can do is a generic answer:

rosbag has a C++ API that can be used to read all of the messages from a bag file, in order. You can use this to get the first (or Nth) message from a bag file, or you can compare the header timestamps if you want a message with a specific timestamp.