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.Dd February 2, 2024
.Dt SCTP_SENDMSG 3
.Os
.Sh NAME
.Nm sctp_sendmsg ,
.Nm sctp_sendmsgx
.Nd send a message from an SCTP socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/sctp.h
.Ft ssize_t
.Fo sctp_sendmsg
.Fa "int s" "const void *msg" "size_t len" "const struct sockaddr *to"
.Fa "socklen_t tolen" "uint32_t ppid" "uint32_t flags" "uint16_t stream_no"
.Fa "uint32_t timetolive" "uint32_t context"
.Fc
.Ft ssize_t
.Fo sctp_sendmsgx
.Fa "int s" "const void *msg" "size_t len" "const struct sockaddr *to"
.Fa "int addrcnt" "uint32_t ppid" "uint32_t flags" "uint16_t stream_no"
.Fa "uint32_t timetolive" "uint32_t context"
.Fc
.Sh DESCRIPTION
The
.Fn sctp_sendmsg
system call
is used to transmit a message to another SCTP endpoint.
The
.Fn 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
.Xr connect 2
system call).
Calling
.Fn sctp_sendmsg
or
.Fn sctp_sendmsgx
on a non-connected one-to-one socket will result in
.Va errno
being set to
.Er ENOTCONN ,
-1 being returned, and the message not being transmitted.
.Pp
The address of the target is given by
.Fa to
with
.Fa tolen
specifying its size.
The length of the message
.Fa msg
is given by
.Fa len .
If the message is too long to pass atomically through the
underlying protocol,
.Va errno
is set to
.Er EMSGSIZE ,
-1 is returned, and
the message is not transmitted.
.Pp
No indication of failure to deliver is implicit in a
.Fn sctp_sendmsg
call.
Locally detected errors are indicated by a return value of -1.
.Pp
If no space is available at the socket to hold
the message to be transmitted, then
.Fn sctp_sendmsg
normally blocks, unless the socket has been placed in
non-blocking I/O mode.
The
.Xr 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.
.Pp
The
.Fa 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
.Xr sctp_recvmsg 3 ) .
Note that the stack passes this value without regard to byte
order.
.Pp
The
.Fa flags
argument may include one or more of the following:
.Bd -literal
#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 */
.Ed
.Pp
The flag
.Dv 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.
.Pp
.Dv SCTP_ABORT
is used to immediately terminate an association.
An abort
is sent to the peer and the local TCB is destroyed.
.Pp
.Dv 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 streams.
.Pp
The flag
.Dv 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
.Fa 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 to.
.Pp
For a one-to-many type (SOCK_SEQPACKET) socket the flag
.Dv 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.
.Pp
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.
.Pp
.Dv SCTP_PR_SCTP_TTL
is used to indicate that a time based lifetime is being applied
to the data.
The
.Fa 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
.Fa timetolive
may expire before the first transmission is ever made.
.Pp
The
.Dv SCTP_PR_SCTP_BUF
based policy transforms the
.Fa 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
.Fa timetolive
bytes is in queue.
.Pp
The
.Dv SCTP_PR_SCTP_RTX
policy transforms the
.Fa 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
.Fa timetolive
retransmissions will be made before the data is skipped.
.Pp
.Fa 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
.Fa 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
.Xr sctp 4 ) .
Normally a user process can use this value to index some application
specific data structure when a send cannot be fulfilled.
.Fn sctp_sendmsgx
is identical to
.Fn sctp_sendmsg
with the exception that it takes an array of sockaddr structures in the
argument
.Fa to
and adds the additional argument
.Fa 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
.Fn sctp_connectx
had been called to set up the association.
.Sh RETURN VALUES
The call returns the number of characters sent, or -1
if an error occurred.
.Sh ERRORS
The
.Fn sctp_sendmsg
system call
fails if:
.Bl -tag -width Er
.It Bq Er EBADF
An invalid descriptor was specified.
.It Bq Er ENOTSOCK
The argument
.Fa s
is not a socket.
.It Bq Er EFAULT
An invalid user space address was specified for an argument.
.It Bq Er EMSGSIZE
The socket requires that message be sent atomically,
and the size of the message to be sent made this impossible.
.It Bq Er EAGAIN
The socket is marked non-blocking and the requested operation
would block.
.It Bq Er ENOBUFS
The system was unable to allocate an internal buffer.
The operation may succeed when buffers become available.
.It Bq Er 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.
.It Bq Er EHOSTUNREACH
The remote host was unreachable.
.It Bq Er ENOTCONN
On a one-to-one style socket no association exists.
.It Bq Er ECONNRESET
An abort was received by the stack while the user was
attempting to send data to the peer.
.It Bq Er ENOENT
On a one-to-many style socket no address is specified
so that the association cannot be located or the
.Dv SCTP_ABORT
flag was specified on a non-existing association.
.It Bq Er EPIPE
The socket is unable to send anymore data
.Dv ( 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.
.El
.Sh SEE ALSO
.Xr connect 2 ,
.Xr getsockopt 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr sendmsg 2 ,
.Xr socket 2 ,
.Xr write 2 ,
.Xr sctp_connectx 3 ,
.Xr sctp 4
.Sh BUGS
Because in the one-to-many style socket
.Fn sctp_sendmsg
or
.Fn sctp_sendmsgx
may have multiple associations under one endpoint, a
select on write will only work for a one-to-one style
socket.