STREAMS Programming Guide

Flushing Priority Band

The bi_flag field is one of FLUSHR, FLUSHW, or FLUSHRW.

The following example shows flushing according to the priority band.

queue_t *rdq;								/* read queue */
queue_t *wrq;								/* write queue */

	case M_FLUSH:
		if (*bp->b_rptr & FLUSHBAND) {
			if (*bp->b_rptr & FLUSHW)
				flushband(wrq, FLUSHDATA, *(bp->b_rptr + 1));
			if (*bp->b_rptr & FLUSHR)
				flushband(rdq, FLUSHDATA, *(bp->b_rptr + 1));
		} else {
			if (*bp->b_rptr & FLUSHW)
				flushq(wrq, FLUSHDATA);
			if (*bp->b_rptr & FLUSHR)
				flushq(rdq, FLUSHDATA);
		 * modules pass the message on;
		 * drivers shut off FLUSHW and loop the message
		 * up the read-side if FLUSHR is set; otherwise,
		 * drivers free the message.

Note that modules and drivers are not required to treat messages as flowing in separate bands. Modules and drivers can view the queue having only two bands of flow, normal and high priority. However, the latter alternative flushes the entire queue whenever an M_FLUSH message is received.

One use of the field b_flag of the msgb structure is provided to give the stream head a way to stop M_FLUSH messages from being reflected forever when the stream is used as a pipe. When the stream head receives an M_FLUSH message, it sets the MSGNOLOOP flag in the b_flag field before reflecting the message down the write-side of the stream. If the stream head receives an M_FLUSH message with this flag set, the message is freed rather than reflected.

Figure 8-3 Interfaces Affecting Drivers


The set of STREAMS utilities available to drivers are listed in Appendix B, STREAMS Utilities. No system-defined macros that manipulate global kernel data or introduce structure-size dependencies are permitted in these utilities. So, some utilities that have been implemented as macros in the prior Solaris system releases are implemented as functions in the SunOS 5 System. This does not preclude the existence of both macro and function versions of these utilities. It is intended that driver source code include a header file that picks up function declarations while the core operating system source includes a header file that defines the macros. With the DKI interface, the following STREAMS utilities are implemented as C programming language functions: datamsg(9F), OTHERQ(9F), putnext(9F), RD(9F), and WR(9F).

Replacing macros such as RD with function equivalents in the driver source code allows driver objects to be insulated from changes in the data structures and their size, increasing the useful lifetime of driver source code and objects. Multithreaded drivers are also protected against changes in implementation-specific STREAMS synchronization.

The DKI defines an interface suitable for drivers and there is no need for drivers to access global kernel data structures directly. The kernel function drv_getparm(9F) fetches information from these structures. This restriction has an important consequence. Since drivers are not permitted to access global kernel data structures directly, changes in the contents/offsets of information within these structures will not break objects.