amn amn - 1 year ago 76
Linux Question

May `epoll_ctl` modify the `epoll_event` structure passed to it?

The manpages linked from, at, declare the

procedure as follows:

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);

As evident, the
parameter is declared as pointing to a
struct epoll_event

The significance of this in the context of this question is that there is no
ahead of the type declaration, and thus, at least "on paper", the procedure thus appears permitted to modify the contents of the passed structure.

Is it an omission of sorts, or is it so that the procedure is made like that by design and we have to assume that the passed structure may be modified inside the procedure?

I understand that the declaration is unambiguous here, but has anyone observed this to be an omission?

I have also taken a look at the relevant source code in kernel 4.6 tree, and I don't see much evidence that the procedure even intends to modify the structure, so there.

amn amn
Answer Source

Nevermind, found a rather conclusive answer on the linux kernel mailing list. For posterity, quoting Davide Libenzi here, chief or sole author of epoll:

From: Davide Libenzi <davidel <at>>
Subject: Re: epoll_ctl and const correctness
Newsgroups: gmane.linux.kernel
Date: 2009-03-25 16:23:21 GMT (7 years, 17 weeks, 1 day, 9 hours and 4 minutes ago)

On Wed, 25 Mar 2009, nicolas sitbon wrote:

> Currently, the prototype of epoll_ctl is :
> int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
> I searched in the man of epoll_ctl and google, and it seems that the
> structure pointed to by event isn't modify, valgrind confirms this
> behaviour, so am I wrong? or the good prototype is
> int epoll_ctl(int epfd, int op, int fd, struct epoll_event const *event);

According to the current ctl operations, yes. But doing that would prevent 
other non-const operations to be added later on.

- Davide

The takeaway is that even though de-facto behavior is not to modify the structure, the interface omits const modifier because other control operations may be added in the future through the same system call, which necessitate potentially modifiable structure as pointed at by event argument.

I should have hit the linux kernel mailing list first, apologies for another perhaps redundant information on SO.

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