SCTP_RECVMSG(3)

NAME
sctp_recvmsg - receive a message from an SCTP socket

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/sctp.h>
ssize_t
sctp_recvmsg(int s, void *msg, size_t len, struct sockaddr * restrict from, socklen_t * restrict fromlen, struct sctp_sndrcvinfo *sinfo, int *flags);

DESCRIPTION
The sctp_recvmsg() system call is used to receive a message from another SCTP endpoint. The sctp_recvmsg() call is used by one-to-one (SOCK_STREAM) type sockets after a successful connect() call or after the application has performed a listen() followed by a successful accept(). For a one-to-many (SOCK_SEQPACKET) type socket, an endpoint may call sctp_recvmsg() after having implicitly started an association via one of the send calls including sctp_sendmsg(), sendto(), and sendmsg(). Or, an application may also receive a message after having called listen() with a positive backlog to enable the reception of new associations.

The address of the sender is held in the from argument with fromlen specifying its size. At the completion of a successful sctp_recvmsg() call from will hold the address of the peer and fromlen will hold the length of that address. Note that the address is bounded by the initial value of fromlen which is used as an in/out variable.

The length of the message msg to be received is bounded by len. If the message is too long to fit in the users receive buffer, then the flags argument will not have the MSG_EOF flag applied. If the message is a complete message then the flags argument will have MSG_EOF set. Locally detected errors are indicated by a return value of -1 with errno set accordingly. The flags argument may also hold the value MSG_NOTIFICATION. When this occurs it indicates that the message received is not from the peer endpoint, but instead is a notification from the SCTP stack (see sctp(4) for more details). Note that no notifications are ever given unless the user subscribes to such notifications using the SCTP_EVENTS socket option.

If no messages are available at the socket then 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 select(2) system call may be used to determine when it is possible to receive a message.

The sinfo argument is defined as follows.

    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 */
    };

The 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.

The sinfo->sinfo_flags field may include the following:

    #define SCTP_UNORDERED 	  0x0400	/* Message is un-ordered */

The 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.

The 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.

The sinfo->sinfo_context field is used only if the local application set an association level context with the 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.

The sinfo->sinfo_ssn field will hold the stream sequence number assigned by the peer endpoint if the message is not unordered. For unordered messages this field holds an undefined value.

The 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.

The 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.

The 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 sctp(4)).

The sinfo->info_timetolive field is not used by sctp_recvmsg().

RETURN VALUES
The call returns the number of bytes received, or -1 if an error occurred.

ERRORS
The sctp_recvmsg() 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 argument.

[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 impossible.

[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 association.

[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.

SEE ALSO
getsockopt(2), recv(2), select(2), sendmsg(2), setsockopt(2), socket(2), write(2), sctp_send(3), sctp_sendmsg(3), sctp(4) RFC, 6458, Sockets API Extensions for the Stream Control Transmission Protocol (SCTP), December 2011.

HISTORY
This function first appeared in NetBSD 9.0.