Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
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).  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, EA-
	      GAIN 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.

   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 ad-
       dress; 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 at-
	      tempting 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_lo-
	      cal_port_range in	ip(7).

       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  re-
	      cipient 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 oc-
	      cur  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
       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 3.74 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
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-08-19			       SEND(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=sendmsg&sektion=2&manpath=Debian+8.1.0>

home | help