Provided by: manpages-dev_2.17-1_all bug

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)  is
       equivalent to sendto(s,buf,len,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
       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 parameter is the bitwise OR of zero or more of the following
       flags.

       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.

       MSG_EOR
              Terminates a record (when  this  notion  is  supported,  as  for
              sockets of type SOCK_SEQPACKET).

       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
              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_NOSIGNAL
              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_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’ll 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_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.

       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 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.
       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(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
              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 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  1003.1-2001.   These  function calls appeared in
       4.2BSD.

       POSIX only describes the MSG_OOB and MSG_EOR  flags.   The  MSG_CONFIRM
       flag is a Linux extension.

NOTE

       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).

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), ip(7), socket(7), tcp(7), udp(7)