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NAME

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

SYNOPSIS

       #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);

DESCRIPTION

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

       MSG_DONTROUTE
              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.

       MSG_OOB
              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.

   sendmsg()
       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.

RETURN VALUE

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

ERRORS

       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.

       EAGAIN or EWOULDBLOCK
              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 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).

       EALREADY
              Another Fast Open is in progress.

       EBADF  sockfd is not a valid open file descriptor.

       ECONNRESET
              Connection reset by peer.

       EDESTADDRREQ
              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.

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

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

       ENOBUFS
              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.

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

       ENOTSOCK
              The file descriptor sockfd does not refer to a socket.

       EOPNOTSUPP
              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.

CONFORMING TO

       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.

NOTES

       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.

BUGS

       Linux may return EPIPE instead of ENOTCONN.

EXAMPLE

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

SEE ALSO

       fcntl(2),  getsockopt(2), recv(2), select(2), sendfile(2), sendmmsg(2), shutdown(2), socket(2), write(2),
       cmsg(3), ip(7), ipv6(7), socket(7), tcp(7), udp(7), unix(7)

COLOPHON

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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       https://www.kernel.org/doc/man-pages/.