epoll_ctl : Operation not permitted error - c program

1 #include <sys/epoll.h>
2 #include <stdio.h>
3 #include <sys/types.h>
4 #include <sys/stat.h>
5 #include <fcntl.h>
6 #include <string.h>
7 #include <sys/uio.h>
8
9 int main() {
10 struct epoll_event event ;
11 int ret,fd, epfd ;
12
13 fd = open("doc", O_RDONLY);
14 if( fd < 0 )
15 perror("open");
16
17 event.data.fd = fd ;
18 event.events = EPOLLIN|EPOLLOUT ;
19
20 epfd = epoll_create(50);
21 printf("%d", epfd );
22
23 if( epfd < 0 )
24 perror("epoll_create");
25
26 ret = epoll_ctl( epfd, EPOLL_CTL_ADD, fd, &event ) ;
27 if( ret < 0 )
28 perror("epoll_ctl");
29
30 }

When compiling this code, there was no errors.
gcc -o epoll epoo.c

but when i tried to execute the program 'epoll', i got the error message
> epoll_ctl:Operation not permitted.

I've tried to change the 'doc' file's mode to 0777 but it was not work.

What is the problem? Thank you :)

From `epoll_ctl(2)`:

EPERM The target file fd does not support epoll.

I'm going to guess that `doc` is a regular file. Regular files are _always_ ready for `read(2)` or `write(2)` operations, thus it doesn't make sense to `epoll(7)` or `select(2)` on regular files.

If `doc` is a pipe or unix domain socket, comment here (so I know to delete my post) and amend your question so others don't make the same mistake I did. :)

In this case you're opening a regular file. `epoll()`, `select()`, and `poll()` don't make sense on regular files.

If it is a pipe or socket, then:

$mkfifo doc