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Dd August 1, 2018
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 * restrict from"
Fa "socklen_t * restrict 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
Dv ( 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
Dv ( 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_EOF
flag applied.
If the message is a complete message then
the
Fa flags
argument will have
Dv MSG_EOF
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
Dv ( 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 EAGAIN
The socket is marked non-blocking and the requested operation
would block.
It Bq Er EBADF
An invalid descriptor was specified.
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 EFAULT
An invalid user space address was specified for an argument.
It Bq Er EHOSTUNREACH
The remote host was unreachable.
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 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.
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 ENOTCONN
On a one-to-one style socket no association exists.
It Bq Er ENOTSOCK
The argument
Fa s
is not a socket.
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
Rs
%R RFC
%N 6458
%T "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)"
%D December 2011
Re
Sh HISTORY
This function first appeared in
Nx 9.0 .

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