sendmmsg.2: Add example program for sendmmsg()

The example uses sendmmsg() to send out a string "onetwo"
on a first datagram, where both halves originate from
distinct buffers and a second datagram contains "three",
coming from a single buffer.

Tested with netcat listening:
root@ubuntu:~# nc -l -u -p 1234
onetwothree

And tcpdump peeking:
root@ubuntu:~# tcpdump -c 2 -s 0 -X -ni lo tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 65535 bytes
18:45:16.632134 IP 127.0.0.1.34715 > 127.0.0.1.1234: UDP, length 6
     0x0000:  4500 0022 c21c 4000 4011 7aac 7f00 0001  E.."..@.@.z.....
     0x0010:  7f00 0001 879b 04d2 000e fe21 6f6e 6574  ...........!onet
     0x0020:  776f                                     wo
18:45:16.633267 IP 127.0.0.1.34715 > 127.0.0.1.1234: UDP, length 5
     0x0000:  4500 0021 c21d 4000 4011 7aac 7f00 0001  E..!..@.@.z.....
     0x0010:  7f00 0001 879b 04d2 000d fe20 7468 7265  ............thre
     0x0020:  65                                       e
2 packets captured
4 packets received by filter
0 packets dropped by kernel

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

1 files changed, 68 insertions, 2 deletions

diff --git a/man2/sendmmsg.2 b/man2/sendmmsg.2
index 12ad3ff0cc..b426b4de09 100644
--- a/man2/sendmmsg.2
+++ b/man2/sendmmsg.2
@@ -23,8 +23,6 @@
 .\" Formatted or processed versions of this manual, if unaccompanied by
 .\" the source, must acknowledge the copyright and authors of this work.
 .\"
-.\" FIXME Adding an example program would improve this page
-.\"
 .TH SENDMMSG 2 2012-02-27 "Linux" "Linux Programmer's Manual"
 .SH NAME
 sendmmsg \- send multiple messages on a socket
@@ -165,6 +163,74 @@ is capped to
 .\"     For error handling an application using sendmmsg needs to retry at
 .\"     the first unsent message, so capping is simpler and requires less
 .\"     application logic than returning EINVAL.
+.SH EXAMPLE
+The example below uses 
+.BR sendmmsg ()
+to send  
+.I onetwo
+and 
+.I three
+in two distinct UDP datagrams using one system call. The contents
+of the first datagram originates from a pair of buffers.
+
+.nf
+#define _GNU_SOURCE
+#include <netinet/ip.h> 
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+
+int 
+main()
+{
+    int sockfd;
+    struct sockaddr_in sa;
+    struct mmsghdr msg[2];
+    struct iovec msg1[2], msg2;
+    int retval;
+
+    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
+    if (sockfd == \-1) {
+        perror("socket()");
+        exit(EXIT_FAILURE);
+    }
+
+    sa.sin_family = AF_INET;             
+    sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);  
+    sa.sin_port = htons(1234);  
+    if (connect(sockfd, (struct sockaddr *) &sa, sizeof(sa))) {
+        perror("connect()");
+        exit(EXIT_FAILURE);
+    }
+
+    memset(msg1, 0, sizeof(msg1)); 
+    msg1[0].iov_base = "one";
+    msg1[0].iov_len = 3;
+    msg1[1].iov_base = "two";
+    msg1[1].iov_len = 3;
+
+    memset(&msg2, 0, sizeof(msg2));
+    msg2.iov_base = "three";
+    msg2.iov_len = 5;
+
+    memset(msg, 0, sizeof(msg));
+    msg[0].msg_hdr.msg_iov = msg1;
+    msg[0].msg_hdr.msg_iovlen = 2;
+
+    msg[1].msg_hdr.msg_iov = &msg2;
+    msg[1].msg_hdr.msg_iovlen = 1;
+
+    retval = sendmmsg(sockfd, msg, 2, 0);
+    if (retval == -1)
+        perror("sendmmsg()");
+    else
+        printf("%d messages sent\\n", retval);
+
+    exit(0);
+}
+.fi
 .SH SEE ALSO
 .BR recvmmsg (2),
 .BR sendmsg (2),