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

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

       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/socket.h>&gt;

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

       ssize_t sendto(int s, const void *buf, size_t len, int flags,
                      const struct sockaddr *to, socklen_t tolen);

       ssize_t sendmsg(int s, 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,
       send(s,buf,len,flags) is equivalent to sendto(s,buf,len,flags,NULL,0).

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

       If sendto() is used on a connection-mode (SOCK_STREAM,  SOCK_SEQPACKET)
       socket,  the  arguments to and tolen 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 to with tolen  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-

       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  return  EAGAIN  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 (Linux 2.3+ only)
              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 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 is returned (this can also be enabled  using  the  O_NON-
              BLOCK 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.

              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  net.core.optmem_max  sysctl;  see

       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.   Addi-
       tional  errors may be generated and returned from the underlying proto-
       col 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 non-blocking and the requested operation
              would block.

       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  sig-

       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  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.

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

              The argument s 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 tolen 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)

       This  page  is  part of release 3.05 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                             2008-07-14                           SEND(2)