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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="sd_pidfd_get_inode_id"
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_pidfd_get_inode_id</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>sd_pidfd_get_inode_id</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_pidfd_get_inode_id</refname>
<refpurpose>Acquire the 64-bit inode ID of a PID file descriptor (PIDFD)</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-daemon.h></funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>sd_pidfd_get_inode_id</function></funcdef>
<paramdef>int <parameter>pidfd</parameter></paramdef>
<paramdef>uint64_t *<parameter>ret</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><function>sd_pidfd_get_inode_id()</function> may be invoked to acquire the 64-bit inode ID of
a PID file descriptor (PIDFD), which can be used to reliably identify a process for the current boot.</para>
<para>As a typical example, the service manager sets <varname>$MAINPIDFDID</varname> and <varname>$MANAGERPIDFDID</varname>
environment variables to the inode IDs of the service main process and the service manager itself, respectively,
if such functionality is supported by the kernel.</para>
<para>On 64-bit architectures, the inode ID can be directly obtained via a call to
<citerefentry project='man-pages'><refentrytitle>fstat</refentrytitle><manvolnum>2</manvolnum></citerefentry>
on a given pidfd. However, on 32-bit architectures <structname>struct stat</structname>'s .st_ino
field is also 32-bit, which similar to PIDs is subject to reuse. Therefore, a second mechanism leveraging
<citerefentry project='man-pages'><refentrytitle>name_to_handle_at</refentrytitle><manvolnum>2</manvolnum></citerefentry>
has been added to kernel in v6.14. This helper is added to simplify downstream handling of pidfd/pidfs internals.
</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>On success, the function returns 0 or a positive integer. On failure, a negative errno-style
error code is returned.</para>
<refsect2>
<title>Errors</title>
<para>Returned errors may indicate the following problems:</para>
<variablelist>
<varlistentry>
<term><constant>-EOPNOTSUPP</constant></term>
<listitem><para>The stable PIDFD inode ID is not supported by the running kernel, or the system
is 32-bit and <function>name_to_handle_at()</function> is unavailable.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EBADF</constant></term>
<listitem><para>The specified file descriptor is invalid, or is not a PIDFD.</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
</refsect1>
<refsect1>
<title>Notes</title>
<xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
</refsect1>
<refsect1>
<title>History</title>
<para><function>sd_pidfd_get_inode_id()</function> was added in version 258.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para><simplelist type="inline">
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_notify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
</simplelist></para>
</refsect1>
</refentry>
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