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SCTP_SENDMSG(3)            Library Functions Manual            SCTP_SENDMSG(3)

     sctp_sendmsg, sctp_sendmsgx - send a message from an SCTP socket

     Standard C Library (libc, -lc)

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

     sctp_sendmsg(int s, const void *msg, size_t len,
         const struct sockaddr *to, socklen_t tolen, uint32_t ppid,
         uint32_t flags, uint16_t stream_no, uint32_t timetolive,
         uint32_t context);

     sctp_sendmsgx(int s, const void *msg, size_t len,
         const struct sockaddr *to, int addrcnt, uint32_t ppid,
         uint32_t flags, uint16_t stream_no, uint32_t timetolive,
         uint32_t context);

     The sctp_sendmsg() system call is used to transmit a message to another
     SCTP endpoint.  The sctp_sendmsg() may be used at any time.  If the
     socket is a one-to-many type (SOCK_SEQPACKET) socket then an attempt to
     send to an address that no association exists to will implicitly create a
     new association.  Data sent in such an instance will result in the data
     being sent on the third leg of the SCTP four-way handshake.  Note that if
     the socket is a one-to-one type (SOCK_STREAM) socket then an association
     must be in existence (by use of the connect(2) system call).  Calling
     sctp_sendmsg() or sctp_sendmsgx() on a non-connected one-to-one socket
     will result in errno being set to ENOTCONN, -1 being returned, and the
     message not being transmitted.

     The address of the target is given by to with tolen specifying its size.
     The length of the message msg is given by len.  If the message is too
     long to pass atomically through the underlying protocol, errno is set to
     EMSGSIZE, -1 is returned, and the message is not transmitted.

     No indication of failure to deliver is implicit in a sctp_sendmsg() call.
     Locally detected errors are indicated by a return value of -1.

     If no space is available at the socket to hold the message to be
     transmitted, then sctp_sendmsg() normally blocks, unless the socket has
     been placed in non-blocking I/O mode.  The select(2) system call may be
     used to determine when it is possible to send more data on one-to-one
     type (SOCK_STREAM) sockets.

     The ppid argument is an opaque 32 bit value that is passed transparently
     through the stack to the peer endpoint.  It will be available on
     reception of a message (see sctp_recvmsg(3)).  Note that the stack passes
     this value without regard to byte order.

     The flags argument may include one or more of the following:

     #define SCTP_EOF          0x0100        /* Start a shutdown procedures */
     #define SCTP_ABORT        0x0200        /* Send an ABORT to peer */
     #define SCTP_UNORDERED    0x0400        /* Message is un-ordered */
     #define SCTP_ADDR_OVER    0x0800        /* Override the primary-address */
     #define SCTP_SENDALL      0x1000        /* Send this on all associations */
                                             /* for the endpoint */
     /* The lower byte is an enumeration of PR-SCTP policies */
     #define SCTP_PR_SCTP_TTL  0x0001        /* Time based PR-SCTP */
     #define SCTP_PR_SCTP_BUF  0x0002        /* Buffer based PR-SCTP */
     #define SCTP_PR_SCTP_RTX  0x0003        /* Number of retransmissions based PR-SCTP */

     The flag SCTP_EOF is used to instruct the SCTP stack to queue this
     message and then start a graceful shutdown of the association.  All
     remaining data in queue will be sent after which the association will be
     shut down.

     SCTP_ABORT is used to immediately terminate an association.  An abort is
     sent to the peer and the local TCB is destroyed.

     SCTP_UNORDERED is used to specify that the message being sent has no
     specific order and should be delivered to the peer application as soon as
     possible.  When this flag is absent messages are delivered in order
     within the stream they are sent, but without respect to order to peer

     The flag SCTP_ADDR_OVER is used to specify that an specific address
     should be used.  Normally SCTP will use only one of a multi-homed peers
     addresses as the primary address to send to.  By default, no matter what
     the to argument is, this primary address is used to send data.  By
     specifying this flag, the user is asking the stack to ignore the primary
     address and instead use the specified address not only as a lookup
     mechanism to find the association but also as the actual address to send

     For a one-to-many type (SOCK_SEQPACKET) socket the flag SCTP_SENDALL can
     be used as a convenient way to make one send call and have all
     associations that are under the socket get a copy of the message.  Note
     that this mechanism is quite efficient and makes only one actual copy of
     the data which is shared by all the associations for sending.

     The remaining flags are used for the partial reliability extension
     (RFC3758) and will only be effective if the peer endpoint supports this
     extension.  This option specifies what local policy the local endpoint
     should use in skipping data.  If none of these options are set, then data
     is never skipped over.

     SCTP_PR_SCTP_TTL is used to indicate that a time based lifetime is being
     applied to the data.  The timetolive argument is then a number of
     milliseconds for which the data is attempted to be transmitted.  If that
     many milliseconds elapse and the peer has not acknowledged the data, the
     data will be skipped and no longer transmitted.  Note that this policy
     does not even assure that the data will ever be sent.  In times of a
     congestion with large amounts of data being queued, the timetolive may
     expire before the first transmission is ever made.

     The SCTP_PR_SCTP_BUF based policy transforms the timetolive field into a
     total number of bytes allowed on the outbound send queue.  If that number
     or more bytes are in queue, then other buffer based sends are looked to
     be removed and skipped.  Note that this policy may also result in the
     data never being sent if no buffer based sends are in queue and the
     maximum specified by timetolive bytes is in queue.

     The SCTP_PR_SCTP_RTX policy transforms the timetolive into a number of
     retransmissions to allow.  This policy always assures that at a minimum
     one send attempt is made of the data.  After which no more than
     timetolive retransmissions will be made before the data is skipped.

     stream_no is the SCTP stream that you wish to send the message on.
     Streams in SCTP are reliable (or partially reliable) flows of ordered
     messages.  The context field is used only in the event the message cannot
     be sent.  This is an opaque value that the stack retains and will give to
     the user when a failed send is given if that notification is enabled (see
     sctp(4)).  Normally a user process can use this value to index some
     application specific data structure when a send cannot be fulfilled.
     sctp_sendmsgx() is identical to sctp_sendmsg() with the exception that it
     takes an array of sockaddr structures in the argument to and adds the
     additional argument addrcnt which specifies how many addresses are in the
     array.  This allows a caller to implicitly set up an association passing
     multiple addresses as if sctp_connectx() had been called to set up the

     The call returns the number of characters sent, or -1 if an error

     The sctp_sendmsg() system call fails if:

     [EAGAIN]           The socket is marked non-blocking and the requested
                        operation would block.

     [EBADF]            An invalid descriptor was specified.

     [ECONNRESET]       An abort was received by the stack while the user was
                        attempting to send data to the peer.

     [EFAULT]           An invalid user space address was specified for an

     [EHOSTUNREACH]     The remote host was unreachable.

     [EMSGSIZE]         The socket requires that message be sent atomically,
                        and the size of the message to be sent made this

     [ENOBUFS]          The system was unable to allocate an internal buffer.
                        The operation may succeed when buffers become
                        available.  This error is also returned when 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.

     [ENOENT]           On a one-to-many style socket no address is specified
                        so that the association cannot be located or the
                        SCTP_ABORT flag was specified on a non-existing

     [ENOTCONN]         On a one-to-one style socket no association exists.

     [ENOTSOCK]         The argument s is not a socket.

     [EPIPE]            The socket is unable to send anymore data
                        (SBS_CANTSENDMORE has been set on the socket).  This
                        typically means that the socket is not connected and
                        is a one-to-one style socket.

     connect(2), getsockopt(2), recv(2), select(2), sendmsg(2), socket(2),
     write(2), sctp_connectx(3), sctp(4)

     Sockets API Extensions for the Stream Control Transmission Protocol
     (SCTP), RFC, 6458, December 2011.

     These functions first appeared in NetBSD 9.0.

     Because in the one-to-many style socket sctp_sendmsg() or sctp_sendmsgx()
     may have multiple associations under one endpoint, a select on write will
     only work for a one-to-one style socket.

NetBSD 10.99                    August 1, 2018                    NetBSD 10.99