sendmmsg(2)

1SENDMMSG(2)                Linux Programmer's Manual               SENDMMSG(2)
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NAME

6       sendmmsg - send multiple messages on a socket
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SYNOPSIS

9       #define _GNU_SOURCE         /* See feature_test_macros(7) */
10       #include <sys/socket.h>
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12       int sendmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
13                    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.
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23       The msgvec argument is a pointer to an  array  of  mmsghdr  structures.
24       The size of this array is specified in vlen.
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26       The mmsghdr structure is defined in <sys/socket.h> as:
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28           struct mmsghdr {
29               struct msghdr msg_hdr;  /* Message header */
30               unsigned int  msg_len;  /* Number of bytes transmitted */
31           };
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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).
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38       The flags argument contains flags ORed together.   The  flags  are  the
39       same as for sendmsg(2).
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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.
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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.
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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.
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55       On error, -1 is returned, and errno is set to indicate the error.
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ERRORS

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

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

66       sendmmsg() is Linux-specific.
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NOTES

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

72       If an error occurs after at least one message has been sent,  the  call
73       succeeds,  and  returns the number of messages sent.  The error code is
74       lost.  The caller can retry the transmission,  starting  at  the  first
75       failed  message,  but  there  is  no  guarantee  that,  if  an error is
76       returned, it will be the same as the one that was lost on the  previous
77       call.
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EXAMPLES

80       The  example below uses sendmmsg() to send onetwo and three in two dis‐
81       tinct UDP datagrams using one system call.  The contents of  the  first
82       datagram originates from a pair of buffers.
83
84       #define _GNU_SOURCE
85       #include <netinet/ip.h>
86       #include <stdio.h>
87       #include <stdlib.h>
88       #include <string.h>
89       #include <sys/types.h>
90       #include <sys/socket.h>
91
92       int
93       main(void)
94       {
95           int sockfd;
96           struct sockaddr_in addr;
97           struct mmsghdr msg[2];
98           struct iovec msg1[2], msg2;
99           int retval;
100
101           sockfd = socket(AF_INET, SOCK_DGRAM, 0);
102           if (sockfd == -1) {
103               perror("socket()");
104               exit(EXIT_FAILURE);
105           }
106
107           addr.sin_family = AF_INET;
108           addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
109           addr.sin_port = htons(1234);
110           if (connect(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
111               perror("connect()");
112               exit(EXIT_FAILURE);
113           }
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115           memset(msg1, 0, sizeof(msg1));
116           msg1[0].iov_base = "one";
117           msg1[0].iov_len = 3;
118           msg1[1].iov_base = "two";
119           msg1[1].iov_len = 3;
120
121           memset(&msg2, 0, sizeof(msg2));
122           msg2.iov_base = "three";
123           msg2.iov_len = 5;
124
125           memset(msg, 0, sizeof(msg));
126           msg[0].msg_hdr.msg_iov = msg1;
127           msg[0].msg_hdr.msg_iovlen = 2;
128
129           msg[1].msg_hdr.msg_iov = &msg2;
130           msg[1].msg_hdr.msg_iovlen = 1;
131
132           retval = sendmmsg(sockfd, msg, 2, 0);
133           if (retval == -1)
134               perror("sendmmsg()");
135           else
136               printf("%d messages sent\n", retval);
137
138           exit(0);
139       }
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COLOPHON

145       This  page  is  part of release 5.07 of the Linux man-pages project.  A
146       description of the project, information about reporting bugs,  and  the
147       latest     version     of     this    page,    can    be    found    at
148       https://www.kernel.org/doc/man-pages/.
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152Linux                             2020-06-09                       SENDMMSG(2)