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Subject[RESEND] RFC: pidfd_getfd(2) manual page
Date
[No response on my mail of a week ago, so I try again; the page
text is unchanged since the draft sent out on 31 March]

Hello Sargun et al.

I've taken a shot at writing a manual page for pidfd_getfd().
I would be happy to receive comments, suggestions for
improvements, etc. The text is as follows (the groff source
is at the foot of this mail):

NAME
pidfd_getfd - obtain a duplicate of another process's file
descriptor

SYNOPSIS
int pidfd_getfd(int pidfd, int targetfd, unsigned int flags);

DESCRIPTION
The pidfd_getfd() system call allocates a new file descriptor in
the calling process. This new file descriptor is a duplicate of
an existing file descriptor, targetfd, in the process referred to
by the PID file descriptor pidfd.

The duplicate file descriptor refers to the same open file
description (see open(2)) as the original file descriptor in the
process referred to by pidfd. The two file descriptors thus share
file status flags and file offset. Furthermore, operations on the
underlying file object (for example, assigning an address to a
socket object using bind(2)) can be equally be performed via the
duplicate file descriptor.

The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) is set on the
file descriptor returned by pidfd_getfd().

The flags argument is reserved for future use. Currently, it must
be specified as 0.

Permission to duplicate another process's file descriptor is gov‐
erned by a ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check
(see ptrace(2)).

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

ERRORS
EBADF pidfd is not a valid PID file descriptor.

EBADF targetfd is not an open file descriptor in the process
referred to by pidfd.

EINVAL flags is not 0.

EMFILE The per-process limit on the number of open file descrip‐
tors has been reached (see the description of RLIMIT_NOFILE
in getrlimit(2)).

ENFILE The system-wide limit on the total number of open files has
been reached.

ESRCH The process referred to by pidfd does not exist (i.e., it
has terminated and been waited on).

VERSIONS
pidfd_getfd() first appeared in Linux 5.6.

CONFORMING TO
pidfd_getfd() is Linux specific.

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

For a description of PID file descriptors, see pidfd_open(2).

SEE ALSO
clone3(2), kcmp(2), pidfd_open(2)

Cheers,

Michael

.\" Copyright (c) 2020 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH PIDFD_GETFD 2 2020-03-31 "Linux" "Linux Programmer's Manual"
.SH NAME
pidfd_getfd \- obtain a duplicate of another process's file descriptor
.SH SYNOPSIS
.nf
.BI "int pidfd_getfd(int " pidfd ", int " targetfd ", unsigned int " flags );
.fi
.SH DESCRIPTION
The
.BR pidfd_getfd ()
system call allocates a new file descriptor in the calling process.
This new file descriptor is a duplicate of an existing file descriptor,
.IR targetfd ,
in the process referred to by the PID file descriptor
.IR pidfd .
.PP
The duplicate file descriptor refers to the same open file description (see
.BR open (2))
as the original file descriptor in the process referred to by
.IR pidfd .
The two file descriptors thus share file status flags and file offset.
Furthermore, operations on the underlying file object
(for example, assigning an address to a socket object using
.BR bind (2))
can be equally be performed via the duplicate file descriptor.
.PP
The close-on-exec flag
.RB ( FD_CLOEXEC ;
see
.BR fcntl (2))
is set on the file descriptor returned by
.BR pidfd_getfd ().
.PP
The
.I flags
argument is reserved for future use.
Currently, it must be specified as 0.
.PP
Permission to duplicate another process's file descriptor
is governed by a ptrace access mode
.B PTRACE_MODE_ATTACH_REALCREDS
check (see
.BR ptrace (2)).
.SH RETURN VALUE
On success,
.BR pidfd_getfd ()
returns a nonnegative file descriptor.
On error, \-1 is returned and
.I errno
is set to indicate the cause of the error.
.SH ERRORS
.TP
.B EBADF
.I pidfd
is not a valid PID file descriptor.
.TP
.B EBADF
.I targetfd
is not an open file descriptor in the process referred to by
.IR pidfd .
.BR
.TP
.B EINVAL
.I flags
is not 0.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached
(see the description of
.BR RLIMIT_NOFILE
in
.BR getrlimit (2)).
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ESRCH
The process referred to by
.I pidfd
does not exist
(i.e., it has terminated and been waited on).
.SH VERSIONS
.BR pidfd_getfd ()
first appeared in Linux 5.6.
.\" commit 8649c322f75c96e7ced2fec201e123b2b073bf09
.SH CONFORMING TO
.BR pidfd_getfd ()
is Linux specific.
.SH NOTES
Currently, there is no glibc wrapper for this system call; call it using
.BR syscall (2).
.PP
For a description of PID file descriptors, see
.BR pidfd_open (2).
.SH SEE ALSO
.BR clone3 (2),
.BR kcmp (2),
.BR pidfd_open (2)

--
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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