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       send, sendto, sendmsg - send a message on a socket


       #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);


       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.
       Otherwise, 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

       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
       nonblocking 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
       determine when it is possible to send more data.

       The  flags  argument is the bitwise OR of zero or more of the following

       MSG_CONFIRM (Since Linux 2.3.15)
              Tell the link layer that forward progress happened:  you  got  a
              successful reply from the other side.  If the link layer doesn’t
              get this it will regularly reprobe the  neighbor  (e.g.,  via  a
              unicast ARP).  Only valid on SOCK_DGRAM and SOCK_RAW sockets and
              currently only implemented for IPv4 and IPv6.   See  arp(7)  for

              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
              protocol 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
              sockets 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
              transmitted  when a call is performed that does not specify this
              flag.   (See  also  the  UDP_CORK  socket  option  described  in

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

              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_controllen  members.   The maximum control buffer length the kernel
       can   process   is   limited   per   socket    by    the    value    in
       /proc/sys/net/core/optmem_max; see socket(7).


       On  success,  these  calls  return  the  number of characters sent.  On
       error, -1 is returned, and errno is set appropriately.


       These  are  some  standard  errors  generated  by  the  socket   layer.
       Additional  errors  may  be  generated and returned from the underlying
       protocol modules; see their respective manual pages.

       EACCES (For Unix domain sockets,  which  are  identified  by  pathname)
              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).)

              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

       EBADF  An invalid descriptor was specified.

              Connection reset by peer.

              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

       EINVAL Invalid argument passed.

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

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

              The  output  queue  for  a  network  interface  was  full.  This
              generally 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.

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

              The argument sockfd is not a socket.

              Some  bit  in the flags argument is inappropriate for the socket

       EPIPE  The local end has  been  shut  down  on  a  connection  oriented
              socket.   In  this  case the process will also receive a SIGPIPE
              unless MSG_NOSIGNAL is set.


       4.4BSD, SVr4, POSIX.1-2001.  These function calls appeared in 4.2BSD.

       POSIX.1-2001  only  describes  the  MSG_OOB  and  MSG_EOR  flags.   The
       MSG_CONFIRM flag is a Linux extension.


       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 (2.4) types
       it as size_t.


       Linux may return EPIPE instead of ENOTCONN.


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


       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)


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