Question

I’m writing a program using epoll_wait to wait for file descriptors on 64-bit Linux, and I try to put some information together with the file descriptor in the epoll_event user data.

I know in practice it’s unlikely that file descriptor will exceed 32 bits. Just wanna know is that kernel guarantees that file descriptors have a specific range, or it just counts from small and unlikely to get very big?

Answer 1

The epoll_ctl(2) interface to add new filedescriptors takes an int fd argument, so you’re already limited to 32-bit range (at least on the Linux platforms I’m familiar with).

You’re further limited by /proc/sys/fs/file-max system-wide limit on the number of open files for all processes; /proc/sys/fs/file-max is currently 595956 on my system.

Each process is further limited via the setrlimit(2) RLIMIT_NOFILE per-process limit on the number of open files. 1024 is a common RLIMIT_NOFILE limit. (It’s very easy to change this limit via /etc/security/limits.conf.)

It’s a rare application that needs more than 1024. The full 32 bits seems unlikely as well, since each open file will take some kernel memory to represent — four billion ~280 byte struct inode structures (at the minimum) is a lot of pinned memory.