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SEND(2)			   Linux Programmer's Manual		       SEND(2)

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.  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‐
       mitted.

       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 nonblock‐
       ing 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 deter‐
       mine 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 proto‐
	      col 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 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 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 sock‐
	      ets 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_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).

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.	 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).)

	      (For  UDP	 sockets)  An  attempt	was  made  to  send  to a net‐
	      work/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 pos‐
	      sibilities.

       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  sig‐
	      nal(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	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.

       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 3.53 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/.

Linux				  2012-04-23			       SEND(2)
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