Does Creating a New Thread Duplicate File Descriptors and Socket Descriptors in Linux?

Question

Everyone knows the classic model of a process listening for connections on a socket and forking a new process to handle each new connection. Normal practice is for the parent process to immediately call close on the newly created socket, decrementing the handle count so that only the child has a handle to the new socket.

I've read that the only difference between a process and a thread in Linux is that threads share the same memory. In this case I'm assuming spawning a new thread to handle a new connection also duplicates file descriptors and would also require the 'parent' thread to close it's copy of the socket?

Answer 1

In principle, Linux clone() can implement not only a new process (like fork()), or a new thread (like pthread_create perhaps), but also anything in between.

In practice, it is only ever used for one or the other. Threads created with pthread_create share the file descriptors with all other threads in the process (not just the parent). This is non-negotiable.

Sharing a file descriptor and having a copy is different. If you have a copy (like fork()) then all copies must be closed before the file handle goes away. If you share the FD in a thread, once one closes it, it's gone.