UNIX for Dummies Questions & Answers

Not completely arbitrary. The lower unused index available is always used when a file is successfully open. I wouldn't call it a tradition as it is a quite established POSIX standard (STDIN_FILENO and so on).

It wouldn't matter if the kernel assigned them randomly, for most purposes. (assuming dup2 still works, of course.) In that respect it's arbitrary.

Yes, stdin/stdout/stderr are a standard, but not one enforced by the kernel. Only to programs are FDs 0,1,2 anything special.

You are right it wouldn't matter that much but the fact is every Unix/Unix like implementation is assigning file descriptor sequentially and is doing so because it is both the most logical and the standard compliant/documented way to do it.

I don't get your std* remark. File descriptors are only relevant when referring to processes. Unix/Unix like kernels are handling files objects with file/vnode/whatever structures but not file descriptors.

I'm just saying there's nothing in particular enforcing stdin/stdout/stderr. They don't get special treatment from open/close/read/write, they're just prepared in advance.