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From: "Michael Kerrisk (man-pages)" <>
To: Christian Brauner <>
Subject: Re: [PATCH] clone.2: add CLONE_PIDFD entry
Date: Thu, 19 Sep 2019 06:43:56 +0200
Message-ID: <> (raw)
In-Reply-To: <>

Hello Christian,

A tweak to one point from my mail of a few minutes ago...

On 9/19/19 6:04 AM, Michael Kerrisk (man-pages) wrote:

> A further question... We now have three ways of getting a
> process file descriptor [*]:
> open() of /proc/PID
> pidfd_open()
> clone()/clone3() with CLONE_PIDFD
> I thought the FD was supposed to be equivalent in all three cases.
> However, if I try (on kernel 5.3) poll() an FD returned by opening
> /proc/PID, poll() tells me POLLNVAL for the FD. Is that difference
> intentional? (I am guessing it is not.)

Okay -- I made a misstep there, although my question remains.
I got the POLLNVAL return because I had opened /proc/PID with
the O_PATH flag. When I open /proc/PID with O_RDONLY, then poll()
on the FD always returns immediately (i.e., before the target
process has terminated), and the returned events are



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:

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Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2019-09-11  8:58 ` Michael Kerrisk (man-pages)
2019-09-16  7:40   ` Christian Brauner
2019-09-18  6:49     ` Michael Kerrisk (man-pages)
2019-09-18  7:14       ` Christian Brauner
2019-09-19  4:04         ` Michael Kerrisk (man-pages)
2019-09-19  4:43           ` Michael Kerrisk (man-pages) [this message]
2019-09-19  6:47           ` Christian Brauner
2019-09-23  8:11             ` Michael Kerrisk (man-pages)
2019-09-23 14:13               ` Christian Brauner

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