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(2) is the presence of flags.  With zero  flags
       parameter,     send()	 is    equivalent    to    write(2).	 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  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.

       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(7).)

       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.

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

COLOPHON
       This  page  is  part of release 2.77 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				  2004-07-01			       SEND(2)