recvmmsg

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,
                    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 5.03 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/.



Linux                             2019-03-06                       RECVMMSG(2)