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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 s, const void *buf, size_t len, int flags);
       ssize_t	sendto(int  s,	const  void *buf, size_t len, int flags, const
       struct sockaddr *to, socklen_t tolen);
       ssize_t sendmsg(int s, 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() is the presence of flags.  With zero flags
       parameter,    send()    is    equivalent	    to	   write().	 Also,
       send(s,buf,len,flags)  is equivalent to sendto(s,buf,len,flags,NULL,0).

       The parameter s is the file descriptor of the sending socket.

       If sendto() is used on a connection-mode (SOCK_STREAM,  SOCK_SEQPACKET)
       socket,	the parameters to and tolen 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 to with tolen	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 non-block-
       ing I/O mode.  In non-blocking mode it  would  return  EAGAIN  in  this
       case.   The select(2) call may be used to determine when it is possible
       to send more data.

       The flags parameter is the bitwise OR of zero or more of the  following
       flags.

       MSG_CONFIRM (Linux 2.3+ only)
	      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 neighbour (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.

       MSG_DONTROUTE
	      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
	      Enables  non-blocking  operation;	 if the operation would block,
	      EAGAIN is returned (this can also be enabled  using  the	O_NON-
	      BLOCK with the F_SETFL fcntl(2)).

       MSG_EOR
	      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
	      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 */
	     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 pro-
       cess is limited per  socket  by	the  net.core.optmem_max  sysctl;  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(2).)

       EAGAIN or EWOULDBLOCK
	      The  socket  is  marked non-blocking and the requested operation
	      would block.

       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 a parameter.

       EINTR  A signal occurred before any data was transmitted.

       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 s 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  only  describes  the  MSG_OOB  and	 MSG_EOR  flags.   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 tolen 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.

BUGS
       Linux may return EPIPE instead of ENOTCONN.

SEE ALSO
       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)



Linux 2.6.7			  2004-07-01			       SEND(2)
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