c - Seeking a simple description regarding 'file descriptor' after fork()

Question

In "Advanced Programming in the Unix Environment", 2nd edition, By W. Richard Stevens. Section 8.3 fork function.

Here's the description:

It is important that the parent and the child share the same file offset.

Consider a process that forks a child, then waits for the child to complete. Assume that both processes write to standard output as part of their normal processing. If the parent has its standard output redirected (by a shell, perhaps) it is essential that the parent's file offset be updated by the child when the child writes to standard output.

My responses:

{1} What does it mean? if parent's std output is redirected to a 'file1' for example, then what should child update after child writes? parent's original std output offset or redirected ouput(i.e file1) offset? Can't be the later, right?

{2} How is the update done? by child explicitly, by OS implicitly, by files descriptor itself? After fork, I thought parent and child went their own ways and has their own COPY of file descriptor. So how does child update offset to parent side?

In this case, the child can write to standard output while the parent is waiting for it; on completion of the child, the parent can continue writing to standard output, knowing that its output will be appended to whatever the child wrote. If the parent and the child did not share the same file offset, this type of interaction would be more difficult to accomplish and would require explicit actions by the parent.

If both parent and child write to the same descriptor, without any form of synchronization, such as having the parent wait for the child, their output will be intermixed (assuming it's a descriptor that was open before the fork). Although this is possible, it's not the normal mode of operation.

There are two normal cases for handling the descriptors after a fork.

1. The parent waits for the child to complete. In this case, the parent does not need to do anything with its descriptors. When the child terminates, any of the shared descriptors that the child read from or wrote to will have their file offsets updated accordingly.

2. Both the parent and the child go their own ways. Here, after the fork, the parent closes the descriptors that it doesn't need, and the child does the same thing. This way, neither interferes with the other's open descriptors. This scenario is often the case with network servers.

My response:

{3} When fork() is invoked, all i understand is that child get a COPY of what parent has, file descriptor in this case, and does its thing. If any offset changes to file descriptor that parent and child share, it can only be because the descriptor remember the offset itself. Am I right?

I am kind of new to the concepts.

Answer 1

It's important to distinguish between the file descriptor, which is a small integer that the process uses in its read and write calls to identify the file, and the file description, which is a structure in the kernel. The file offset is part of the file description. It lives in the kernel.

As an example, let's use this program:

#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>

int main(void)
{
    int fd;

    fd = open("output", O_CREAT|O_TRUNC|O_WRONLY, 0666);

    if(!fork()) {
        /* child */
        write(fd, "hello ", 6);
        _exit(0);
    } else {
        /* parent */
        int status;

        wait(&status);
        write(fd, "world
", 6);
    }
}

(All error checking has been omitted)

If we compile this program, call it hello, and run it like this:

./hello

here's what happens:

The program opens the output file, creating it if it didn't exist already or truncating it to zero size if it did exist. The kernel creates a file description (in the Linux kernel this is a struct file) and associates it with a file descriptor for the calling process (the lowest non-negative integer not already in use in that process's file descriptor table). The file descriptor is returned and assigned to fd in the program. For the sake of argument suppose that fd is 3.

The program does a fork(). The new child process gets a copy of its parent's file descriptor table, but the file description is not copied. Entry number 3 in both processes' file tables points to the same struct file.

The parent process waits while the child process writes. The child's write causes the first half of "hello world " to be stored in the file, and advances the file offset by 6. The file offset is in the struct file!

The child exits, the parent's wait() finishes, and the parent writes, using fd 3 which is still associated with the same file description that had its file offset updated by the child's write(). So the second half of the message is stored after the first part, not overwriting it as it would have done if the parent had a file offset of zero, which would be the case if the file description was not shared.

Finally the parent exits, and the kernel sees that the struct file is no longer in use and frees it.