xenial (2) recvmmsg.2.gz

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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,
                    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.

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 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);
       }

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.

SEE ALSO

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

COLOPHON

       This  page  is  part  of  release  4.04  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
       http://www.kernel.org/doc/man-pages/.