recvmmsg(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | BUGS | EXAMPLES | SEE ALSO | COLOPHON

RECVMMSG(2)             Linux Programmer's Manual            RECVMMSG(2)

NAME         top

       recvmmsg - receive multiple messages on a socket

SYNOPSIS         top

       #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         top

       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         top

       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         top

       Errors are as for recvmsg(2).  In addition, the following error
       can occur:

       EINVAL timeout is invalid.

       See also BUGS.

VERSIONS         top

       The recvmmsg() system call was added in Linux 2.6.33.  Support in
       glibc was added in version 2.12.

CONFORMING TO         top

       recvmmsg() is Linux-specific.

BUGS         top

       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         top

       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);
           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         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2020-11-01                    RECVMMSG(2)

Pages that refer to this page: recv(2)sendmmsg(2)syscalls(2)signal(7)