RECVMMSG(2)               (2020-11-01)                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 opera-
          tion.

          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 follow-
          ing 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 nanosec-
          onds) 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

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          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 sub-
          fields of msg_hdr are updated as described in recvmsg(2).
          The return value of the call indicates the number of ele-
          ments 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.  Sup-
          port 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 mes-
          sages received.  The error code is expected to be returned
          on a subsequent call to recvmmsg().  In the current imple-
          mentation, 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

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          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;
              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);

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              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 (int 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 (int 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 5.10 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/.

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