sendmmsg(2)

1SENDMMSG(2)                Linux Programmer's Manual               SENDMMSG(2)
2
3
4

NAME

6       sendmmsg - send multiple messages on a socket
7

SYNOPSIS

9       #define _GNU_SOURCE
10       #include <sys/socket.h>
11
12       int sendmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
13                    unsigned int flags);
14

DESCRIPTION

16       The  sendmmsg()  system  call is an extension of sendmsg(2) that allows
17       the caller to transmit multiple messages on a  socket  using  a  single
18       system call.  (This has performance benefits for some applications.)
19
20       The  sockfd argument is the file descriptor of the socket on which data
21       is to be transmitted.
22
23       The msgvec argument is a pointer to an  array  of  mmsghdr  structures.
24       The size of this array is specified in vlen.
25
26       The mmsghdr structure is defined in <sys/socket.h> as:
27
28           struct mmsghdr {
29               struct msghdr msg_hdr;  /* Message header */
30               unsigned int  msg_len;  /* Number of bytes transmitted */
31           };
32
33       The  msg_hdr  field  is a msghdr structure, as described in sendmsg(2).
34       The msg_len field is used to return the number of bytes sent  from  the
35       message  in  msg_hdr  (i.e., the same as the return value from a single
36       sendmsg(2) call).
37
38       The flags argument contains flags ORed together.   The  flags  are  the
39       same as for sendmsg(2).
40
41       A  blocking  sendmmsg() call blocks until vlen messages have been sent.
42       A nonblocking call sends as many messages as possible (up to the  limit
43       specified by vlen) and returns immediately.
44
45       On return from sendmmsg(), the msg_len fields of successive elements of
46       msgvec are updated to contain the number of bytes transmitted from  the
47       corresponding msg_hdr.  The return value of the call indicates the num‐
48       ber of elements of msgvec that have been updated.
49

RETURN VALUE

51       On success, sendmmsg() returns the number of messages sent from msgvec;
52       if  this  is  less than vlen, the caller can retry with a further send‐
53       mmsg() call to send the remaining messages.
54
55       On error, -1 is returned, and errno is set to indicate the error.
56

ERRORS

58       Errors are as for sendmsg(2).  An error is returned only  if  no  data‐
59       grams could be sent.
60

VERSIONS

62       The  sendmmsg()  system  call was added in Linux 3.0.  Support in glibc
63       was added in version 2.14.
64

CONFORMING TO

66       sendmmsg() is Linux-specific.
67

NOTES

69       The value specified in vlen is capped to UIO_MAXIOV (1024).
70

EXAMPLE

72       The example below uses sendmmsg() to send onetwo and three in two  dis‐
73       tinct  UDP  datagrams using one system call.  The contents of the first
74       datagram originates from a pair of buffers.
75
76       #define _GNU_SOURCE
77       #include <netinet/ip.h>
78       #include <stdio.h>
79       #include <stdlib.h>
80       #include <string.h>
81       #include <sys/types.h>
82       #include <sys/socket.h>
83
84       int
85       main(void)
86       {
87           int sockfd;
88           struct sockaddr_in sa;
89           struct mmsghdr msg[2];
90           struct iovec msg1[2], msg2;
91           int retval;
92
93           sockfd = socket(AF_INET, SOCK_DGRAM, 0);
94           if (sockfd == -1) {
95               perror("socket()");
96               exit(EXIT_FAILURE);
97           }
98
99           sa.sin_family = AF_INET;
100           sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
101           sa.sin_port = htons(1234);
102           if (connect(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
103               perror("connect()");
104               exit(EXIT_FAILURE);
105           }
106
107           memset(msg1, 0, sizeof(msg1));
108           msg1[0].iov_base = "one";
109           msg1[0].iov_len = 3;
110           msg1[1].iov_base = "two";
111           msg1[1].iov_len = 3;
112
113           memset(&msg2, 0, sizeof(msg2));
114           msg2.iov_base = "three";
115           msg2.iov_len = 5;
116
117           memset(msg, 0, sizeof(msg));
118           msg[0].msg_hdr.msg_iov = msg1;
119           msg[0].msg_hdr.msg_iovlen = 2;
120
121           msg[1].msg_hdr.msg_iov = &msg2;
122           msg[1].msg_hdr.msg_iovlen = 1;
123
124           retval = sendmmsg(sockfd, msg, 2, 0);
125           if (retval == -1)
126               perror("sendmmsg()");
127           else
128               printf("%d messages sent\n", retval);
129
130           exit(0);
131       }
132

COLOPHON

137       This page is part of release 3.53 of the Linux  man-pages  project.   A
138       description  of  the project, and information about reporting bugs, can
139       be found at http://www.kernel.org/doc/man-pages/.
140
141
142
143Linux                             2012-12-16                       SENDMMSG(2)