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

       recvmmsg - receive multiple messages on a socket

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

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

       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 sin-
       gle 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 operation
       (but see BUGS!).  (This interval will be rounded up to the system clock
       granularity, and kernel scheduling delays mean that the blocking inter-
       val may overrun by a small amount.)  If timeout is NULL, then the oper-
       ation 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.

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

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

       EINVAL timeout is invalid.

       See also BUGS.

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

       recvmmsg() is Linux-specific.

       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  data-
       grams  are  received  before  the  timeout expires, but then no further
       datagrams are received, the call will block forever.

       If an error occurs after at least one message has  been  received,  the
       call  succeeds, and returns the number of messages received.  The error
       code is expected to be returned on a subsequent call to recvmmsq().  In
       the  current implementation, however, the error code can be overwritten
       in the meantime by an unrelated network event on a socket, for  example
       an incoming ICMP packet.

       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/;
                 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>

       #define VLEN 10
       #define BUFSIZE 200
       #define TIMEOUT 1
           int sockfd, retval, i;
           struct sockaddr_in addr;
           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) {

           addr.sin_family = AF_INET;
           addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
           addr.sin_port = htons(1234);
           if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {

           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) {

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

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

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Linux                             2018-02-02                       RECVMMSG(2)
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