Thread: Shared fd after a fork()

Hello guys,

I've seen examples as follows:

Code:

    int pfds[2];
    char buf[30];
    pipe(pfds);
    if (!fork()) {
        printf(" CHILD: writing to the pipe\n");
        close(pfds[0]);
        write(pfds[1], "test", 5);
        printf(" CHILD: exiting\n");
        exit(0);
    } else {
        printf("PARENT: reading from pipe\n");
        read(pfds[0], buf, 5);
        printf("PARENT: read \"%s\"\n", buf);
        wait(NULL);
    }

So the childprocess gets a copy of filedescriptors from the parents. But if the child closes a pfds[0], won't that pfds[0] in the parent process be useless? (appearently not!)
It's a bit confusing to me, cause I thought closing a filedescriptor, will close also one end of the pipe.

Thank you.

My understanding is that when the child receives a copy, it's completely duplicated - the memory pointed to by pointers is also duplicated (that's why malloc'ed memory can't be shared between two process). So the two file descriptors, even though they reference the same 'file', are completely separate entities. One can indeed be closed, and the other would still work. Pipes are intended to work this way. The parent process closes the end it doesn't use --only as far as the parent process is concerned--, and the child process does the same --only as far as the child process is concerned--.

That's why you need file locking and things like that - because 2 different programs can access the same file in 2 different ways - pipes included, I guess!

Sean,

Thank you for your reply.
I now understand that after forking the filedescriptor will be duplicated, it's like calling the dup() function, right. But you say that pointers are also duplicated, meaning that the address pointed to the malloc'ed memory will be the same (like normal variables have the same value as the parent form the moment the child is being forked).

So I don't really understand your line "that's why malloc'ed memory can't be shared between two process". Cause as long as the pointer (whatever pointer it is) holds the address of a malloc'ed memory, I can reach it from everywhere (theoretically). Unless the kernel provided me my own peace of memory which is isolated from the others, so the malloc'ed mem-address is more kind of relative address?

So how can I share, for example, my own defined structure between childprocesses, if I can't use the pointer to it (without discussing the synchronization stuff like semaphore)?

Thank you, sorry for the silly questions, I really do read a book about it, but I still need some time to understand the concept, as I was used to work with ready made MFC classes for windose. Thumbs down