Two file descriptor from different process point to the same entry in open file table
c, file-io, linux, linux-kernel, unix
Solution
When child process does some read on the file, would the offset of the same file change in parent process?
Yes, since the offset is stored system-wide file table. You could get a similar effect using `dup` or `dup2`.
If 1 is true, for two processes, is there a convenient way that I can get the same effect of `fork` on same file? That means two processes share a position(offset) information on the same file.
There is a technique called "passing the file descriptor" using Unix domain sockets. Look for "ancillary" data in `sendmsg`.
Is there a way to fork so that both processes have totally unrelated tables, like two unrelated processes only that they opened same files.
You have to `open` the file again to achieve this. Although it doesn't do what you want, you should also look for the `FD_CLOEXEC` flag.
Problem
Unix kernel represents open files using three data structures: `Descriptor table`, `File table`, and `v-node table`. When a process opens a file twice, it gets two different descriptors in the `descriptor table`, two entries in the `file table`(so that they have different positions in the same file), and they both point to one entry in the `v-node table`. And child process inherits parent process's `descriptor table`, so kernel maintains one `descriptor table` for each process respectively. But two descriptor from different processes point to the same entry in `open file table`. So - When child process does some read on the file, would the offset of the same file change in parent process? - If 1 is true, for two processes, is there a convenient way that I can get the same effect of `fork` on same file? That means two processes share a position(offset) information on the same file. - Is there a way to fork so that both processes have totally unrelated tables, like two unrelated processes only that they opened same files.