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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.	  Oth‐
       erwise,	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 trans‐

       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-block‐
       ing 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	deter‐
       mine when it is possible to send more data.

       The  flags  argument is the bitwise OR of zero or more of the following

       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

	      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 proto‐
	      col 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 sock‐
	      ets 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	trans‐
	      mitted 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	 sock‐
	      ets  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_con‐
       trollen members.	 The maximum control  buffer  length  the  kernel  can
       process	is  limited per socket by the value in /proc/sys/net/core/opt‐
       mem_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.	 Addi‐
       tional errors may be generated and returned from the underlying	proto‐
       col 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 pos‐

       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  sig‐

       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	gener‐
	      ally  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

       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

Linux				  2009-02-23			       SEND(2)

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