Re: [PATCH 1/4] sock_diag.7: New page documenting NETLINK_SOCK_DIAG interface

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hello Dmitry,

On 12/07/2016 10:36 AM, Dmitry V. Levin wrote:
> Hi,
> 
> On Mon, Apr 04, 2016 at 08:34:47PM +1200, Michael Kerrisk (man-pages) wrote:
>> Hello Dmitry
>>
>> Thanks for taking the time to work on this!
>>
>> I will probably have more comments, for a future draft, but here are
>> a few initial comments. Could you take a look and send a new draft?
> 
> Unfortunately, the new draft took much longer than I expected,
> but here it is.

Thank you! I have just a few more questions. The page could
still do with some polishing, but it's got a lot of
valuable stuff, and I'll accept it already, but would be
happy if you could look at the new questions, and perhaps
lightly revise the page.

Cheers,

Michael

> .\" Copyright (c) 2016 Pavel Emelyanov <xemul-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
> .\" Copyright (c) 2016 Dmitry V. Levin <ldv-u2l5PoMzF/Vg9hUCZPvPmw@public.gmane.org>
> .\"
> .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
> .\" This is free documentation; you can redistribute it and/or
> .\" modify it under the terms of the GNU General Public License as
> .\" published by the Free Software Foundation; either version 2 of
> .\" the License, or (at your option) any later version.
> .\"
> .\" The GNU General Public License's references to "object code"
> .\" and "executables" are to be interpreted as the output of any
> .\" document formatting or typesetting system, including
> .\" intermediate and printed output.
> .\"
> .\" This manual is distributed in the hope that it will be useful,
> .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
> .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> .\" GNU General Public License for more details.
> .\"
> .\" You should have received a copy of the GNU General Public
> .\" License along with this manual; if not, see
> .\" <http://www.gnu.org/licenses/>.
> .\" %%%LICENSE_END
> .TH SOCK_DIAG 7 2016-12-06 "Linux" "Linux Programmer's Manual"
> .SH NAME
> sock_diag \- obtaining information about sockets
> .SH SYNOPSIS
> .nf
> .B #include <sys/socket.h>
> .B #include <linux/sock_diag.h>
> .BR "#include <linux/unix_diag.h>" " /* for UNIX domain sockets */"
> .BR "#include <linux/inet_diag.h>" " /* for IPv4 and IPv6 sockets */"
> 
> .BI "diag_socket = socket(AF_NETLINK, " socket_type ", NETLINK_SOCK_DIAG);"
> .fi
> .SH DESCRIPTION
> The sock_diag netlink subsystem provides a mechanism for obtaining
> information about sockets of various protocol families from the kernel.
> This subsystem can be used to obtain information about individual
> sockets or request a list of sockets.
> 
> In the request the caller can specify an additional information it would
> like to obtain about the socket, e.g. memory information or
> family-specific stuff.
> 
> When requesting a list of sockets, the caller can specify filters that
> would be applied by the kernel to select a subset of sockets to report.
> For now there is only an ability to filter sockets by state (connected,
> listening, etc.)
> 
> Note that sock_diag reports only those sockets that have a name,
> i.e. either bound explicitly with
> .BR bind (2)
> or auto-bound ones (e.g. connected).  This is the same set of sockets

s/auto-bound ones (e.g. connected/
 /sockets that were automatically bound to an address (e.g., by connect(2)/

> that is available via
> .IR /proc/net/unix ,
> .IR /proc/net/tcp ,
> .IR /proc/net/udp ,
> etc.
> 
> .SS Request
> The request starts with
> .I "struct nlmsghdr"
> header described in
> .BR netlink (7)
> with
> .I nlmsg_type
> field set to
> .BR SOCK_DIAG_BY_FAMILY .
> It is followed by a protocol family specific header that starts with
> a common part shared by all protocol families:
> 
> .in +4n
> .nf
> struct sock_diag_req {
>     __u8 sdiag_family;
>     __u8 sdiag_protocol;
> };
> .fi
> .in
> .PP
> The fields of this structure are as follows:
> .TP
> .I sdiag_family
> A protocol family.  It should be set to the appropriate
> .B AF_*
> constant.
> .TP
> .I sdiag_protocol
> Depends on
> .IR sdiag_family .
> It should be set to the appropriate
> .B IPPROTO_*
> constant for
> .B AF_INET
> and
> .BR AF_INET6,
> and to 0 otherwise.
> .PP
> If
> .I nlmsg_flags
> field of the
> .I "struct nlmsghdr"
> header has
> .BR NLM_F_DUMP
> flag set, it means that a list of sockets is being requested,
> otherwise it is a query about an individual socket.
> 
> .SS Response
> The response starts with
> .I "struct nlmsghdr"
> header and is followed by an array of family-specific objects.
> The array is to be accessed with the standard
> .B NLMSG_*
> macros from
> .BR netlink (3)
> API.
> .PP
> Each object is the NLA (netlink attributes) list that is to be accessed
> with the
> .B RTA_*
> macros from
> .BR rtnetlink (3)
> API.
> 
> .SS UNIX domain sockets
> For UNIX domain sockets the request is represented in the following structure:
> 
> .in +4n
> .nf
> struct unix_diag_req {
>     __u8    sdiag_family;
>     __u8    sdiag_protocol;
>     __u16   pad;
>     __u32   udiag_states;
>     __u32   udiag_ino;
>     __u32   udiag_show;
>     __u32   udiag_cookie[2];
> };
> .fi
> .in
> .PP
> The fields of this structure are as follows:
> .TP
> .I sdiag_family
> The protocol family, it should be set to
> .BR AF_UNIX .
> .PP
> .I sdiag_protocol
> .PD 0
> .TP
> .PD
> .I pad
> These fields should be set to 0.
> .TP
> .I udiag_states
> This is a bit mask that defines a filter of sockets states.
> Only those sockets whose states are in this mask will be reported.
> Ignored when querying for an individual socket.
> Supported values are:
> .PD 0
> .RS
> .IP "" 2
> 1 <<
> .B TCP_ESTABLISHED
> .IP
> 1 <<
> .B TCP_LISTEN
> .RE
> .PD
> .TP
> .I udiag_ino
> This is an inode number when querying for an individual socket.
> Ignored when querying for a list of sockets.
> .TP
> .I udiag_show
> This is a set of flags defining what kind of information to report.
> Each requested kind of information is reported back as a netlink
> attribute as described below:
> .RS
> .IP "" 2
> .B UDIAG_SHOW_NAME
> .RS 4
> The attribute reported in answer to this request is
> .BR UNIX_DIAG_NAME .
> The payload associated with this attribute is the name of the socket
> to which it was bound (a sequence of bytes up to

I think the last two lines should be:

The payload associated with this attribute is the pathname to which
the socket was bound (a sequence of bytes up to

> .B UNIX_PATH_MAX
> length).
> .RE
> .IP "" 2
> .B UDIAG_SHOW_VFS
> .RS 4
> The attribute reported in answer to this request is
> .BR UNIX_DIAG_VFS .
> The payload associated with this attribute is represented in the following
> structure:
> 
> .in +4n
> .nf
> struct unix_diag_vfs {
>     __u32 udiag_vfs_dev;
>     __u32 udiag_vfs_ino;
> };
> .fi
> .in
> 
> The fields of this structure are as follows:
> .PD 0
> .RS 2
> .IP "" 2
> .I udiag_vfs_dev
> The device number of the corresponding on-disk socket node.

s/node/inode/ ?

> .IP
> .I udiag_vfs_ino
> The inode number of the corresponding on-disk socket node.

s/node/inode/ ?

> .RE
> .PD
> .RE
> .IP
> .B UDIAG_SHOW_PEER
> .RS 4
> The attribute reported in answer to this request is
> .BR UNIX_DIAG_PEER .
> The payload associated with this attribute is a __u32 value
> which is the peer's inode number.
> This attribute is reported for connected sockets only.
> .RE
> .IP
> .B UDIAG_SHOW_ICONS
> .RS 4
> The attribute reported in answer to this request is
> .BR UNIX_DIAG_ICONS .
> The payload associated with this attribute is an array of __u32 values
> which are inode numbers of sockets that has passed the
> .BR connect (2)
> call, but hasn't been processed with
> .BR accept (2)
> yet.  This attribute is reported for listening sockets only.
> .RE
> .IP
> .B UDIAG_SHOW_RQLEN
> .RS 4
> The attribute reported in answer to this request is
> .BR UNIX_DIAG_RQLEN .
> The payload associated with this attribute is represented in the following
> structure:
> 
> .in +4n
> .nf
> struct unix_diag_rqlen {
>     __u32 udiag_rqueue;
>     __u32 udiag_wqueue;
> };
> .fi
> .in
> 
> The fields of this structure are as follows:
> .PD 0
> .RS 2
> .IP "" 2
> .I udiag_rqueue
> .RS 6
> .IP "listening sockets:" 2
> The number of pending connections which equals to
> .B UNIX_DIAG_ICONS
> array length.

The meaning of the last sentence isn't clear to me.
Can you elaborate this text?

> .IP "established sockets:"
> The amount of data in incoming queue.
> .RE
> .IP
> .I udiag_wqueue
> .RS 6
> .IP "listening sockets:" 2
> The backlog length which equals to the value passed as the second argument to
> .BR listen (2).
> .IP "established sockets:"
> The amount of memory available for sending.
> .RE
> .RE
> .PD
> .RE
> .IP
> .B UDIAG_SHOW_MEMINFO
> .RS 4
> The attribute reported in answer to this request is
> .BR UNIX_DIAG_MEMINFO .
> The payload associated with this attribute is an array of __u32 values
> described below in "Socket memory information" subsection.
> .RE
> .IP
> .RE
> .RS
> The following attributes are reported back without any specific request:
> .IP "" 2
> .BR UNIX_DIAG_SHUTDOWN .
> The payload associated with this attribute is __u8 value which represents
> bits of
> .BR shutdown (2)
> state.
> .RE
> .TP
> .I udiag_cookie
> This is an array of opaque identifiers that could be used along with
> .I udiag_ino
> to specify an individual socket.  It is ignored when querying for a list
> of sockets, as well as when all its elements are set to \-1.
> .PP
> The response to a query for UNIX domain sockets is represented as an array of
> 
> .in +4n
> .nf
> struct unix_diag_msg {
>     __u8    udiag_family;
>     __u8    udiag_type;
>     __u8    udiag_state;
>     __u8    pad;
>     __u32   udiag_ino;
>     __u32   udiag_cookie[2];
> };
> .fi
> .in
> 
> followed by netlink attributes.
> .PP
> The fields of this structure are as follows:
> .TP
> .I udiag_family
> This field has the same meaning as in
> .IR "struct unix_diag_req" .
> .TP
> .I udiag_type
> This is set to one of the following constants:
> .PD 0
> .RS
> .IP "" 2
> .B SOCK_PACKET
> .IP
> .B SOCK_STREAM
> .IP
> .B SOCK_SEQPACKET
> .RE
> .PD
> .TP
> .I udiag_state
> This is set to one of the following constants:
> .PD 0
> .RS
> .IP "" 2
> .B TCP_LISTEN
> .IP
> .B TCP_ESTABLISHED
> .RE
> .PD
> .TP
> .I pad
> This field is set to 0.
> .TP
> .I udiag_ino
> This is the socket inode number.
> .TP
> .I udiag_cookie
> This is an array of opaque identifiers that could be used in subsequent
> queries.
> 
> .SS IPv4 and IPv6 sockets
> For IPv4 and IPv6 sockets the request is represented in the following structure:
> 
> .in +4n
> .nf
> struct inet_diag_req_v2 {
>     __u8    sdiag_family;
>     __u8    sdiag_protocol;
>     __u8    idiag_ext;
>     __u8    pad;
>     __u32   idiag_states;
>     struct inet_diag_sockid id;
> };
> .fi
> .in
> 
> where
> .I "struct inet_diag_sockid"
> is defined as follows:
> 
> .in +4n
> .nf
> struct inet_diag_sockid {
>     __be16  idiag_sport;
>     __be16  idiag_dport;
>     __be32  idiag_src[4];
>     __be32  idiag_dst[4];
>     __u32   idiag_if;
>     __u32   idiag_cookie[2];
> };
> .fi
> .in
> .PP
> The fields of
> .I "struct inet_diag_req_v2"
> are as follows:
> .TP
> .I sdiag_family
> This should be set to either
> .B AF_INET
> or
> .B AF_INET6
> for
> .B IPv4
> or
> .B IPv6
> sockets respectively.
> .TP
> .I sdiag_protocol
> This should be set to one of the following constants:
> .PD 0
> .RS
> .IP "" 2
> .B IPPROTO_TCP
> .IP
> .B IPPROTO_UDP
> .IP
> .B IPPROTO_UDPLITE
> .RE
> .PD
> .TP
> .I idiag_ext
> This is a set of flags defining what kind of extended information to report.
> Each requested kind of information is reported back as a netlink attribute
> as described below:
> .RS
> .TP
> .B INET_DIAG_TOS
> The payload associated with this attribute is a __u8 value
> which is the TOS of the socket.
> .TP
> .B INET_DIAG_TCLASS
> The payload associated with this attribute is a __u8 value
> which is the TClass of the socket.  IPv6 sockets only.
> For LISTEN and CLOSE sockets this is followed by
> .B INET_DIAG_SKV6ONLY
> attribute with associated __u8 payload value meaning whether the socket
> is IPv6-only or not.
> .TP
> .B INET_DIAG_MEMINFO
> The payload associated with this attribute is represented in the following
> structure:
> 
> .in +4n
> .nf
> struct inet_diag_meminfo {
>     __u32 idiag_rmem;
>     __u32 idiag_wmem;
>     __u32 idiag_fmem;
>     __u32 idiag_tmem;
> };
> .fi
> .in
> 
> The fields of this structure are as follows:
> .RS
> .TP 12
> .I idiag_rmem
> The amount of data in the receive queue.
> .TP
> .I idiag_wmem
> The amount of data that is queued by TCP but not yet sent.
> .TP
> .I idiag_fmem
> The amount of memory scheduled for future use (TCP only).
> .TP
> .I idiag_tmem
> The amount of data in send queue.
> .RE
> .TP
> .B INET_DIAG_SKMEMINFO
> The payload associated with this attribute is an array of __u32 values
> described below in "Socket memory information" subsection.
> .TP
> .B INET_DIAG_INFO
> The payload associated with this attribute is protocol specific.
> For TCP sockets it is an object of type
> .IR "struct tcp_info" .
> .TP
> .B INET_DIAG_CONG
> The payload associated with this attribute is a string that describes the
> congestion control algorithm used.  For TCP sockets only.
> .RE
> .TP
> .I pad
> This should be set to 0.
> .TP
> .I idiag_states
> This is a bit mask that defines a filter of sockets states.
> Only those sockets whose states are in this mask will be reported.
> Ignored when querying for an individual socket.
> .TP
> .I id
> This is a socket id object that is used in dump requests, in queries
> about individual sockets, and is reported back in each response.
> Unlike UNIX domain sockets, IPv4 and IPv6 sockets are identified
> using addresses and ports.  All values are in network byte order.
> .PP
> The fields of
> .I "struct inet_diag_sockid"
> are as follows:
> .TP
> .I idiag_sport
> The source port.
> .TP
> .I idiag_dport
> The destination port.
> .TP
> .I idiag_src
> The source address.
> .TP
> .I idiag_dst
> The destination address.
> .TP
> .I idiag_if
> The interface number the socket is bound to.
> .TP
> .I idiag_cookie
> This is an array of opaque identifiers that could be used along with
> other fields of this structure to specify an individual socket.
> It is ignored when querying for a list of sockets, as well as
> when all its elements are set to \-1.
> .PP
> The response to a query for IPv4 or IPv6 sockets is represented as an array of
> 
> .in +4n
> .nf
> struct inet_diag_msg {
>     __u8    idiag_family;
>     __u8    idiag_state;
>     __u8    idiag_timer;
>     __u8    idiag_retrans;
> 
>     struct inet_diag_sockid id;
> 
>     __u32   idiag_expires;
>     __u32   idiag_rqueue;
>     __u32   idiag_wqueue;
>     __u32   idiag_uid;
>     __u32   idiag_inode;
> };
> .fi
> .in
> 
> followed by netlink attributes.
> .PP
> The fields of this structure are as follows:
> .TP
> .I idiag_family
> This is the same field as in
> .IR "struct inet_diag_req_v2" .
> .TP
> .I idiag_state
> This denotes socket state as in
> .IR "struct inet_diag_req_v2" .
> .TP
> .I idiag_timer
> For TCP sockets, this field describes the type of timer that is currently
> active for the socket.  It is set to one of the following constants:
> .RS
> .TP
> .B 0
> no timer is active
> .TP
> .B 1
> a retransmit timer
> .TP
> .B 2
> a keep-alive timer
> .TP
> .B 3
> a TIME_WAIT timer
> .TP
> .B 4
> a zero window probe timer
> .RE
> .IP
> .RS
> For non-TCP sockets this field is set to 0.
> .RE
> .TP
> .I idiag_retrans
> For
> .I idiag_timer
> values 1, 2, and 4 this field contains the number of retransmits.  For other
> .I idiag_timer
> values this field is set to 0.
> .TP
> .I idiag_expires
> For TCP sockets that have an active timer this field describes its expiration
> time in milliseconds.  For other sockets this field is set to 0.
> .TP
> .I idiag_rqueue
> .RS 7
> .IP "listening sockets:" 2
> The number of pending connections.
> .IP "other sockets:"
> The amount of data in incoming queue.
> .RE
> .TP
> .I idiag_wqueue
> .RS 7
> .IP "listening sockets:" 2
> The backlog length.
> .IP "other sockets:"
> The amount of memory available for sending.
> .RE
> .TP
> .I idiag_uid
> This is the socket owner UID.
> .TP
> .I idiag_inode
> This is the socket inode number.
> 
> .SS Socket memory information
> The payload associated with
> .B UNIX_DIAG_MEMINFO
> and
> .BR INET_DIAG_SKMEMINFO
> netlink attributes is an array of the following __u32 values:
> .TP
> .B SK_MEMINFO_RMEM_ALLOC
> The amount of data in receive queue.
> .TP
> .B SK_MEMINFO_RCVBUF
> The receive socket buffer as set by
> .BR SO_RCVBUF .
> .TP
> .B SK_MEMINFO_WMEM_ALLOC
> The amount of data in send queue.
> .TP
> .B SK_MEMINFO_SNDBUF
> The send socket buffer as set by
> .BR SO_SNDBUF .
> .TP
> .B SK_MEMINFO_FWD_ALLOC
> The amount of memory scheduled for future use (TCP only).
> .TP
> .B SK_MEMINFO_WMEM_QUEUED
> The amount of data queued by TCP, but not yet sent.
> .TP
> .B SK_MEMINFO_OPTMEM
> The amount of memory allocated for socket's service needs (e.g. socket
> filter).
> .TP
> .B SK_MEMINFO_BACKLOG
> The amount of packets in the backlog (not yet processed).
> .SH CONFORMING TO
> The NETLINK_SOCK_DIAG API is Linux-specific.
> .SH VERSIONS
> .B NETLINK_INET_DIAG
> was introduced in Linux 2.6.14 and supported
> .B AF_INET
> and
> .B AF_INET6
> sockets only.  In Linux 3.3 it was renamed to
> .B NETLINK_SOCK_DIAG
> and extended to support
> .B AF_UNIX
> sockets.
> .PP
> .B UNIX_DIAG_MEMINFO
> and
> .BR INET_DIAG_SKMEMINFO
> were introduced in Linux 3.6.
> .SH EXAMPLE
> The following example program prints inode number, peer's inode number,
> and name of all UNIX domain sockets in the current namespace.
> 
> .nf
> #include <errno.h>
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/socket.h>
> #include <sys/un.h>
> #include <linux/netlink.h>
> #include <linux/rtnetlink.h>
> #include <linux/sock_diag.h>
> #include <linux/unix_diag.h>

For memcpy(), please add

#include <string.h>

> 
> static int
> send_query(int fd)
> {
>     struct sockaddr_nl nladdr = {
>         .nl_family = AF_NETLINK
>     };
>     struct
>     {
>         struct nlmsghdr nlh;
>         struct unix_diag_req udr;
>     } req = {
>         .nlh = {
>             .nlmsg_len = sizeof(req),
>             .nlmsg_type = SOCK_DIAG_BY_FAMILY,
>             .nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP
>         },
>         .udr = {
>             .sdiag_family = AF_UNIX,
>             .udiag_states = \-1,
>             .udiag_show = UDIAG_SHOW_NAME | UDIAG_SHOW_PEER
>         }
>     };
>     struct iovec iov = {
>         .iov_base = &req,
>         .iov_len = sizeof(req)
>     };
>     struct msghdr msg = {
>         .msg_name = (void *) &nladdr,
>         .msg_namelen = sizeof(nladdr),
>         .msg_iov = &iov,
>         .msg_iovlen = 1
>     };
> 
>     for (;;) {
>         if (sendmsg(fd, &msg, 0) < 0) {
>             if (errno == EINTR)
>                 continue;
>             perror("sendmsg");
>             return \-1;
>         }
>         return 0;
>     }
> }
> 
> static int
> print_diag(const struct unix_diag_msg *diag, unsigned int len)
> {
>     if (len < NLMSG_LENGTH(sizeof(*diag))) {
>         fputs("short response\\n", stderr);
>         return \-1;
>     }
>     if (diag\->udiag_family != AF_UNIX) {
>         fprintf(stderr, "unexpected family %u\\n", diag\->udiag_family);
>         return \-1;
>     }
> 
>     struct rtattr *attr;
>     unsigned int rta_len = len \- NLMSG_LENGTH(sizeof(*diag));
>     unsigned int peer = 0;
>     size_t path_len = 0;
>     char path[sizeof(((struct sockaddr_un *) 0)\->sun_path) + 1];
> 
>     for (attr = (struct rtattr *) (diag + 1);
>              RTA_OK(attr, rta_len); attr = RTA_NEXT(attr, rta_len)) {
>         switch (attr\->rta_type) {
>             case UNIX_DIAG_NAME:
>                 if (!path_len) {
>                     path_len = RTA_PAYLOAD(attr);
>                     if (path_len > sizeof(path) \- 1)
>                         path_len = sizeof(path) \- 1;
>                     memcpy(path, RTA_DATA(attr), path_len);
>                     path[path_len] = '\\0';
>                 }
>                 break;
>             case UNIX_DIAG_PEER:
>                 if (RTA_PAYLOAD(attr) >= sizeof(peer))
>                     peer = *(unsigned int *) RTA_DATA(attr);
>                 break;
>         }
>     }
> 
>     printf("inode=%u", diag->udiag_ino);
> 
>     if (peer)
>         printf(", peer=%u", peer);
> 
>     if (path_len)
>         printf(", name=%s%s", *path ? "" : "@", *path ? path : path + 1);
> 
>     putchar('\\n');
>     return 0;
> }
> 
> static int
> receive_responses(int fd)
> {
>     long buf[8192 / sizeof(long)];
>     struct sockaddr_nl nladdr = {
>         .nl_family = AF_NETLINK
>     };
>     struct iovec iov = {
>         .iov_base = buf,
>         .iov_len = sizeof(buf)
>     };
>     int flags = 0;
> 
>     for (;;) {
>         struct msghdr msg = {
>             .msg_name = (void *) &nladdr,
>             .msg_namelen = sizeof(nladdr),
>             .msg_iov = &iov,
>             .msg_iovlen = 1
>         };
> 
>         ssize_t ret = recvmsg(fd, &msg, flags);
> 
>         if (ret < 0) {
>             if (errno == EINTR)
>                 continue;
>             perror("recvmsg");
>             return \-1;
>         }
>         if (ret == 0)
>             return 0;
> 
>         const struct nlmsghdr *h = (struct nlmsghdr *) buf;
> 
>         if (!NLMSG_OK(h, ret)) {
>             fputs("!NLMSG_OK\\n", stderr);
>             return \-1;
>         }
>         for (; NLMSG_OK(h, ret); h = NLMSG_NEXT(h, ret)) {
>             if (h\->nlmsg_type == NLMSG_DONE)
>                 return 0;
>             if (h\->nlmsg_type == NLMSG_ERROR) {
>                 const struct nlmsgerr *err = NLMSG_DATA(h);
> 
>                 if (h\->nlmsg_len < NLMSG_LENGTH(sizeof(*err))) {
>                     fputs("NLMSG_ERROR\\n", stderr);
>                 } else {
>                     errno = \-err\->error;
>                     perror("NLMSG_ERROR");
>                 }
>                 return \-1;
>             }
>             if (h\->nlmsg_type != SOCK_DIAG_BY_FAMILY) {
>                 fprintf(stderr, "unexpected nlmsg_type %u\\n",
>                         (unsigned) h\->nlmsg_type);
>                 return \-1;
>             }
> 
>             if (print_diag(NLMSG_DATA(h), h\->nlmsg_len))
>                 return \-1;
>         }
>     }
> }
> 
> int
> main(void)
> {
>     int fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_SOCK_DIAG);
> 
>     if (fd < 0) {
>         perror("socket");
>         return 1;
>     }
> 
>     int ret = send_query(fd) || receive_responses(fd);
> 
>     close(fd);
>     return ret;
> }
> .fi
> .SH SEE ALSO
> .BR netlink (3),
> .BR rtnetlink (3),
> .BR netlink (7),
> .BR tcp (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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