How to determining if other processes are using a file descriptor?

I'm writing an application in C++ for a linux environment which will operate in a multi-process setup (i.e. the program calls fork()). As part of the application I am creating some file descriptors (sockets in this case), I am wondering if it is possible for the program to determine how many other processes have that file descriptor open. In 开发者_开发技巧this case I want to perform special actions before closing the last copy of the file descriptor.

I know that the implementation of close for file descriptors like TCP sockets seem to accomplish this but I'm not sure how.

Edit: I'm looking for a way to accomplish this without access to the main or fork code. The code is in a library providing these sockets to other applications.

Example code:

void handle_socket(int socket_fd) {
    // determine number of processes with this socket_fd open.
    if (num_proc == 1) {
        // Special code
    }
    close(socket_fd);
}

int main(int argc, char *argv[]) {
    int socket_fd = socket(...);
    if (fork() == 0) {
        //action 1
    } else {
       // action 2
    }
    handle_socket(socket_fd);
    return 0;
}

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If you are writing the main() that spawns the child processes, you should be waiting for those children to finish.