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  a  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 nonblock-
       ing I/O mode.  In nonblocking mode it would fail with the error  EAGAIN
       or  EWOULDBLOCK in this case.  The select(2) call may be used to deter-

       MSG_DONTROUTE
              Don't use a gateway to send out the packet, send to  hosts  only
              on  directly  connected  networks.  This is usually used only by
              diagnostic or routing programs.  This is defined only for proto-
              col families that route; packet sockets don't.

       MSG_DONTWAIT (since Linux 2.2)
              Enables  nonblocking  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 transmitted
              only 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 */
               size_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-
              Write  permission  is  denied on the destination socket file, or
              search permission is denied for one of the directories the  path
              prefix.  (See path_resolution(7).)

              (For  UDP  sockets)  An  attempt  was  made  to  send  to a net-
              work/broadcast address as though it was a unicast address.

       EAGAIN or EWOULDBLOCK
              The socket is marked nonblocking  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
       flag is a Linux extension.

NOTES
       The  prototypes  given  above  follow the Single UNIX Specification, as
       glibc2 also does; the flags argument was int in 4.x BSD,  but  unsigned
       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 types it as
       size_t.

       See sendmmsg(2) for information about a Linux-specific system call that
       can be used to transmit multiple datagrams in a single call.

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),  sendmmsg(2),
       shutdown(2),  socket(2),  write(2),  cmsg(3), ip(7), socket(7), tcp(7),
       udp(7)

COLOPHON
       This page is part of release 3.54 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                             2012-04-23                           SEND(2)
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