Advanced Programming in the UNIX Environment: Second Edition [Electronic resources] نسخه متنی

16.7. Out-of-Band Data

Sections 3.14 and 14.6.2, we saw that we could use the F_SETOWN command to fcntl to set the ownership of a socket. If the third argument to fcntl is positive, it specifies a process ID. If it is a negative value other than -1, it represents the process group ID. Thus, we can arrange that our process receive signals from a socket by calling

fcntl(sockfd, F_SETOWN, pid);

The F_GETOWN command can be used to retrieve the current socket ownership. As with the F_SETOWN command, a negative value represents a process group ID, and a positive value represents a process ID. Thus, the call

owner = fcntl(sockfd, F_GETOWN, 0);

will return with owner equal to the ID of the process configured to receive signals from the socket if owner is positive and with the absolute value of owner equal to the ID of the process group configured to receive signals from the socket if owner is negative.

TCP supports the notion of an

urgent mark : the point in the normal data stream where the urgent data would go. We can choose to receive the urgent data inline with the normal data if we use the SO_OOBINLINE socket option. To help us identify when we have reached the urgent mark, we can use the sockatmark function.

When the next byte to be read is where the urgent mark is located, sockatmark will return 1.

When out-of-band data is present in a socket's read queue, the select function (Section 14.5.1) will return the file descriptor as having an exception condition pending. We can choose to receive the urgent data inline with the normal data, or we can use the MSG_OOB flag with one of the recv functions to receive the urgent data ahead of any other queue data. TCP queues only one byte of urgent data. If another urgent byte arrives before we receive the current one, the existing one is discarded.