Does each Unix file description have its own read/write buffers?

Question

In reference to this question about read() and write(), I'm wondering if each open file description has its own read and write buffers or if perhaps there's a single read and write buffer for a file when it has been opened multiple times at once. I'm curious because this would have an effect on what exactly happens with overlapping writes to the same file. Perhaps this is something that varies among Unixes?

(To my understanding, "file description" refers to the info/options about an open file, such as the current marker position. "File descriptor", in contrast, refers to just the number used in a process to refer to a description.)

Answer 1

This depends a bit on whether you are talking about sockets or actual files.

Strictly speaking, a descriptor never has its own buffers; it's just a handle to a deeper abstraction.

File system objects have their "own" buffers, at least when they are required. That is, if a program writes less than the file system block size, the kernel has no choice but to read a FS block and merge the write with the existing data.

This buffer is attached to the vnode and at a lower level, possibly an inode. It's owned by the file and not the descriptor. It may be kept around for a long time if memory is available.

In the case of a socket, then a stream, but not specifically a single descriptor, does actually have buffers that it owns.