NAME

       recvmmsg - receive multiple messages on a socket

LIBRARY

       Standard C library (libc, -lc)

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.

STANDARDS

       Linux.

HISTORY

       Linux 2.6.33, glibc 2.12.

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.

EXAMPLES

       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 <arpa/inet.h>
       #include <netinet/in.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/socket.h>
       #include <time.h>

       int
       main(void)
       {
       #define VLEN 10
       #define BUFSIZE 200
       #define TIMEOUT 1
           int                 sockfd, retval;
           char                bufs[VLEN][BUFSIZE+1];
           struct iovec        iovecs[VLEN];
           struct mmsghdr      msgs[VLEN];
           struct timespec     timeout;
           struct sockaddr_in  addr;

           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 (size_t 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 (size_t i = 0; i < retval; i++) {
               bufs[i][msgs[i].msg_len] = 0;
               printf("%zu %s", i+1, bufs[i]);
           }
           exit(EXIT_SUCCESS);
       }

SEE ALSO

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