sendto(2)



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 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,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 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 non-block-
       ing 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 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_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  proto-
              col 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_NON-
              BLOCK with the F_SETFL fcntl(2)).

       MSG_EOR
              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 only trans-
              mitted 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 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.

       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_con-
       trollen  members. The maximum control buffer length the kernel can pro-
       cess 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.  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(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 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 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.

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)

Linux 2.6.7                       2004-07-01                           SEND(2)

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