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

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

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

       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 pro-
       cess  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

Linux                             2008-07-14                           SEND(2)

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