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 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.

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.

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 4.09 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/.

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