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  a  zero
   flags  argument, send() is equivalent to write(2).  Also, the following

       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
   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).  Valid only on SOCK_DGRAM and SOCK_RAW sockets and
          currently  implemented  only  for IPv4 and IPv6.  See arp(7) for

          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
          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  provides  similar
          behavior  to  setting  the  O_NONBLOCK  flag  (via  the fcntl(2)
          F_SETFL operation), but differs in that MSG_DONTWAIT is  a  per-
          call  option,  whereas  O_NONBLOCK is a setting on the open file
          description (see open(2)), which will affect all threads in  the
          calling  process  and  as well as other processes that hold file
          descriptors referring to the same open file description.

   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  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)
          Don't generate a SIGPIPE signal if the peer on a stream-oriented
          socket  has  closed  the  connection.   The EPIPE error is still
          returned.  This provides similar behavior to using  sigaction(2)
          to  ignore  SIGPIPE,  but,  whereas  MSG_NOSIGNAL  is a per-call
          feature, ignoring SIGPIPE sets a process attribute that  affects
          all threads in the process.

          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

       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 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).)

          (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

   EAGAIN (Internet  domain  datagram  sockets)  The socket referred to by
          sockfd had not previously 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  currently
          in        use.         See        the        discussion       of
          /proc/sys/net/ipv4/ip_local_port_range in ip(7).

   EBADF  sockfd is not a valid open file descriptor.

          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 file descriptor sockfd does not refer to 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 interfaces first appeared in 4.2BSD.

   POSIX.1-2001   describes   only   the   MSG_OOB   and   MSG_EOR  flags.
   POSIX.1-2008 adds a specification  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

   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),


   This page is part of release 4.09 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

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