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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 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  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 tar-
       get 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  informa-

       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 non-blocking I/O mode.  In  non-block-
       ing  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 regu-
              larly 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.

              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 pro-
              grams.  This is only defined for protocol families that route; packet  sock-
              ets don't.

       MSG_DONTWAIT (since Linux 2.2)
              Enables  non-blocking  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

       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.

              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 */
               socklen_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).

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

              The socket is marked non-blocking 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.

              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 speci-
              fied.  (Now either this error is returned, or the recipient specification is

              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 con-
              gestion.  (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 only describes the MSG_OOB and MSG_EOR flags.  The MSG_CONFIRM flag is
       a Linux extension.

       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 (2.4) types it as size_t.

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2009-02-23                           SEND(2)

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