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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 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

       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 argu‐
       ments 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 speci‐
       fying  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 transmitted.

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

       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 implemented only for IPv4 and IPv6.  See arp(7) for details.

              Don't use a gateway to send out the packet, send to hosts  only  on  directly  con‐
              nected  networks.   This  is  usually  used only by diagnostic or routing programs.
              This is defined only 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

       MSG_EOR (since Linux 2.2)
              Terminates a record (when  this  notion  is  supported,  as  for  sockets  of  type

       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 ker‐
              nel to package all of the data sent in calls with this flag set into a single data‐
              gram  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 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 employed by sendmsg() is as follows:

           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 (unused) */

       The msg_name field is used on an unconnected socket to specify the target  address  for  a
       datagram.   It  points to a buffer containing the address; the msg_namelen field should be
       set to the size of the address.  For a connected socket, these fields should be  specified
       as NULL and 0, respectively.

       The msg_iov and msg_iovlen fields specify scatter-gather locations, as for writev(2).

       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).

       The msg_flags field is ignored.

       On  success,  these  calls return the number of bytes 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

       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).)

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

              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 possibilities.

       EAGAIN (Internet domain datagram sockets) The socket referred to by sockfd had not  previ‐
              ously  been  bound  to  an  address and, upon attempting to bind it to an ephemeral
              port, it was determined that all port numbers in the ephemeral port range are  cur‐
              rently  in  use.   See  the discussion of /proc/sys/net/ipv4/ip_local_port_range in

       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 signal(7).

       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 mes‐
              sage 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 type.

       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 describes only the MSG_OOB and MSG_EOR flags.  POSIX.1-2008 adds a specifica‐
       tion of MSG_NOSIGNAL.  The MSG_CONFIRM flag is a Linux extension.

       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.

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

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-08-19                                    SEND(2)

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