STREAMS Programming Guide

Chapter 13 STREAMS Multiplex Drivers

This chapter describes how STREAMS multiplexing configurations are created and also discusses multiplexing drivers. A STREAMS multiplexer is a driver with multiple streams connected to it. The primary function of the multiplexing driver is to switch messages among the connected streams. Multiplexer configurations are created from the user level by system calls.

This chapter contains the following information:

STREAMS Multiplexers

STREAMS-related system calls are used to set up the “plumbing,” or stream interconnections, for multiplexing drivers. The subset of these calls that allows a user to connect (and disconnect) streams below a driver is referred to as multiplexing. This type of connection is referred to as a one-to-M, or lower, multiplexer configuration. This configuration must always contain a multiplexing driver, which is recognized by STREAMS as having special characteristics.

Multiple streams can be connected above a driver by open(2) calls. This accommodates the loop-around driver and the driver that handled multiple minor devices in Chapter 9, STREAMS Drivers. There is no difference between the connections to these drivers. Only the functions performed by the driver are different. In the multiplexing case, the driver routes data between multiple streams. In the device driver case, the driver routes data between user processes and associated physical ports. Multiplexing with streams connected above is referred to as an N-to-1, or upper, multiplexer. STREAMS does not provide any facilities beyond open and close to connect or disconnect upper streams for multiplexing.

From the driver's perspective, upper and lower configurations differ only in the way they are initially connected to the driver. The implementation requirements are the same: route the data and handle flow control. All multiplexer drivers require special developer-provided software to perform the multiplexing data routing and to handle flow control. STREAMS does not directly support flow control among multiplexed streams. M-to-N multiplexing configurations are implemented by using both of these mechanisms in a driver.

As discussed in Chapter 9, STREAMS Drivers, the multiple streams that represent minor devices are actually distinct streams in which the driver keeps track of each stream attached to it. The STREAMS subsystem does not recognize any relationship between the streams. The same is true for STREAMS multiplexers of any configuration. The multiplexed streams are distinct and the driver must be implemented to do most of the work.

In addition to upper and lower multiplexers, more complex configurations can be created by connecting streams containing multiplexers to other multiplexer drivers. With such a diversity of needs for multiplexers, providing general-purpose multiplexer drivers is not possible. Rather, STREAMS provides a general purpose multiplexing facility. The facility enables you to set up the intermodule or driver plumbing to create multiplexer configurations of generally unlimited interconnection.

Building a Multiplexer

The example in this section builds a protocol multiplexer with the multiplexing configuration shown in Figure 13–1. To free users from the need to know about the underlying protocol structure, a user-level daemon process is built to maintain the multiplexing configuration. Users can then access the transport protocol directly by opening the transport protocol (TP) driver device node.

An internetworking protocol driver (IP) routes data from a single upper stream to one of two lower streams. This driver supports two STREAMS connections beneath it. These connections are to two distinct networks; one for the IEEE 802.3 standard through the 802.3 driver, and another to the IEEE 802.4 standard through the 802.4 driver. The TP driver multiplexes upper streams over a single stream to the IP driver.

Figure 13–1 Protocol Multiplexer

Diagram shows the multiplexing used in the code example in this section.

Example 13–1 shows how this daemon process sets up the protocol multiplexer. The necessary declarations and initialization for the daemon program follow.

Example 13–1 Protocol Daemon

#include <fcntl.h>
#include <stropts.h>
		int	fd_802_4,
		/* daemon-ize this process */

		switch (fork()) {
			case 0:
			case -1:
				perror("fork failed");

This multilevel multiplexed stream configuration is built from the bottom up. The example begins by first constructing the IP multiplexer. This multiplexing device driver is treated like any other software driver. It owns a node in the Solaris file system and is opened just like any other STREAMS device driver.

The first step is to open the multiplexing driver and the 802.4 driver, thus creating separate streams above each driver as shown in Figure 13–2. The stream to the 802.4 driver may now be connected below the multiplexing IP driver using the I_LINK ioctl(2).

Figure 13–2 Streams Before Link

Diagram continues the example. It shows the streams created after the drivers are opened.

The sequence of instructions to this point is:

	if ((fd_802_4 = open("/dev/802_4", O_RDWR)) < 0) {
			perror("open of /dev/802_4 failed");
		if ((fd_ip = open("/dev/ip", O_RDWR)) < 0) {
			perror("open of /dev/ip failed");
		/* now link 802.4 to underside of IP */
		if (ioctl(fd_ip, I_LINK, fd_802_4) < 0) {
			perror("I_LINK ioctl failed");

I_LINK takes two file descriptors as arguments. The first file descriptor, fd_ip, is the stream connected to the multiplexing driver, and the second file descriptor, fd_802_4, is the stream to be connected below the multiplexer. The complete stream to the 802.4 driver is connected below the IP driver. The stream head's queues of the 802.4 driver are used by the IP driver to manage the lower half of the multiplexer.

I_LINK returns an integer value, muxid, which is used by the multiplexing driver to identify the stream just connected below it. muxid is ignored in the example, but it is useful for dismantling a multiplexer or routing data through the multiplexer. Its significance is discussed in Dismantling a Multiplexer.

The following sequence of system calls continues building the Internetworking Protocol multiplexer (IP):

	if ((fd_802_3 = open("/dev/802_3", O_RDWR)) < 0) {
			perror("open of /dev/802_3 failed");
		if (ioctl(fd_ip, I_LINK, fd_802_3) < 0) {
			perror("I_LINK ioctl failed");

The stream above the multiplexing driver used to establish the lower connections is the controlling stream and has special significance when dismantling the multiplexing configuration. This is illustrated in Dismantling a Multiplexer. The stream referenced by fd_ip is the controlling stream for the IP multiplexer.

The order in which the streams in the multiplexing configuration are opened is unimportant. If intermediate modules in the stream are necessary between the IP driver and media drivers, these modules must be added to the streams associated with the media drivers (using I_PUSH) before the media drivers are attached below the multiplexer.

The number of streams that can be linked to a multiplexer is restricted by the design of the particular multiplexer. The manual page describing each driver describes such restrictions (see SunOS Reference Manual, Intro(7)) . However, only one I_LINK operation is allowed for each lower stream; a single stream cannot be linked below two multiplexers simultaneously.

Continuing with the example, the IP driver is now linked below the transport protocol (TP) multiplexing driver. As seen in Figure 13–1, only one link is supported below the transport driver. This link is formed by the following sequence of system calls:

	if ((fd_tp = open("/dev/tp", O_RDWR)) < 0) {
			perror("open of /dev/tp failed");
		if (ioctl(fd_tp, I_LINK, fd_ip) < 0) {
			perror("I_LINK ioctl failed");

Because the controlling stream of the IP multiplexer has been linked below the TP multiplexer, the controlling stream for the new multilevel multiplexer configuration is the stream above the TP multiplexer.

At this point, the file descriptors associated with the lower drivers can be closed without affecting the operation of the multiplexer. If these file descriptors are not closed, all subsequent read(2), write(2), ioctl(2), poll(2), getmsg(2), and putmsg(2) calls issued to them fail. That is because I_LINK associates the stream head of each linked stream with the multiplexer, so the user may not access that stream directly for the duration of the link.

The following sequence of system calls completes the daemon example:

		/* Hold multiplexer open forever or at least til this process
      is terminated by an external UNIX signal */

The transport driver supports several simultaneous streams. These streams are multiplexed over the single stream connected to the IP multiplexer. The mechanism for establishing multiple streams above the transport multiplexer is actually a by-product of the way in which streams are created between a user process and a driver. By opening different minor devices of a STREAMS driver, separate streams will be connected to that driver. The driver must be designed with the intelligence to route data from the single lower stream to the appropriate upper stream.

The daemon process maintains the multiplexed stream configuration through an open stream (the controlling stream) to the transport driver. Meanwhile, other users can access the services of the transport protocol by opening new streams to the transport driver; they are freed from the need for any unnecessary knowledge of the underlying protocol configurations and subnetworks that support the transport service.

Multilevel multiplexing configurations should be assembled from the bottom up. That is because the passing of ioctl(2) through the multiplexer is determined by the nature of the multiplexing driver and cannot generally be relied on.

Dismantling a Multiplexer

Streams connected to a multiplexing driver from above with open(2), can be dismantled by closing each stream with close(2). The mechanism for dismantling streams that have been linked below a multiplexing driver is less obvious, and is described in Disconnecting Lower Streams.

I_UNLINK ioctl(2) disconnects each multiplexer link below a multiplexing driver individually. This command has the form:

ioctl(fd, I_UNLINK, muxid);

where fd is a file descriptor associated with a stream connected to the multiplexing driver from above, and muxid is the identifier that was returned by I_LINK when a driver was linked below the multiplexer. Each lower driver may be disconnected individually in this way, or a special muxid value of MUXID_ALL can be used to disconnect all drivers from the multiplexer simultaneously.

In the multiplexing daemon program, the multiplexer is never explicitly dismantled. That is because all links associated with a multiplexing driver are automatically dismantled when the controlling stream associated with that multiplexer is closed. Because the controlling stream is open to a driver, only the final call of close for that stream will close it. In this case, the daemon is the only process that has opened the controlling stream, so the multiplexing configuration will be dismantled when the daemon exits.

For the automatic dismantling mechanism to work in the multilevel, multiplexed stream configuration, the controlling stream for each multiplexer at each level must be linked under the next higher-level multiplexer. In the example, the controlling stream for the IP driver was linked under the TP driver. This resulted in a single controlling stream for the full, multilevel configuration. Because the multiplexing program relied on closing the controlling stream to dismantle the multiplexed stream configuration instead of using explicit I_UNLINK calls, the muxid values returned by I_LINK could be ignored.

An important side effect of automatic dismantling on the close is that a process cannot build a multiplexing configuration with I_LINK and then exit. exit(2) closes all files associated with the process, including the controlling stream. To keep the configuration intact, the process must exist for the life of that multiplexer. That is the motivation for implementing the multiplexer as a daemon processs, see Multiplexing Driver Example.

If the process uses persistent links through I_PLINK ioctl(2), the multiplexer configuration remains intact after the process exits. These links are described in Persistent Links.

Routing Data Through a Multiplexer

As demonstrated, STREAMS provides a mechanism for building multiplexed stream configurations. However, the criteria by which a multiplexer routes data are driver dependent. For example, the protocol multiplexer might use address information found in a protocol header to determine the subnetwork over which data should be routed. You must define its routing criteria.

One routing option available to the multiplexer is to use the muxid value to determine the stream to which data is routed (remember that each multiplexer link has a muxid). I_LINK passes the muxid value to the driver and returns this value to the user. The driver can therefore specify that the muxid value accompany data routed through it. For example, if a multiplexer routed data from a single upper stream to one of several lower streams (as did the IP driver), the multiplexer can require the user to insert the muxid of the desired lower stream into the first four bytes of each message passed to it. The driver can then match the muxid in each message with the muxid of each lower stream, and route the data accordingly.

Connecting And Disconnecting Lower Streams

Multiple streams are created above a driver/multiplexer by use of the open system call on either different minor device, or on a cloneable device file. Note that any driver that handles more than one minor device is considered an upper multiplexer.

To connect streams below a multiplexer requires additional software in the multiplexer. The main difference between STREAMS lower multiplexers and STREAMS device drivers is that multiplexers are pseudo-devices and multiplexers have two additional qinit structures, pointed to by fields in streamtab(9S): the lower half read-side qinit(9S) and the lower half write-side qinit(9S).

The multiplexer is divided into two parts: the lower half and the upper half. The multiplexer queue structures allocated when the multiplexer was opened use the usual qinit entries from the multiplexer's streamtab(9S). This is the same as any open of the STREAMS device. When a lower stream is linked beneath the multiplexer, the qinit structures at the stream head are substituted by the lower half qinit(9S) structures identified in the streamstab for the multiplexers. Once the linkage is made, the multiplexer switches messages between upper and lower streams. When messages reach the top of the lower stream, they are handled by put and service routines specified in the bottom half of the multiplexer.

Connecting Lower Streams

A lower multiplexer is connected as follows: the initial open to a multiplexing driver creates a stream, as in any other driver. open uses the st_rdinit and st_wrinit elements of the streamtab structure to initialize the driver queues.. At this point, the only distinguishing characteristics of this stream are non-NULL entries in the streamtab(9S) st_muxrinit and st_muxwinit fields.

These fields are ignored by open. Any other stream subsequently opened to this driver will have the same streamtab and thereby the same mux fields.

Next, another file is opened to create a (soon-to-be) lower stream. The driver for the lower stream is typically a device driver This stream has no distinguishing characteristics. It can include any driver compatible with the multiplexer. Any modules required on the lower stream must be pushed onto it now.

Next, this lower stream is connected below the multiplexing driver with an I_LINK ioctl(2) (see streamio(7I)). The stream head points to the stream head routines as its procedures (through its queue). An I_LINK to the upper stream, referencing the lower stream, causes STREAMS to modify the contents of the stream head's queues in the lower stream. The pointers to the stream head routines, and other values, in the stream head's queues are replaced with those contained in the mux fields of the multiplexing driver's streamtab. Changing the stream head routines on the lower stream means that all subsequent messages sent upstream by the lower stream's driver are passed to the put procedure designated in st_muxrinit, the multiplexing driver. The I_LINK also establishes this upper stream as the control stream for this lower stream. STREAMS remembers the relationship between these two streams until the upper stream is closed or the lower stream is unlinked.

Finally, the stream head sends an M_IOCTL message with ioc_cmd set to I_LINK to the multiplexing driver. The M_DATA part of the M_IOCTL contains a linkblk(9S) structure. The multiplexing driver stores information from the linkblk(9S) structure in private storage and returns an M_IOCACK acknowledgement. l_index is returned to the process requesting the I_LINK. This value is used later by the process to disconnect the stream.

An I_LINK is required for each lower stream connected to the driver. Additional upper streams can be connected to the multiplexing driver by open calls. Any message type can be sent from a lower stream to user processes along any of the upper streams. The upper streams provide the only interface between the user processes and the multiplexer.

No direct data structure linkage is established for the linked streams. The read queue's q_next is NULL and the write queue's q_next points to the first entity on the lower stream. Messages flowing upstream from a lower driver (a device driver or another multiplexer) will enter the multiplexing driver put procedure with the queue represented in l_qbot as the queue_t for the put procedure. The multiplexing driver has to route the messages to the appropriate upper (or lower) stream. Similarly, a message coming downstream from user space on any upper stream has to be processed and routed, if required, by the driver.

Note –

It is the responsibility of the driver to handle routing of messages between the upper and lower streams, or between any lateral stream that is part of the multiplexer. This operation is not handled by the STREAMS framework.

In general, multiplexing drivers should be implemented so that new streams can be dynamically connected to (and existing streams disconnected from) the driver without interfering with its ongoing operation. The number of streams that can be connected to a multiplexer is implementation dependent.

Disconnecting Lower Streams

Dismantling a lower multiplexer is accomplished by disconnecting (unlinking) the lower streams. Unlinking can be initiated in three ways:

As in the link, an unlink sends a linkblk(9S) structure to the driver in an M_IOCTL message. The I_UNLINK call, which unlinks a single stream, uses the l_index value returned in the I_LINK to specify the lower stream to be unlinked. The latter two calls must designate a file corresponding to a control stream, which causes all the lower streams that were previously linked by this control stream to be unlinked. However, the driver sees a series of individual unlinks.

If no open references exist for a lower stream, a subsequent unlink will automatically close the stream. Otherwise, the lower stream must be closed by close(2) following the unlink. STREAMS automatically dismantles all cascaded multiplexers (below other multiplexing streams) if their controlling stream is closed. An I_UNLINK leaves lower, cascaded multiplexing streams intact unless the stream file descriptor was previously closed.

Multiplexer Construction Example

This section describes an example of multiplexer construction and usage. Multiple upper and lower streams interface to the multiplexer driver.

The Ethernet, LAPB, and IEEE 802.2 device drivers terminate links to other nodes. The multiplexer driver is an Internet Protocol (IP) multiplexer that switches data among the various nodes or sends data upstream to users in the system. The net modules typically provide a convergence function that matches the multiplexer driver and device driver interface.

Streams A, B, and C are opened by the process, and modules are pushed as needed. Two upper streams are opened to the IP multiplexer. The rightmost stream represents multiple streams, each connected to a process using the network. The stream second from the right provides a direct path to the multiplexer for supervisory functions. The control stream, leading to a process, sets up and supervises this configuration. It is always directly connected to the IP driver. Although not shown, modules can be pushed on the control stream.

After the streams are opened, the supervisory process typically transfers routing information to the IP drivers (and any other multiplexers above the IP), and initializes the links. As each link becomes operational, its stream is connected below the IP driver. If a more complex multiplexing configuration is required, the IP multiplexer stream with all its connected links can be connected below another multiplexer driver.

Multiplexing Driver Example

This section contains an example of a multiplexing driver that implements an N-to-1 configuration. This configuration might be used for terminal windows, where each transmission to or from the terminal identifies the window. This resembles a typical device driver, with two differences: the device-handling functions are performed by a separate driver, connected as a lower stream, and the device information (that is, relevant user process) is contained in the input data rather than in an interrupt call.

Each upper stream is created by open(2). A single lower stream is opened and then it is linked by use of the multiplexing facility. This lower stream might connect to the TTY driver. The implementation of this example is a foundation for an M-to-N multiplexer.

As in the loop-around driver (Chapter 9, STREAMS Drivers), flow control requires the use of standard and special code because physical connectivity among the streams is broken at the driver. Different approaches are used for flow control on the lower stream, for messages coming upstream from the device driver, and on the upper streams, for messages coming downstream from the user processes.

Note –

The code presented here for the multiplexing driver represents a single-threaded, uniprocessor implementation. See Chapter 12, Multithreaded STREAMS for details on multiprocessor and multithreading issues such as locking for data corruption and to prevent race conditions.

Example 13–2 is of multiplexer declarations:

Example 13–2 Multiplexer Declarations

#include <sys/types.h>
#include <sys/param.h>
#include <sys/stream.h>
#include <sys/stropts.h>
#include <sys/errno.h>
#include <sys/cred.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>

static int muxopen (queue_t*, dev_t*, int, int, cred_t*);
static int muxclose (queue_t*, int, cred_t*);
static int muxuwput (queue_t*, mblk_t*);
static int muxlwsrv (queue_t*);
static int muxlrput (queue_t*, mblk_t*);
static int muxuwsrv (queue_t*);

static struct module_info info = {
	0xaabb, "mux", 0, INFPSZ, 512, 128 };

static struct qinit urinit = {							/* upper read */
	NULL, NULL, muxopen, muxclose, NULL, &info, NULL };

static struct qinit uwinit = {							/* upper write */
	muxuwput, muxuwsrv, NULL, NULL, NULL, &info, NULL };

static struct qinit lrinit = { /* lower read */
	muxlrput, NULL, NULL, NULL, NULL, &info, NULL };

static struct qinit lwinit = { /* lower write */
	NULL, muxlwsrv, NULL, NULL, NULL, &info, NULL };

struct streamtab muxinfo = {
	&urinit, &uwinit, &lrinit, &lwinit };

struct mux {
	queue_t *qptr;					/* back pointer to read queue */
	int		bufcid;					/* bufcall return value */
extern struct mux mux_mux[];
extern int mux_cnt;     /* max number of muxes */

static queue_t *muxbot; 			/* linked lower queue */
static int muxerr;					/* set if error of hangup on
lower strm */

The four streamtab entries correspond to the upper read, upper write, lower read, and lower write qinit structures. The multiplexing qinit structures replace those in each lower stream head (in this case there is only one) after the I_LINK has concluded successfully. In a multiplexing configuration, the processing performed by the multiplexing driver can be partitioned between the upper and lower queues. There must be an upper-stream write put procedure and lower-stream read put procedure. If the queue procedures of the opposite upper/lower queue are not needed, the queue can be skipped, and the message put to the following queue.

In the example, the upper read-side procedures are not used. The lower-stream read queue put procedure transfers the message directly to the read queue upstream from the multiplexer. There is no lower write put procedure because the upper write put procedure directly feeds the lower write queue downstream from the multiplexer.

The driver uses a private data structure, mux. mux_mux[dev] points back to the opened upper read queue. This is used to route messages coming upstream from the driver to the appropriate upper queue. It is also used to find a free major or minor device for a CLONEOPEN driver open case.

Example 13–3, the upper queue open, contains the canonical driver open code.

Example 13–3 Upper Queue Open

static int
muxopen(queue_t *q, dev_t *devp, int flag,
				 int sflag, cred_t *credp)
	struct mux *mux;
	minor_t device;

	if (q->q_ptr)

	if (sflag == CLONEOPEN) {
			for (device = 0; device < mux_cnt; device++)
				if (mux_mux[device].qptr == 0)

			*devp=makedevice(getmajor(*devp), device);
	else {
			device = getminor(*devp);
			if (device >= mux_cnt)
				return ENXIO;

	mux = &mux_mux[device];
	mux->qptr = q;
	q->q_ptr = (char *) mux;
	WR(q)->q_ptr = (char *) mux;
	return (0);

muxopen checks for a clone or ordinary open call. It initializes q_ptr to point at the mux_mux[] structure.

The core multiplexer processing is as follows: downstream data written to an upper stream is queued on the corresponding upper write message queue if the lower stream is flow controlled. This allows flow control to propagate toward the stream head for each upper stream. A lower write service procedure, rather than a write put procedure, is used so that flow control, coming up from the driver below, may be handled.

On the lower read side, data coming up the lower stream are passed to the lower read put procedure. The procedure routes the data to an upper stream based on the first byte of the message. This byte holds the minor device number of an upper stream. The put procedure handles flow control by testing the upper stream at the first upper read queue beyond the driver.

Upper Write put Procedure Sample

muxuwput, the upper-queue write put procedure, traps ioctl calls, in particular I_LINK and I_UNLINK:

Example 13–4 bufcall Callback Routine

static int
* This is our callback routine used by bufcall() to inform us
* when buffers become available
static void mux_qenable(long ql)
		queue_t *q = (queue_t *ql);
		struct mux *mux;

		mux = (struct mux *)(q->q_ptr);
		mux->bufcid = 0;
muxuwput(queue_t *q, mblk_t *mp)
	struct mux *mux;

	mux = (struct mux *)q->q_ptr;
	switch (mp->b_datap->db_type) {
	case M_IOCTL: {
			struct iocblk *iocp;
			struct linkblk *linkp;
			 * ioctl. Only channel 0 can do ioctls. Two
			 * calls are recognized: LINK, and UNLINK
			if (mux != mux_mux)
				goto iocnak;

			iocp = (struct iocblk *) mp->b_rptr;
			switch (iocp->ioc_cmd) {
			case I_LINK:
				 *Link. The data contains a linkblk structure
				 *Remember the bottom queue in muxbot.
				if (muxbot != NULL)
						goto iocnak;

				linkp=(struct linkblk *) mp->b_cont->b_rptr;
				muxbot = linkp->l_qbot;
				muxerr = 0;

				mp->b_datap->db_type = M_IOCACK;
				iocp->ioc_count = 0;
				qreply(q, mp);
			case I_UNLINK:
				 * Unlink. The data contains a linkblk struct.
				 * Should not fail an unlink. Null out muxbot.
				linkp=(struct linkblk *) mp->b_cont->b_rptr;
				muxbot = NULL;
				mp->b_datap->db_type = M_IOCACK;
				iocp->ioc_count = 0;
				qreply(q, mp);
				/* fail ioctl */
				mp->b_datap->db_type = M_IOCNAK;
				qreply(q, mp);
	case M_FLUSH:
			if (*mp->b_rptr & FLUSHW)
				flushq(q, FLUSHDATA);
			if (*mp->b_rptr & FLUSHR) {
				*mp->b_rptr &= ~FLUSHW;
				qreply(q, mp);
			} else

	case M_DATA:{
			 * Data. If we have no lower queue --> fail
			 * Otherwise, queue the data and invoke the lower
			 * service procedure.
		mblk_t *bp;
			if (muxerr || muxbot == NULL)
				goto bad;
		if ((bp = allocb(1, BPRI_MED)) == NULL) {
				putbq(q, mp);
				mux->bufcid = bufcall(1, BPRI_MED,
					mux_qenable, (long)q);
			*bp->b_wptr++ = (struct mux *)q->ptr - mux_mux;
			bp->b_cont = mp;
			putq(q, bp);
			 * Send an error message upstream.
			mp->b_datap->db_type = M_ERROR;
			mp->b_rptr = mp->b_wptr = mp->b_datap->db_base;
			*mp->b_wptr++ = EINVAL;
			qreply(q, mp);

First, there is a check to enforce that the stream associated with minor device 0 will be the single, controlling stream. The ioctls are only accepted on this stream. As described previously, a controlling stream is the one that issues the I_LINK. There should be only a single control stream. I_LINK and I_UNLINK include a linkblk structure containing the following fields:

l_qtop is the upper write queue from which the ioctl(2) comes. It always equals q for an I_LINK, and NULL for I_PLINK.

l_qbot is the new lower write queue. It is the former stream head write queue and is where the multiplexer gets and puts its data.

l_index is a unique (system-wide) identifier for the link. It can be used for routing or during selective unlinks. Since the example only supports a single link, l_index is not used.

For I_LINK, l_qbot is saved in muxbot and a positive acknowledgement is generated. From this point on, until an I_UNLINK occurs, data from upper queues will be routed through muxbot. Note that when an I_LINK, is received, the lower stream has already been connected. This enables the driver to send messages downstream to perform any initialization functions. Returning an M_IOCNAK message (negative acknowledgement) in response to an I_LINK causes the lower stream to be disconnected.

The I_UNLINK handling code nulls out muxbot and generates a positive acknowledgement. A negative acknowledgement should not be returned to an I_UNLINK. The stream head ensures that the lower stream is connected to a multiplexer before sending an I_UNLINK M_IOCTL.

Drivers can handle the persistent link requests I_PLINK and I_PUNLINK ioctl(2) in the same manner, except that l_qtop in the linkblk structure passed to the put routine is NULL instead of identifying the controlling stream.

muxuwput handles M_FLUSH messages as a normal driver does, except that there are no messages queued on the upper read queue, so there is no need to call flushq if FLUSHR is set.

M_DATA messages are not placed on the lower write message queue. They are queued on the upper write message queue. When flow control subsides on the lower stream, the lower service procedure, muxlwsrv, is scheduled to start output. This is similar to starting output on a device driver.

Upper Write service Procedure Sample

The following example shows the code for the upper multiplexer write service procedure:

Example 13–5 Upper Multiplexer Write Service Procedure

static int muxuwsrv(queue_t *q)
	mblk_t *mp;
	struct mux *muxp;
	muxp = (struct mux *)q->q_ptr;

	if (!muxbot) {
			flushq(q, FLUSHALL);
			return (0);
	if (muxerr) {
			flushq(q, FLUSHALL);
			return (0);
	while (mp = getq(q)) {
			if (canputnext(muxbot))
				putnext(muxbot, mp);
			else {
				putbq(q, mp);
	return (0);

As long as there is a stream still linked under the multiplexer and there are no errors, the service procedure will take a message off the queue and send it downstream, if flow control allows.

Lower Write service Procedure

muxlwsrv, the lower (linked) queue write service procedure is scheduled as a result of flow control subsiding downstream (it is back-enabled).

static int muxlwsrv(queue_t *q)
	int i;

	for (i = 0; i < mux_cnt; i++)
			if (mux_mux[i].qptr && mux_mux[i].qptr->q_first)
	return (0);

muxlwsrv steps through all possible upper queues. If a queue is active and there are messages on the queue, then its upper write service procedure is enabled through qenable.

Lower Read put Procedure

The lower (linked) queue read put procedure is shown in the following example:

Example 13–6 Lower Read put Procedure

static int
muxlrput(queue_t *q, mblk_t *mp)
	queue_t *uq;
	int device;

	if(muxerr) {
			return (0);

	switch(mp->b_datap->db_type) {
	case M_FLUSH:
			 * Flush queues. NOTE: sense of tests is reversed
			 * since we are acting like a "stream head"
			if (*mp->b_rptr & FLUSHW) {
				*mp->b_rptr &= ~FLUSHR;
				qreply(q, mp);
			} else
	case M_ERROR:
	case M_HANGUP:
			muxerr = 1;
	case M_DATA:
			 * Route message. First byte indicates
			 * device to send to. No flow control.
			 * Extract and delete device number. If the
			 * leading block is now empty and more blocks
			 * follow, strip the leading block.
			device = *mp->b_rptr++;

			/* Sanity check. Device must be in range */
			if (device < 0 || device >= mux_cnt) {
			 * If upper stream is open and not backed up,
			 * send the message there, otherwise discard it.
			uq = mux_mux[device].qptr;
			if (uq != NULL && canputnext(uq))
				putnext(uq, mp);
	return (0);

muxlrput receives messages from the linked stream. In this case, it is acting as a stream head and handles M_FLUSH messages. The code is the reverse of a driver, handling M_FLUSH messages from upstream. There is no need to flush the read queue because no data is ever placed in it.

muxlrput also handles M_ERROR and M_HANGUP messages. If one is received, it locks up the upper streams by setting muxerr.

M_DATA messages are routed by checking the first data byte of the message. This byte contains the minor device of the upper stream. Several checks examine whether:

This multiplexer does not support flow control on the read side; it is merely a router. If the message passes all checks, it is put to the proper upper queue. Otherwise, the message is discarded.

The upper stream close routine clears the mux entry so this queue will no longer be found. Outstanding bufcalls are not cleared.

* Upper queue close
static int
muxclose(queue_t *q, int flag, cred_t *credp)
	struct mux *mux;

	mux = (struct mux *) q->q_ptr;
	if (mux->bufcid != 0)
	mux->bufcid = 0;
	mux->ptr = NULL;
	q->q_ptr = NULL;
	WR(q)->q_ptr = NULL;

Persistent Links

Keeping a process running merely to hold the multiplexer configuration together is not always desirable, so, “free standing” links below a multiplexer are needed. A persistent link is such a link. It is similar to a STREAMS multiplexer link except that a process is not needed to hold the links together. After the multiplexer has been set up, the process may close all file descriptors and exit, and the multiplexer remains intact.

With I_LINK and I_UNLINK ioctl(2) the file descriptor associated with the stream above the multiplexer used to set up the lower multiplexer connections must remain open for the duration of the configuration. Closing the file descriptor associated with the controlling stream dismantles the whole multiplexing configuration.

Two ioctl(2)s, I_PLINK and I_PUNLINK, are used to create and remove persistent links that are associated with the stream above the multiplexer. close(2) and I_UNLINK are not able to disconnect the persistent links (see strconf(1) and strchg(1)).

The format of I_PLINK is:

ioctl(fd0, I_PLINK, fd1)

The first file descriptor, fd0, must reference the stream connected to the multiplexing driver and the second file descriptor, fd1, must reference the stream to be connected below the multiplexer. The persistent link can be created as follows:

upper_stream_fd = open("/dev/mux", O_RDWR);
lower_stream_fd = open("/dev/driver", O_RDWR);
muxid = ioctl(upper_stream_fd, I_PLINK, lower_stream_fd);
 * save muxid in a file

The persistent link can still exist even if the file descriptor associated with the upper stream to the multiplexing driver is closed. The I_PLINK ioctl(2) returns an integer value, muxid, that can be used for dismantling the multiplexing configuration. If the process that created the persistent link still exists, it may pass the muxid value to some other process to dismantle the link, if the dismantling is desired, or it can leave the muxid value in a file so that other processes may find it later.

Several users can open the MUX driver and send data to Driver1 since the persistent link to Driver1 remains intact.

The I_PUNLINK ioctl(2) is used to dismantle the persistent link. Its format is:

ioctl(fd0, I_PUNLINK, muxid)

where fd0 is the file descriptor associated with stream connected to the multiplexing driver from above. The muxid is returned by the I_PLINK ioctl(2) for the stream that was connected below the multiplexer. I_PUNLINK removes the persistent link between the multiplexer referenced by fd0 and the stream to the driver designated by the muxid. Each of the bottom persistent links can be disconnected individually. An I_PUNLINK ioctl(2) with the muxid value of MUXID_ALL will remove all persistent links below the multiplexing driver referenced by fd0.

The following code example shows how to dismantle the previously given configuration:

fd = open("/dev/mux", O_RDWR);
 * retrieve muxid from the file
ioctl(fd, I_PUNLINK, muxid);

Do not use the I_PLINK and I_PUNLINK ioctls with I_LINK and I_UNLINK. Any attempt to unlink a regular link with I_PUNLINK or to unlink a persistent link with the I_UNLINK ioctl(2) causes the errno value of EINVAL to be returned.

Because multilevel multiplexing configurations are allowed in STREAMS, persistent links could exist below a multiplexer whose stream is connected to the above multiplexer by regular links. Closing the file descriptor associated with the controlling stream will remove the regular link but not the persistent links below it. On the other hand, regular links are allowed to exist below a multiplexer whose stream is connected to the above multiplexer with persistent links. In this case, the regular links will be removed if the persistent link above is removed and no other references to the lower streams exist.

The construction of cycles is not allowed when creating links. A cycle could be constructed by:

  1. Creating a persistent link of multiplexer 2 below multiplexer 1

  2. Closing the controlling file descriptor associated with the multiplexer 2

  3. Reopening the file descriptor again

  4. Linking the multiplexer 1 below the multiplexer 2

This is not allowed. The operating system prevents a multiplexer configuration from containing a cycle to ensure that messages cannot be routed infinitely, which would create an infinite loop or overflow the kernel stack.

Design Guidelines

The following are general multiplexer design guidelines: