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
       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).  Valid only 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 provides similar behav-
	      ior  to  setting	the  O_NONBLOCK flag (via the fcntl(2) F_SETFL
	      operation), but differs  in  that	 MSG_DONTWAIT  is  a  per-call
	      option,  whereas	O_NONBLOCK  is	a  setting  on	the  open file
	      description (see open(2)), which will affect all threads in  the
	      calling  process	and  as well as other processes that hold file
	      descriptors referring to the same open file description.

       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)
	      Don't generate a SIGPIPE signal if the peer on a stream-oriented
	      socket  has  closed  the	connection.   The EPIPE error is still
	      returned.	 This provides similar behavior to using  sigaction(2)
	      to  ignore SIGPIPE, but, whereas MSG_NOSIGNAL is a per-call fea-
	      ture, ignoring SIGPIPE sets a process attribute that affects all
	      threads in the process.

       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.

   sendmsg()
       The definition of the msghdr structure employed by sendmsg() is as fol-
       lows:

	   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 (unused) */
	   };

       The msg_name field is used on an unconnected socket to specify the tar-
       get  address  for  a  datagram.	 It  points to a buffer containing the
       address; the msg_namelen field  should  be  set	to  the	 size  of  the
       address.	  For  a connected socket, these fields should be specified as
       NULL and 0, respectively.

       The msg_iov and msg_iovlen fields specify scatter-gather locations,  as
       for writev(2).

       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).

       The msg_flags field is ignored.

RETURN VALUE
       On success, these calls return the number of bytes 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.

       EAGAIN (Internet	 domain	 datagram  sockets)  The socket referred to by
	      sockfd had not previously been bound to  an  address  and,  upon
	      attempting  to  bind  it to an ephemeral port, it was determined
	      that all port numbers in the ephemeral port range are  currently
	      in	use.	     See	the	   discussion	    of
	      /proc/sys/net/ipv4/ip_local_port_range in ip(7).

       EBADF  sockfd is not a valid open file descriptor.

       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 file descriptor sockfd does not refer to 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 interfaces first 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
       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), ipv6(7), socket(7),
       tcp(7), udp(7), unix(7)

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			       SEND(2)