Manual Pages  — SCTP_RECVMSG

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_EOR flag applied. If the message is a complete message then the flags argument will have MSG_EOR 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().