Ubuntu Manpage: send, sendto, sendmsg - send a message on a socket

NAME

       send, sendto, sendmsg - send a message on a socket

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/socket.h>

       ssize_t send(int sockfd, const void buf[.len], size_t len, int flags);
       ssize_t sendto(int sockfd, const void buf[.len], 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.  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 nonblocking 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
       determine 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).  Valid only 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 protocol families that route; packet sockets don't.

       MSG_DONTWAIT (since Linux 2.2)
              Enables nonblocking operation; if the operation would block, EAGAIN or  EWOULDBLOCK
              is  returned.   This  provides similar behavior to setting the O_NONBLOCK flag (via
              the fcntl(2) F_SETFL operation), but differs in that  MSG_DONTWAIT  is  a  per-call
              option, whereas O_NONBLOCK is a setting on the open file description (see open(2)),
              which will affect all threads in the calling process as  well  as  other  processes
              that hold file descriptors referring to the same open file description.

       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 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)
              Don't  generate a SIGPIPE signal if the peer on a stream-oriented socket has closed
              the connection.  The EPIPE error is still returned.  This provides similar behavior
              to  using  sigaction(2)  to ignore SIGPIPE, but, whereas MSG_NOSIGNAL is a per-call
              feature, ignoring SIGPIPE sets a process attribute that affects all threads in  the
              process.

       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_FASTOPEN (since Linux 3.7)
              Attempts TCP Fast Open (RFC7413) and sends data in the SYN like  a  combination  of
              connect(2) and write(2), by performing an implicit connect(2) operation.  It blocks
              until the data is buffered and the handshake has  completed.   For  a  non-blocking
              socket, it returns the number of bytes buffered and sent in the SYN packet.  If the
              cookie is not available locally, it returns EINPROGRESS, and sends  a  SYN  with  a
              Fast  Open  cookie request automatically.  The caller needs to write the data again
              when the socket is connected.  On errors, it sets the same errno as  connect(2)  if
              the  handshake  fails.  This flag requires enabling TCP Fast Open client support on
              sysctl net.ipv4.tcp_fastopen.

              Refer to TCP_FASTOPEN_CONNECT socket option in tcp(7) for an alternative approach.

   sendmsg()
       The definition of the msghdr structure employed by sendmsg() is as follows:

           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 target  address  for  a
       datagram.   It  points to a buffer containing the address; 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 (ancillary data) 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).  For further information on
       the use of ancillary data in various socket domains, see unix(7) and ip(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 to indicate the error.

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).)

(For UDP sockets) An attempt was made to send to a network/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 possibilities.

EAGAIN (Internet domain datagram sockets) The socket referred to by sockfd had not
previously been bound to an address and, upon attempting 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_local_port_range in
ip(7).

EALREADY
Another Fast Open is in progress.

EBADF sockfd is not a valid open file descriptor.

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 file descriptor sockfd does not refer to 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.

VERSIONS

       According  to  POSIX.1-2001,  the  msg_controllen  field of the msghdr structure should be
       typed as socklen_t, and the msg_iovlen field should be typed as int, but  glibc  currently
       types both as size_t.

STANDARDS

       POSIX.1-2008.

       MSG_CONFIRM is a Linux extension.

HISTORY

       4.4BSD, SVr4, POSIX.1-2001.  (first 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.

NOTES

       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.

EXAMPLES

       An example of the use of sendto() is shown in getaddrinfo(3).