fstat() on epoll fd returns a value in st_mode which indicates that it is a regular file. Is this file type correct for an epoll fd?

Bikku · 02-04-2020, 12:18 AM

I am trying to understand file type of an epoll fd returned from epoll_create() system call. When I call fstat() on epoll fd, the value returned in statbuf.st_mode indicates that the epoll fd represents a regular file. Is this a correct behavior?

Thank you in advance.

berndbausch · 02-04-2020, 05:32 PM

I don't have a a definite answer, but what makes you think it's not correct?

First of all, who/what defines what is correct behaviour? The epoll interface is not standardized as far as I know; all we have is the manual page, and it doesn't mention the type of the file descriptor returned by epoll_create.

Next, which of the following types would be better adapted in your opinion (from the inode(7) man page):

Code:

           S_IFSOCK   0140000   socket
           S_IFLNK    0120000   symbolic link
           S_IFREG    0100000   regular file
           S_IFBLK    0060000   block device
           S_IFDIR    0040000   directory
           S_IFCHR    0020000   character device
           S_IFIFO    0010000   FIFO

EDIT: https://idea.popcount.org/2017-02-20...lly-broken-12/ (apart from the title, I did not read the blog post)

Bikku · 02-04-2020, 11:59 PM

When fstat() returns file type as regular file then I interpreted it as disk file. So I thought, epoll fd type should not be a disk file.
ls -l /prod/pid/fd/ shows epoll fd as eventpoll. So, wondering if fstat() returning epoll fd type as a regular file is correct.

[usr1@VFM1012 ~]$ ls -l /proc/99922/fd/5
lrwx------ 1 usr1 group1 64 Feb 5 10:24 5 -> anon_inode:[eventpoll]

berndbausch · 02-05-2020, 12:13 AM

I doubt that the filetype in square brackets [eventpoll] corresponds to the st_mode in the stat structure. In any case, stat.h has no eventpoll type.

Bikku · 02-05-2020, 06:59 AM

It shows anon_inode:[eventpoll].
This anon_inode's type seems to be S_IFREG. If this is the case, then all pseudo files'fd (like epoll fd) file type appears to be regular file in Linux.