Question

I am using std::map to implement my local hash table, which will be accessed by multiple threads at the same time.
I did some research and found that std::map is not thread safe.
So I will use a mutex for insert and delete operations on the map.
I plan to have separate mutex(es), one for each map entry so that they can be modified independently.

Do I need to put find operation also under critical section?
Will find operation be affected by insert/delete operations?
Is there any better implementation than using std::map that can take care of everything?

Answer 1

You will in deed need to put find in a critical section, but you might want to have two different locks, one for writing and one for reading. The write lock is exclusive but if no thread holds the write lock several threads may read concurrently with no problems.

Such an implementation would work with most STL implementations but it would not be standards compliant, however. std::map is usually implemented using a red-black tree which doesn’t change when elements are read. If the map was implemented using a splay tree instead, the tree would change during lookup and only one thread could read at a time.

For most purposes I would recommend using two locks.