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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
       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_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
              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_EOR
              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
              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  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(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
              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.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 3.01 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/.