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.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.TH netlink 3 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
netlink \- Netlink macros
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <asm/types.h>
.B #include <linux/netlink.h>
.P
.BI "int NLMSG_ALIGN(size_t " size );
.BI "int NLMSG_LENGTH(size_t " size );
.BI "int NLMSG_SPACE(size_t " size );
.BI "void *NLMSG_DATA(struct nlmsghdr *" nlh );
.BI "struct nlmsghdr *NLMSG_NEXT(struct nlmsghdr *" nlh ", int " size );
.BI "int NLMSG_OK(struct nlmsghdr *" nlh ", int " size );
.BI "int NLMSG_PAYLOAD(struct nlmsghdr *" nlh ", int " size );
.fi
.SH DESCRIPTION
.I <linux/netlink.h>
defines several standard macros to access or create a netlink datagram.
They are similar in spirit to the macros defined in
.BR cmsg (3)
for auxiliary data.
The buffer passed to and from a netlink socket should
be accessed using only these macros.
.TP
.BR NLMSG_ALIGN ()
Round the size of a netlink message up to align it properly.
.TP
.BR NLMSG_LENGTH ()
Given the payload size,
.IR size ,
this macro returns the aligned size to store in the
.I nlmsg_len
field of the
.IR nlmsghdr .
.TP
.BR NLMSG_SPACE ()
Return the number of bytes that a netlink message with payload of
.I size
would occupy.
.TP
.BR NLMSG_DATA ()
Return a pointer to the payload associated with the passed
.IR nlmsghdr .
.TP
.\" this is bizarre, maybe the interface should be fixed.
.BR NLMSG_NEXT ()
Get the next
.I nlmsghdr
in a multipart message.
The caller must check if the current
.I nlmsghdr
didn't have the
.B NLMSG_DONE
set\[em]this function doesn't return NULL on end.
The
.I size
argument is an lvalue containing the remaining size
of the message buffer.
This macro decrements it by the size of the message header.
.TP
.BR NLMSG_OK ()
Return true if the netlink message is not truncated and
is in a form suitable for parsing.
.TP
.BR NLMSG_PAYLOAD ()
Return the size of the payload associated with the
.IR nlmsghdr .
.SH VERSIONS
It is often better to use netlink via
.I libnetlink
than via the low-level kernel interface.
.SH STANDARDS
Linux.
.SH SEE ALSO
.BR libnetlink (3),
.BR netlink (7)
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