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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 sin-
     gle 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 inter-
     val may overrun by a small amount.)  If timeout is NULL, then the oper-
     ation 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  data-
     grams  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); }