Question

I am making a server and I use fork() for creating child processes but I have doubts about that. Here are some:

1. Why do you need to close main socket in a child process and new connection accepted socket in the parent process? (after accepting a new connection) I thought that sockets are only integers with some id which are used to access opened sockets in some system-wide object which is only acceptable through system functions calls. In that case fork would only copy the integer but would have no affect on socket opened.
2. I have checked and found out that if I fork a process inside a class method, all members are copied. Well, I have found out it is copy-on-edit so does it mean that my server class will be copied in each child that uses a non-constant function? How do I make some memory shared between all such processes (like a list of tasks and each child puts something into it while the parent is reading stuff from it)? I guess fork is not the right function for that. What is the best way?

P.S. I am pretty sure I know the answer to the second question, which is clone(), but just wanted to make sure that’s the right function.

Answer 1

1. Sockets in Unix are File descriptors and they are indeed integers, as seen by the user, but they really are indexes into a table that the kernel maintains per process. In this table each file descriptor (FD) refers to an open file description (OFD) that are system-wide objects maintained in kernel. When you do fork() the opened file descriptors are duplicated and both child’s and parent’s point to the same OFD. Having two FDs that refer to the same OFD is not usually a problem, but particularly with sockets they can have subtle problems, as the connection is closed only when you close all the FDs that refer to it.

2. You should really consider using threads (do not close the sockets if you use threads!). clone is a linux system call and is not intended to be used directly. Your alternative is to use shared memory but it is kind of more complex.