Is it guaranteed that socket descriptors in linux/freebsd will have almost sequential or comparable numbers?

Question

I understand that there is nothing preventing linux to assign descriptors any random numbers in range 0...2^32 when I create new socket. But what reality is? In my application (web server) I need a mapping structure that maps descriptor into a "connection structure". I understand that some kind of RB-Tree (int -> connection_ptr*) will work, but linear array of connection_ptr pointers (where each pointer placed at offset (index) = descriptor value) would be a little faster.

Answer 1

File descriptors are always assigned from the lowest available number. This is guaranteed by POSIX.

Answer 2

Even if it were implemented that way, if you care for portability and reliability, you cannot rely on that.

To get almost constant access complexity, you can use a hashed container, like std:unordered_map . You can even write a custom hasher, so the storage will be optimized for the file descriptor number distribution.

struct SocketHasher {
    size_t operator()(uint32_t key) {
        return key & 0xFFFF;
    }
}
std::unordered_map<uint32_t, connection_ptr, SocketHasher> connectionPool;