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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,
                    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 operation (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.

       See also BUGS.

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.

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.

       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 recvmmsg().  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.

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 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) {
               perror("socket()");
               exit(EXIT_FAILURE);
           }

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

SEE ALSO

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

COLOPHON

       This  page  is  part of release 5.02 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 https://www.kernel.org/doc/man-pages/.