C/C++ MultiMap Library

I have store 111 million key-value pairs (one key can have multiple values - maximum 2/3) whose key are 50 bit Integers and values are 32 bit (maximum) Integers. Now, my requirements are:

1. Fast Insertion of (Key, Value) pair [allowing duplicates]


2. Fast retrieving of value/values based on key.

Is there any library for C/C++ which solves this issue ? I can provide 5/6 GB main memory for that. For more info: my previous post.

---