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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  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  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 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.

       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 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.

       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   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

       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.

       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), socket(7), tcp(7),
       udp(7)

COLOPHON

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



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