Message-ID: <90399dee-53d8-a82c-3871-9ec8f94601ce@gmail.com>
Date:   Mon, 23 Sep 2019 11:11:53 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     mtk.manpages@...il.com, Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: For review: pidfd_open(2) manual page

Hello Christian and all,

Below, I have the rendered version of the current draft of
the pidfd_open(2) manual page that I have written.
The page source can be found in a Git branch at:
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_pidfd

I would be pleased to receive corrections and notes on any
details that should be added. (For example, are there error
cases that I have missed?)

Would you be able to review please?

Thanks,

Michael


NAME
       pidfd_open - obtain a file descriptor that refers to a process

SYNOPSIS
       int pidfd_open(pid_t pid, unsigned int flags);

DESCRIPTION
       The  pidfd_open()  system creates a file descriptor that refers to
       the process whose PID is specified in pid.  The file descriptor is
       returned  as the function result; the close-on-exec flag is set on
       the file descriptor.

       The flags argument is reserved for  future  use;  currently,  this
       argument must be specified as 0.

RETURN VALUE
       On  success,  pidfd_open()  returns a nonnegative file descriptor.
       On success, -1 is returned and errno is set to indicate the  cause
       of the error.

ERRORS
       EINVAL flags is not 0.

       EINVAL pid is not valid.

       ESRCH  The process specified by pid does not exist.

VERSIONS
       pidfd_open() first appeared in Linux 5.3.

CONFORMING TO
       pidfd_open() is Linux specific.

NOTES
       Currently, there is no glibc wrapper for this system call; call it
       using syscall(2).

       The pidfd_send_signal(2) system call can be used to send a  signal
       to the process referred to by a PID file descriptor.

       A  PID  file descriptor can be monitored using poll(2), select(2),
       and epoll(7).  When the process that it refers to terminates,  the
       file descriptor indicates as readable.  Note, however, that in the
       current implementation, nothing can be read from the file descrip‐
       tor.

       The  pidfd_open()  system call is the preferred way of obtaining a
       PID file descriptor.  The alternative is to obtain a file descrip‐
       tor by opening a /proc/[pid] directory.  However, the latter tech‐
       nique is possible only if the proc(5) file system is mounted; fur‐
       thermore,  the  file  descriptor  obtained in this way is not pol‐
       lable.

       See also the discussion of the CLONE_PIDFD flag in clone(2).

EXAMPLE
       The program below opens a PID  file  descriptor  for  the  process
       whose PID is specified as its command-line argument.  It then mon‐
       itors the file descriptor for readability (POLLIN) using  poll(2).
       When  the  process  with  the specified by PID terminates, poll(2)
       returns, and indicates that the file descriptor is readable.

   Program source

       #define _GNU_SOURCE
       #include <sys/syscall.h>
       #include <unistd.h>
       #include <poll.h>
       #include <stdlib.h>
       #include <stdio.h>

       #ifndef __NR_pidfd_open
       #define __NR_pidfd_open 434
       #endif

       static
       int pidfd_open(pid_t pid, unsigned int flags)
       {
           return syscall(__NR_pidfd_open, pid, flags);
       }

       int
       main(int argc, char *argv[])
       {
           struct pollfd pollfd;
           int pidfd, ready;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s <pid>\n", argv[0]);
               exit(EXIT_SUCCESS);
           }

           pidfd = pidfd_open(atoi(argv[1]), 0);
           if (pidfd == -1) {
               perror("pidfd_open");
               exit(EXIT_FAILURE);
           }

           pollfd.fd = pidfd;
           pollfd.events = POLLIN;

           ready = poll(&pollfd, 1, -1);
           if (ready == -1) {
               perror("poll");
               exit(EXIT_FAILURE);
           }

           printf("Events (0x%x): POLLIN is %sset\n", pollfd.revents,
                   (pollfd.revents & POLLIN) ? "" : "not ");

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       clone(2),  kill(2),  pidfd_send_signal(2),   poll(2),   select(2),
       epoll(7)


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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