NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       recvmmsg - receive multiple messages on a socket

SYNOPSIS         top

       #define _GNU_SOURCE
       #include <sys/socket.h>

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

DESCRIPTION         top

       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.  (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         top

       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         top

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

       EINVAL timeout is invalid.

VERSIONS         top

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

CONFORMING TO         top

       recvmmsg() is Linux-specific.

EXAMPLE         top

       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\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         top

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

COLOPHON         top

       This page is part of release 3.64 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2012-12-24                      RECVMMSG(2)