SEND(2)                   (2020-11-01)                    SEND(2)

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

          #include <sys/types.h>
          #include <sys/socket.h>

          ssize_t send(int sockfd, const void *buf, size_t len

          ssize_t sendto(int sockfd, const void *buf, size_t len
                         const struct sockaddr *dest_addr, socklen_t addrlen);

          ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags

          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 pres-
          ence of flags. With a zero flags argument, send() is equiva-
          lent 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

          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 tar-
          get is given by msg.msg_name, with msg.msg_namelen specify-
          ing 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 infor-

          If the message is too long to pass atomically through the

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          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

          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 imple-
               mented only for IPv4 and IPv6.  See arp(7) for details.

               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 pro-
               vides 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 and as well as other processes that
               hold file descriptors referring to the same open file

          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

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               TCP_CORK socket option (see tcp(7)), with the differ-
               ence 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 per-
               formed 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.

               Sends out-of-band data on sockets that support this
               notion (e.g., of type SOCK_STREAM); the underlying pro-
               tocol must also support out-of-band data.

          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 spec-
          ify 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, respec-

          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

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          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.

          On success, these calls return the number of bytes sent.  On
          error, -1 is returned, and errno is set appropriately.

          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

               (For UNIX domain sockets, which are identified by path-
               name) Write permission is denied on the destination
               socket file, or search permission is denied for one of
               the directories the path prefix.  (See

               (For UDP sockets) An attempt was made to send to a
               network/broadcast address as though it was a unicast

               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 possibili-

               (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

               Another Fast Open is in progress.

               sockfd is not a valid open file descriptor.

               Connection reset by peer.

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               The socket is not connection-mode, and no peer address
               is set.

               An invalid user space address was specified for an

               A signal occurred before any data was transmitted; see

               Invalid argument passed.

               The connection-mode socket was connected already but a
               recipient was specified.  (Now either this error is
               returned, or the recipient specification is ignored.)

               The socket type requires that message be sent atomi-
               cally, and the size of the message to be sent made this

               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.)

               No memory available.

               The socket is not connected, and no target has been

               The file descriptor sockfd does not refer to a socket.

               Some bit in the flags argument is inappropriate for the
               socket type.

               The local end has been shut down on a connection ori-
               ented socket.  In this case, the process will also
               receive a SIGPIPE unless MSG_NOSIGNAL is set.


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          4.4BSD, SVr4, POSIX.1-2001.  These interfaces 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.  The
          MSG_CONFIRM flag is a Linux extension.

          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.

          See sendmmsg(2) for information about a Linux-specific sys-
          tem call that can be used to transmit multiple datagrams in
          a single call.

          Linux may return EPIPE instead of ENOTCONN.

          An example of the use of sendto() is shown in

          fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2),
          sendmmsg(2), shutdown(2), socket(2), write(2), cmsg(3),
          ip(7), ipv6(7), socket(7), tcp(7), udp(7), unix(7)

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at

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