The stream socket abstraction includes the concept of out-of-band data. Out-of-band data is a logically independent transmission channel associated with each pair of connected stream sockets. Out-of-band data can be delivered to the socket independently of the normal receive queue or within the receive queue depending upon the status of the SO_OOBINLINE socket-level option. The abstraction defines that the out-of-band data facilities must support the reliable delivery of at least one out-of-band message at a time. This message must contain at least one byte of data, and at least one message can be pending delivery to the user at any one time.
For communication protocols that support only in-band signaling (that is, the urgent data is delivered in sequence with the normal data), the operating system normally extracts the data from the normal data stream and stores it separately. This allows users to choose between receiving the urgent data in order and receiving it out of sequence without having to buffer all the intervening data.
If multiple sockets have out-of-band data awaiting delivery, an application program can use a select() call for exceptional conditions to determine those sockets with such data pending. The select() call does not indicate the actual arrival of the out-of-band data, but only notification that it is pending.
In addition to the information passed, a logical mark is placed in the data stream to indicate the point at which the out-of-band data was sent. When a signal flushes any pending output, all data up to the mark in the data stream is discarded.
To send an out-of-band message, the MSG_OOB flag is supplied to a send() or sendto() call. To receive out-of-band data, an application program must set the MSG_OOB flag when performing a recvfrom() or recv() call.
An application program can determine if the read pointer is currently pointing at the logical mark in the data stream, by using the SIOCATMARK ioctl() call.
A process can also read or peek at the out-of-band data without first reading up to the logical mark. This is more difficult when the underlying protocol delivers the urgent data in-band with the normal data, and only sends notification of its presence ahead of time (that is, the TCP protocol used to implement streams in the internet domain). With such protocols, the out-of-band byte may not have arrived when a recv() call is performed with the MSG_OOB flag. In that case, the call will return an SOCEWOULDBLOCK error code. There may be enough in-band data in the input buffer for normal flow control to prevent the peer from sending the urgent data until the buffer is cleared. The process must then read enough of the queued data so that the urgent data can be delivered.
Certain programs that use multiple bytes of urgent data, and that must handle multiple urgent signals, need to retain the position of urgent data within the stream. The socket-level option, SO_OOBINLINE provides the capability. With this option, the position of the urgent data (the logical mark) is retained. The urgent data immediately follows the mark within the normal data stream that is returned without the MSG_OOB flag. Reception of multiple urgent indications causes the mark to move, but no out-of-band data is lost.