SIMPLE SOLUTIONS

SENDMMSG - reference manual online

Send multiple messages on a socket.

Chapter
2014-07-08
SENDMMSG(2)                         Linux Programmer's Manual                         SENDMMSG(2)

NAME sendmmsg - send multiple messages on a socket
SYNOPSIS #define _GNU_SOURCE /* See feature_test_macros(7) */ #include <sys/socket.h> int sendmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen, unsigned int flags);
DESCRIPTION The sendmmsg() system call is an extension of sendmsg(2) that allows the caller to trans‐ mit multiple messages on a socket using a single system call. (This has performance bene‐ fits for some applications.) The sockfd argument is the file descriptor of the socket on which data is to be transmit‐ ted. The msgvec argument is a pointer to an array of mmsghdr structures. The size of this array is specified in vlen. The mmsghdr structure is defined in <sys/socket.h> as: struct mmsghdr { struct msghdr msg_hdr; /* Message header */ unsigned int msg_len; /* Number of bytes transmitted */ }; The msg_hdr field is a msghdr structure, as described in sendmsg(2). The msg_len field is used to return the number of bytes sent from the message in msg_hdr (i.e., the same as the return value from a single sendmsg(2) call). The flags argument contains flags ORed together. The flags are the same as for sendmsg(2). A blocking sendmmsg() call blocks until vlen messages have been sent. A nonblocking call sends as many messages as possible (up to the limit specified by vlen) and returns immedi‐ ately. On return from sendmmsg(), the msg_len fields of successive elements of msgvec are updated to contain the number of bytes transmitted from the corresponding msg_hdr. The return value of the call indicates the number of elements of msgvec that have been updated.
RETURN VALUE On success, sendmmsg() returns the number of messages sent from msgvec; if this is less than vlen, the caller can retry with a further sendmmsg() call to send the remaining mes‐ sages. On error, -1 is returned, and errno is set to indicate the error.
ERRORS Errors are as for sendmsg(2). An error is returned only if no datagrams could be sent.
VERSIONS The sendmmsg() system call was added in Linux 3.0. Support in glibc was added in version 2.14.
CONFORMING TO sendmmsg() is Linux-specific.
NOTES The value specified in vlen is capped to UIO_MAXIOV (1024).
EXAMPLE The example below uses sendmmsg() to send onetwo and three in two distinct UDP datagrams using one system call. The contents of the first datagram originates from a pair of buf‐ fers. #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(void) { 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)) == -1) { 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); }
SEE ALSO recvmmsg(2), sendmsg(2), socket(2), socket(7)
COLOPHON This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
Linux 2014-07-08 SENDMMSG(2)
This manual Reference Other manuals
sendmmsg(2) referred by recvmmsg(2) | send(2) | stress-ng(1) | syscalls(2)
refer to feature_test_macros(7) | recvmmsg(2) | send(2) | socket(2) | socket(7)