sendmsg

SYNOPSIS
       #include <sys/types.h>
       #include <sys/socket.h>

       ssize_t send(int sockfd, const void *buf, size_t len, int flags);

       ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
                      const struct sockaddr *dest_addr, socklen_t addrlen);

       ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

DESCRIPTION
       The system calls send(), sendto(), and sendmsg() are used to transmit a
       message to another socket.

       The send() call may be used only when the  socket  is  in  a  connected
       state  (so  that the intended recipient is known).  The only difference
       between send() and write(2) is the presence of flags.  With zero  flags
       argument, send() is equivalent to write(2).  Also, the following call

           send(sockfd, buf, len, flags);

       is equivalent to

           sendto(sockfd, buf, len, flags, NULL, 0);

       The argument sockfd is the file descriptor of the sending socket.

       If  sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET)
       socket, the arguments dest_addr and addrlen are ignored (and the  error
       EISCONN  may  be  returned when they are not NULL and 0), and the error
       ENOTCONN is returned when the socket was not actually connected.   Oth-
       erwise,  the  address  of the target is given by dest_addr with addrlen
       specifying its size.  For sendmsg(), the address of the target is given
       by msg.msg_name, with msg.msg_namelen specifying its size.

       For  send()  and  sendto(),  the message is found in buf and has length
       len.  For sendmsg(), the message is pointed to by the elements  of  the
       array  msg.msg_iov.   The  sendmsg() call also allows sending ancillary
       data (also known as control information).

       If the message is too long to pass atomically  through  the  underlying
       protocol, the error EMSGSIZE is returned, and the message is not trans-
       mitted.

       No indication of failure to deliver is implicit in a  send().   Locally
       detected errors are indicated by a return value of -1.

       When  the  message  does  not  fit  into the send buffer of the socket,
       send() normally blocks, unless the socket has been placed in non-block-
       ing I/O mode.  In non-blocking mode it would fail with the error EAGAIN
       or EWOULDBLOCK in this case.  The select(2) call may be used to  deter-
       mine when it is possible to send more data.
       MSG_DONTROUTE
              Don't  use  a gateway to send out the packet, only send to hosts
              on directly connected networks.  This is usually  used  only  by
              diagnostic or routing programs.  This is only defined for proto-
              col families that route; packet sockets don't.

       MSG_DONTWAIT (since Linux 2.2)
              Enables non-blocking operation; if the  operation  would  block,
              EAGAIN  or  EWOULDBLOCK  is  returned  (this can also be enabled
              using the O_NONBLOCK flag with the F_SETFL fcntl(2)).

       MSG_EOR (since Linux 2.2)
              Terminates a record (when this notion is supported, as for sock-
              ets of type SOCK_SEQPACKET).

       MSG_MORE (Since Linux 2.4.4)
              The  caller  has  more data to send.  This flag is used with TCP
              sockets to obtain the same effect as the TCP_CORK socket  option
              (see tcp(7)), with the difference that this flag can be set on a
              per-call basis.

              Since Linux 2.6, this flag is also supported  for  UDP  sockets,
              and  informs the kernel to package all of the data sent in calls
              with this flag set into a single datagram which is  only  trans-
              mitted when a call is performed that does not specify this flag.
              (See also the UDP_CORK socket option described in udp(7).)

       MSG_NOSIGNAL (since Linux 2.2)
              Requests not to send SIGPIPE on errors on stream oriented  sock-
              ets  when  the other end breaks the connection.  The EPIPE error
              is still returned.

       MSG_OOB
              Sends out-of-band data  on  sockets  that  support  this  notion
              (e.g.,  of  type SOCK_STREAM); the underlying protocol must also
              support out-of-band data.

       The definition of the msghdr structure follows.  See recv(2) and  below
       for an exact description of its fields.

           struct msghdr {
               void         *msg_name;       /* optional address */
               socklen_t     msg_namelen;    /* size of address */
               struct iovec *msg_iov;        /* scatter/gather array */
               size_t        msg_iovlen;     /* # elements in msg_iov */
               void         *msg_control;    /* ancillary data, see below */
               socklen_t     msg_controllen; /* ancillary data buffer len */
               int           msg_flags;      /* flags on received message */
           };

       You  may  send  control  information using the msg_control and msg_con-
       trollen members.  The maximum control  buffer  length  the  kernel  can
       process  is  limited per socket by the value in /proc/sys/net/core/opt-
       mem_max; see socket(7).
              search  permission is denied for one of the directories the path
              prefix.  (See path_resolution(7).)

       EAGAIN or EWOULDBLOCK
              The socket is marked non-blocking and  the  requested  operation
              would  block.   POSIX.1-2001  allows either error to be returned
              for this case, and does not require these constants to have  the
              same value, so a portable application should check for both pos-
              sibilities.

       EBADF  An invalid descriptor was specified.

       ECONNRESET
              Connection reset by peer.

       EDESTADDRREQ
              The socket is not connection-mode, and no peer address is set.

       EFAULT An invalid user space address was specified for an argument.

       EINTR  A signal occurred before any  data  was  transmitted;  see  sig-
              nal(7).

       EINVAL Invalid argument passed.

       EISCONN
              The connection-mode socket was connected already but a recipient
              was specified.  (Now either  this  error  is  returned,  or  the
              recipient specification is ignored.)

       EMSGSIZE
              The  socket  type  requires that message be sent atomically, and
              the size of the message to be sent made this impossible.

       ENOBUFS
              The output queue for a network interface was full.  This  gener-
              ally  indicates  that the interface has stopped sending, but may
              be caused by transient congestion.   (Normally,  this  does  not
              occur in Linux.  Packets are just silently dropped when a device
              queue overflows.)

       ENOMEM No memory available.

       ENOTCONN
              The socket is not connected, and no target has been given.

       ENOTSOCK
              The argument sockfd is not a socket.

       EOPNOTSUPP
              Some bit in the flags argument is inappropriate for  the  socket
              type.

       EPIPE  The  local  end  has  been  shut  down  on a connection oriented
       int in libc4 and libc5; the len argument was int in 4.x BSD and  libc4,
       but  size_t in libc5; the addrlen argument was int in 4.x BSD and libc4
       and libc5.  See also accept(2).

       According to POSIX.1-2001,  the  msg_controllen  field  of  the  msghdr
       structure should be typed as socklen_t, but glibc currently (2.4) types
       it as size_t.

BUGS
       Linux may return EPIPE instead of ENOTCONN.

EXAMPLE
       An example of the use of sendto() is shown in getaddrinfo(3).

SEE ALSO
       fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2),  shutdown(2),
       socket(2), write(2), cmsg(3), ip(7), socket(7), tcp(7), udp(7)

COLOPHON
       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-02-23                           SEND(2)