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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  ENOT-
       CONN  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 non-blocking I/O mode.  In non-blocking 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.

              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 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 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

       MSG_NOSIGNAL (since Linux 2.2)
              Requests not to send SIGPIPE on errors on stream oriented sockets when the other end breaks the  connec-
              tion.  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

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

       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 destina-
              tion socket file, or search permission is denied for one of  the  directories  the  path  prefix.   (See

              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 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 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  informa-
       tion about reporting bugs, can be found at

Linux                             2009-02-23                           SEND(2)