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

       MSG_DONTROUTE
              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 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
              fcntl(2)).

       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 only transmitted 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.

       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.

       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 */
               size_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_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).

RETURN VALUE

       On  success,  these calls return the number of characters 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).)

       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.

       EBADF  An invalid descriptor was specified.

       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 argument sockfd is not 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 function calls appeared in 4.2BSD.

       POSIX.1-2001   only  describes  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

       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 addrlen 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 types it as size_t.

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

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.