Question

I’m studying for my operating systems midterm and was wondering if I can get some help.

Can someone explain the checks and what the kernel does during the open() system call?

Thanks!

Answer 1

Very roughly, you can think of the following steps:

1. Translate the file name into an inode, which is the actual file system object describing the contents of the file, by traversing the filesystem data structures.
2. During this traversal, the kernel will check that you have sufficient access through the directory path to the file, and check access on the file itself. The precise checks depend on what modes were passed to open.
3. Create what’s sometimes called an open file descriptor within the kernel. There is one of these objects for each file the kernel has opened on behalf of any process.
4. Allocate an unused index in the per-process file descriptor table, and point it at the open file descriptor.
5. Return this index from the system call as the file descriptor.

This description should be essentially correct for opening plain files and/or directories, but things are different for various sorts of special files, in particular for devices.