RECVMMSG(2)						     Linux Programmer's Manual						       RECVMMSG(2)

NAME

       recvmmsg - receive multiple messages on a socket

SYNOPSIS

       #define _GNU_SOURCE	   /* See feature_test_macros(7) */
       #include <sys/socket.h>

       int recvmmsg(int sockfd, struct mmsghdr *msgvec, unsigned int vlen,
		    unsigned int flags, struct timespec *timeout);

DESCRIPTION

       The  recvmmsg()	system call is an extension of recvmsg(2) that allows the caller to receive multiple messages from a socket using a single
       system call.  (This has performance benefits for some applications.)  A further extension over recvmsg(2) is support for a timeout  on  the
       receive operation.

       The sockfd argument is the file descriptor of the socket to receive data from.

       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 received bytes for header */
	   };

       The  msg_hdr field is a msghdr structure, as described in recvmsg(2).  The msg_len field is the number of bytes returned for the message in
       the entry.  This field has the same value as the return value of a single recvmsg(2) on the header.

       The flags argument contains flags ORed together.  The flags are the same as documented for recvmsg(2), with the following addition:

       MSG_WAITFORONE (since Linux 2.6.34)
	      Turns on MSG_DONTWAIT after the first message has been received.

       The timeout argument points to a struct timespec (see clock_gettime(2)) defining a timeout (seconds plus nanoseconds) for the receive oper-
       ation  (but  see  BUGS!).   (This  interval  will be rounded up to the system clock granularity, and kernel scheduling delays mean that the
       blocking interval may overrun by a small amount.)  If timeout is NULL, then the operation blocks indefinitely.

       A blocking recvmmsg() call blocks until vlen messages have been received or until the timeout expires.  A nonblocking call  reads  as  many
       messages as are available (up to the limit specified by vlen) and returns immediately.

       On  return  from recvmmsg(), successive elements of msgvec are updated to contain information about each received message: msg_len contains
       the size of the received message; the subfields of msg_hdr are updated as described in recvmsg(2).  The return value of the call  indicates
       the number of elements of msgvec that have been updated.

RETURN VALUE

       On success, recvmmsg() returns the number of messages received in msgvec; on error, -1 is returned, and errno is set to indicate the error.

ERRORS

       Errors are as for recvmsg(2).  In addition, the following error can occur:

       EINVAL timeout is invalid.

VERSIONS

       The recvmmsg() system call was added in Linux 2.6.33.  Support in glibc was added in version 2.12.

CONFORMING TO

       recvmmsg() is Linux-specific.

EXAMPLE

       The  following  program	uses recvmmsg() to receive multiple messages on a socket and stores them in multiple buffers.  The call returns if
       all buffers are filled or if the timeout specified has expired.

       The following snippet periodically generates UDP datagrams containing a random number:

	   $ while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234;
		 sleep 0.25; done

       These datagrams are read by the example application, which can give the following output:

	   $ ./a.out
	   5 messages received
	   1 11782
	   2 11345
	   3 304
	   4 13514
	   5 28421

   Program source

       #define _GNU_SOURCE
       #include <netinet/ip.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/socket.h>

       int
       main(void)
       {
       #define VLEN 10
       #define BUFSIZE 200
       #define TIMEOUT 1
	   int sockfd, retval, i;
	   struct sockaddr_in sa;
	   struct mmsghdr msgs[VLEN];
	   struct iovec iovecs[VLEN];
	   char bufs[VLEN][BUFSIZE+1];
	   struct timespec timeout;

	   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 (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
	       perror("bind()");
	       exit(EXIT_FAILURE);
	   }

	   memset(msgs, 0, sizeof(msgs));
	   for (i = 0; i < VLEN; i++) {
	       iovecs[i].iov_base	  = bufs[i];
	       iovecs[i].iov_len	  = BUFSIZE;
	       msgs[i].msg_hdr.msg_iov	  = &iovecs[i];
	       msgs[i].msg_hdr.msg_iovlen = 1;
	   }

	   timeout.tv_sec = TIMEOUT;
	   timeout.tv_nsec = 0;

	   retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
	   if (retval == -1) {
	       perror("recvmmsg()");
	       exit(EXIT_FAILURE);
	   }

	   printf("%d messages received
", retval);
	   for (i = 0; i < retval; i++) {
	       bufs[i][msgs[i].msg_len] = 0;
	       printf("%d %s", i+1, bufs[i]);
	   }
	   exit(EXIT_SUCCESS);
       }

BUGS

       The timeout argument does not work as intended.	The timeout is checked only after the receipt of each datagram, so that if  up	to  vlen-1
       datagrams are received before the timeout expires, but then no further datagrams are received, the call will block forever.

SEE ALSO

       clock_gettime(2), recvmsg(2), sendmmsg(2), sendmsg(2), socket(2), socket(7)

Linux								    2014-06-13							       RECVMMSG(2)