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.Dd May 2, 2024
.Dt SCTP_RECVMSG 3
.Os
.Sh NAME
.Nm sctp_recvmsg
.Nd receive 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_recvmsg
.Fa "int s" "void *msg" "size_t len" "struct sockaddr *from"
.Fa "socklen_t *fromlen" "struct sctp_sndrcvinfo *sinfo" "int *flags"
.Fc
.Sh DESCRIPTION
The
.Fn sctp_recvmsg
system call
is used to receive a message from another SCTP endpoint.
The
.Fn sctp_recvmsg
call is used by one-to-one (SOCK_STREAM) type sockets after a
successful
.Fn connect
call or after the application has performed a
.Fn listen
followed by a successful
.Fn accept .
For a one-to-many (SOCK_SEQPACKET) type socket, an endpoint may call
.Fn sctp_recvmsg
after having implicitly started an association via one
of the send calls including
.Fn sctp_sendmsg ,
.Fn sendto
and
.Fn sendmsg .
Or, an application may also receive a message after having
called
.Fn listen
with a positive backlog to enable the reception of new associations.
.Pp
The address of the sender is held in the
.Fa from
argument with
.Fa fromlen
specifying its size.
At the completion of a successful
.Fn sctp_recvmsg
call
.Fa from
will hold the address of the peer and
.Fa fromlen
will hold the length of that address.
Note that
the address is bounded by the initial value of
.Fa fromlen
which is used as an in/out variable.
.Pp
The length of the message
.Fa msg
to be received is bounded by
.Fa len .
If the message is too long to fit in the users
receive buffer, then the
.Fa flags
argument will
.Em not
have the
.Dv MSG_EOR
flag applied.
If the message is a complete message then
the
.Fa flags
argument will have
.Dv MSG_EOR
set.
Locally detected errors are
indicated by a return value of -1 with
.Va errno
set accordingly.
The
.Fa flags
argument may also hold the value
.Dv MSG_NOTIFICATION .
When this
occurs it indicates that the message received is
.Em not
from
the peer endpoint, but instead is a notification from the
SCTP stack (see
.Xr sctp 4
for more details).
Note that no notifications are ever
given unless the user subscribes to such notifications using
the
.Dv SCTP_EVENTS
socket option.
.Pp
If no messages are available at the socket then
.Fn sctp_recvmsg
normally blocks on the reception of a message or NOTIFICATION, 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
receive a message.
.Pp
The
.Fa sinfo
argument is defined as follows.
.Bd -literal
struct sctp_sndrcvinfo {
	uint16_t sinfo_stream;  /* Stream arriving on */
	uint16_t sinfo_ssn;     /* Stream Sequence Number */
	uint16_t sinfo_flags;   /* Flags on the incoming message */
	uint32_t sinfo_ppid;    /* The ppid field */
	uint32_t sinfo_context; /* context field */
	uint32_t sinfo_timetolive; /* not used by sctp_recvmsg */
	uint32_t sinfo_tsn;        /* The transport sequence number */
	uint32_t sinfo_cumtsn;     /* The cumulative acknowledgment point  */
	sctp_assoc_t sinfo_assoc_id; /* The association id of the peer */
};
.Ed
.Pp
The
.Fa sinfo->sinfo_ppid
field is an opaque 32 bit value that is passed transparently
through the stack from the peer endpoint.
Note that the stack passes this value without regard to byte
order.
.Pp
The
.Fa sinfo->sinfo_flags
field may include the following:
.Bd -literal
#define SCTP_UNORDERED 	  0x0400	/* Message is un-ordered */
.Ed
.Pp
The
.Dv SCTP_UNORDERED
flag is used to specify that the message arrived with no
specific order and was delivered to the peer application
as soon as possible.
When this flag is absent the message
was delivered in order within the stream it was received.
.Pp
The
.Fa sinfo->sinfo_stream
field is the SCTP stream that the message was received on.
Streams in SCTP are reliable (or partially reliable) flows of ordered
messages.
.Pp
The
.Fa sinfo->sinfo_context
field is used only if the local application set an association level
context with the
.Dv SCTP_CONTEXT
socket option.
Optionally a user process can use this value to index some application
specific data structure for all data coming from a specific
association.
.Pp
The
.Fa sinfo->sinfo_ssn
field will hold the stream sequence number assigned
by the peer endpoint if the message is
.Em not
unordered.
For unordered messages this field holds an undefined value.
.Pp
The
.Fa sinfo->sinfo_tsn
field holds a transport sequence number (TSN) that was assigned
to this message by the peer endpoint.
For messages that fit in or less
than the path MTU this will be the only TSN assigned.
Note that for messages that span multiple TSNs this
value will be one of the TSNs that was used on the
message.
.Pp
The
.Fa sinfo->sinfo_cumtsn
field holds the current cumulative acknowledgment point of
the transport association.
Note that this may be larger
or smaller than the TSN assigned to the message itself.
.Pp
The
.Fa sinfo->sinfo_assoc_id
is the unique association identification that was assigned
to the association.
For one-to-many (SOCK_SEQPACKET) type
sockets this value can be used to send data to the peer without
the use of an address field.
It is also quite useful in
setting various socket options on the specific association
(see
.Xr sctp 4 ) .
.Pp
The
.Fa sinfo->info_timetolive
field is not used by
.Fn sctp_recvmsg .
.Sh RETURN VALUES
The call returns the number of bytes received, or -1
if an error occurred.
.Sh ERRORS
The
.Fn sctp_recvmsg
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
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 getsockopt 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr sendmsg 2 ,
.Xr setsockopt 2 ,
.Xr socket 2 ,
.Xr write 2 ,
.Xr sctp_send 3 ,
.Xr sctp_sendmsg 3 ,
.Xr sctp 4