• 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 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_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.
  Otherwise, 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 transmitted.
  
  
  
  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-blocking 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
  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 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 protocol
  families that route; packet sockets don't.
  

  MSG_DONTWAIT " (since Linux 2.2)"
  

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

  MSG_EOR " (since Linux 2.2)"
  

  Terminates a record (when this notion is supported, as for sockets 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 transmitted 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 sockets 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_controllen members. The maximum control buffer length the kernel can process is limited per socket by the value in /proc/sys/net/core/optmem_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. Additional errors may be generated and returned from the underlying protocol 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 an argument.

  EINTR

  A signal occurred before any data was transmitted; see signal(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 generally 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 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 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 (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 3.19 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/.

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