STREAMS Programming Guide

Entry Points

The q_qinfo component points to a qinit structure. This structure defines the module's entry point procedures for each queue, which include the following:

int        (*qi_putp)();       /* put procedure */
int        (*qi_srvp)();       /* service procedure */
int        (*qi_qopen)();      /* open procedure */
int        (*qi_qclose)();     /* close procedure */
struct module_info *qi_minfo;  /* module information structure */

There is generally a unique q_init structure associated with the read queue and the write queue. qi_putp identifies the put procedure for the module. qi_srvp identifies the optional service procedure for the module.

The open and close entry points are required for the read-side queue. The put procedure is generally required on both queues and the service procedure is optional.

If the put procedure is not defined and a subsequent put is done to the module, a panic occurs. As a precaution, putnext should be declared as the module's put procedure.

If a module only requires a service procedure, putq(9F) can be used as the module's put procedure. If the service procedure is not defined, the module's put procedure must not queue data (putq(9F)).

The qi_qopen member of the read-side qinit structure identifies the open(9E) entry point of the module. The qi_qclose member of the read-side qinit structure identifies the close(9E) entry point of the module.

The qi_minfo member points to the module_info(9S) structure.

struct module_info {
		ushort   mi_idnum;          /* module id number */
		char     *mi_idname;        /* module name */
		ssize_t  mi_minpsz;         /* min packet size accepted */
		ssize_t  mi_maxpsz;         /* max packet size accepted */
		size_t   mi_hiwat;          /* hi-water mark */
		size_t   mi_lowat;          /* lo-water mark */

mi_idnum is the module's unique identifier defined by the developer and used in strlog(9F). mi_idname is an ASCII string containing the name of the module. mi_minpsz is the initial minimum packet size of the queue. mi_maxpsz is the initial maximum packet size of the queue. mi_hiwat is the initial high-water mark of the queue. mi_lowat is the initial low-water mark of the queue.

open Routine

The open routine of a device is called once for the initial open of the device, then is called again on subsequent reopens of the Stream. Module open routines are called once for the initial push onto the Stream and again on subsequent reopens of the Stream. See open(9E).

Figure 7-8 Order of a Module's open Procedure


The Stream is analogous to a stack. Initially the driver is opened and, as modules are pushed onto the Stream, their open routines are invoked. Once the Stream is built, this order reverses if a reopen of the Stream occurs. For example, while building the Stream shown in Figure 7-8, device A's open routine is called, followed by B's and C's when they are pushed onto the Stream. If the Stream is reopened, Module C's open routine is called first, followed by B's, and finally by A's.

Usually the module or driver does not check this, but the issue is raised so that dependencies on the order of open routines are not introduced by the programmer. Note that although an open can happen more than once, close is only called once. See the next section on the close routine for more details. If a file is duplicated (dup(2)) the Stream is not reopened.

The syntax of the open entry point is:

int prefix open(queue_t *q, dev_t *devp, int oflag, int sflag, cred_t *cred_p)

q -- Pointer to the read queue of this module.

devp -- Pointer to a device number that is always associated with the device at the end of the Stream. Modules cannot modify this value, but drivers can, as described in Chapter 9, STREAMS Drivers.

oflag -- For devices, oflag can contain the following bit mask values: FEXCL, FNDELAY, FREAD, and FWRITE. See Chapter 9, STREAMS Drivers for more information on drivers.

sflag -- When the open is associated with a driver, sflag is set to 0 or CLONEOPEN, see Chapter 9, STREAMS Drivers, "Cloning" for more details. If the open is associated with a module, sflag contains the value MODOPEN.

cred_p Pointer to the user credentials structure.

the open routines to devices are serialized (if more than one process attempts to open the device, only one proceeds and the others wait until the first finishes). Interrupts are not blocked during an open. So the driver's interrupt and open routines must allow for this. See Chapter 9, STREAMS Drivers for more information.

The open routines for both drivers and modules have user context. For example, they can do blocking operations, but the blocking routine should return in the event of a signal. In other words, q_wait_sig is allowed, but q_wait is not.

If the module or driver is to allocate a controlling terminal, it should send an M_SETOPTS message with SO_ISTTY set to the Stream head.

The open routine usually initializes the q_ptr member of the queue. q_ptr is generally initialized to some private data structure that contains various state information private to the module or driver. The module's close routine is responsible for freeing resources allocated by the module including q_ptr. Example 7-1 shows a simple open routine.

Example 7-1 A Simple open Routine

/* example of a module open */
int xx_open(queue_t *q, dev_t *devp, int oflag, int sflag, cred_t *crp)
	struct xxstr *xx_ptr;

	xx_ptr = kmemzalloc(sizeof(struct xxstr), KM_SLEEP);
	xx_ptr->xx_foo = 1;
	q->q_ptr = WR(q)->q_ptr = xx_ptr;
	return (0);

In a multithreaded environment, data can flow up the Stream during the open. A module receiving this data before its open routine finishes initialization can panic. To eliminate this problem, modules and drivers are not linked into the Stream until qprocson(9F) is called. In other words, messages flow around the module until qprocson(9F) is called. Figure 7-9 illustrates this process.

Figure 7-9 Messages Flowing Around the Module Before qprocson


The module or driver instance is guaranteed to be single-threaded before qprocson(9F) is called, except for interrupts or callbacks that must be handled separately. qprocson(9F) must be called before calling qbufcall(9F), qtimeout(9F), qwait(9F), or qwait_sig(9F) .

close Routine

The close routine of devices is called only during the last close of the device. Module close routines are called during the last close or if the module is popped.

The syntax of the close entry point is:

int prefix close (queue *q, int flag, cred_t * cred_p)

q is a pointer to the read queue of the module. flag is analogous to the oflag parameter of the open entry point. If FNBLOCK or FNDELAY is set, then the module should attempt to avoid blocking during the close. cred_p is a pointer to the user credential structure.

Like open, the close entry point has user context and can block. Likewise, the blocking routines should return in the event of a signal. Device drivers must take into consideration that interrupts are not blocked during close. The close routine must cancel all pending timeout(9F) and qbufcall(9F) callbacks, and process any remaining data on its service queue.

The open and close procedures are only used on the read side of the queue and can be set to NULL in the write-side qinit structure initialization. Example 7-3 shows an example of a module close routine.

Example 7-2 Example of a Module Close

/* example of a module close */
static int
xx_close(queue_t *, *rq, int flag, cred_t *credp)
		struct xxstr   *xxp;
    * Disable xxput() and xxsrv() procedures on this queue.
		xxp = (struct xxstr *) rq->q_ptr;

    * Cancel any pending timeout.
    * This example assumes that the timeout was issued
    * against the write queue.

		if (xxp->xx_timeoutid != 0) {
			(void) quntimeout(WR(rq), xxp->xx_timeoutid);
    * Cancel any pending bufcalls.
    * This example assumes that the bufcall was issued
    * against the write queue.
		if (xxp->xx_bufcallid !=0) {
			(void) qunbufcall(WR(rq), xxp->xx_bufcallid);
 		xxp->xx_bufcallid = 0;
		rq->q_ptr = WR(rq)->q_ptr = NULL;

    * Free resources allocated during open
		kmem_free (xxp, sizeof (struct xxstr));
		return (0);

The put Procedure

The put procedure passes messages from the queue of a module to the queue of the next module. The queue's put procedure is invoked by the preceding module to process a message immediately (see put(9F) and putnext(9F)). Almost all modules and drivers must have a put routine. The exception is that the read-side driver does not need a put routine because there can be no downstream component to call the put.

A driver's put procedure must do one of:

All M_IOCTL type messages must be acknowledged through M_IOACK or rejected through M_IOCNACK. M_IOCTL messages should not be freed. Drivers must free any unrecognized message.

A module's put procedure must do one of the following as shown in Example 7-3:

Unrecognized messages are passed to the next module or driver. The Stream operates more efficiently when messages are processed in the put procedure. Processing a message with the service procedure imposes some latency on the message.

Example 7-3 Flow of put Procedure


If the next module is flow controlled (see canput(9F) and canputnext(9F)), the put procedure can queue the message for processing by the next service procedure (see putq(9F)). The put routine is always called before the component's corresponding srv(9E) service routine, so always use put for immediate message processing.

The preferred naming convention for a put procedure reflects the direction of the message flow. The read put procedure is suffixed by r (rput), and the write procedure by w (wput). For example, the read-side put procedure for module xx is declared as int xxrput (queue_t *q, mblk_t *mp). The write-side put procedure is declared as: int xxwput(queue_t *q, mblk_t *mp), where q points to the corresponding read or write queue and mp points to the message to be processed.

Although high-priority messages can be placed on the service queue, it is better to process them immediately in the put procedure. Place ordinary or priority-band messages on the service queue (putq(9F)) if:

If other messages already exist on the queue, and the put procedure does not queue new messages (provided they are not high-priority), messages are reordered. If the next module is flow controlled (see canput(9F) and canputnext(9F)). the put procedure can queue the message for processing by the service procedure (see putq(9F)).

/*example of a module put procedure */
xxrput(queue_t *,mblk_t, *mp)
    * If the message is a high-priority message or
    * the next module is not flow controlled and we have not
    * already deferred processing, then:

    if (mp->b_datap->db_type >= QPCTL ||
             (canputnext(q) && q->q_first == NULL)) {
         * Process message

    } else {
          * put message on service queue
     return (0);

A module need not process the message immediately, and can queue it for later processing by the service procedure (see putq(9F)).

The SunOS STREAMS framework is multithreaded. Unsafe (nonmultithreaded) modules are not supported. To make multithreading of modules easier, the SunOS STREAMS framework uses perimeters (see "MT STREAMS Perimeters" in Chapter 12, MultiThreaded STREAMS for information). Perimeters are a facility fore specifing that the framework provide exclusive access for the entire module, queue pair, or an individual queue. Perimeters make it easier to deal with multithreaded issues, such as message ordering and recursive locking.

Caution - Caution -

Mutex locks must not be held across a call to put(9F), putnext(9F), or qreply(9F).

Because of the asynchronous nature of STREAMS, don't assume that a module's put procedure has been called just because put(9F), putnext(9F), or qreply(9F) has returned.

service Procedure

A queue's service procedure is invoked to process messages on the queue. It removes successive messages from the queue, processes them, and calls the put procedure of the next module in the Stream to give the processed message to the next queue.

The service procedure is optional. A module or driver can use a service procedure for the following reasons:

The service procedure is invoked by the STREAMS scheduler. A STREAMS service procedure is scheduled to run if:

A service procedure usually processes all messages on its queue (getq(9F)) or takes appropriate action to ensure it is reenabled (qenable(9F)) at a later time. Figure 7-11 shows the flow of a service procedure.

Caution - Caution -

High-priority messages must never be placed back on a service queue (putbq(9F)); this can cause an infinite loop.

Figure 7-10 Flow of service Procedure


Example 7-4 shows a module service procedure.

Example 7-4 A Module service Procedure

/*example of a module service procedure */
xxrsrv(queue_t *q)
		mblk_t *mp;
    * While there are still messages on our service queue
		while ((mp = getq(q) != NULL) {
       * We check for high priority messages, but
       * none is ever seen since the put procedure
       * never queues them.
       * If the next module is not flow controlled, then
      if (mp->b_datap->db_type >= QPCTL || (canputnext (q)) {
				 * process message
				putnext (q, mp);
			} else {
          * put message back on service queue
		return (0);

qband(9S) Structure

The queue flow information for each band, other than band 0, is contained in a qband structure. This structure is not visible to other modules. For accessible information see strqget(9F) and strqset(9F). qband(9S) is defined as follows:

typedef struct qband {
		struct  qband    *qb_next;   /* next band's info */
		size_t  qb_count;            /* number of bytes in band */
		struct  msgb     *qb_first;  /* beginning of band's data */
		struct  msgb     *qb_last;   /* end of band's data */
		size_t  qb_hiwat;            /* high- water mark for band */
		size_t  qb_lowat;            /* low-water mark for band */
		uint    qb_flag;             /* see below */
} qband_t;/

The structure contains pointers to the linked list of messages in the queue. These pointers, qb_first and qb_last, denote the beginning and end of messages for the particular band. The qb_count field is analogous to the queue's q_count field. However, qb_count only applies to the messages in the queue in the band of data flow represented by the corresponding qband structure. In contrast, q_count only contains information regarding normal and high-priority messages.

Each band has a separate high and low watermark, qb_hiwat and qb_lowat. These are initially set to the queue's q_hiwat and q_lowat respectively. Modules and drivers can change these values through the strqset(9F) function. One flag defined for qb_flag is QB_FULL, which denotes that the particular band is full.

The qband(9S) structures are not preallocated per queue. Rather, they are allocated when a message with a priority greater than zero is placed in the queue using putq(9F), putbq(9F), or insq(9F). Since band allocation can fail, these routines return 0 on failure and 1 on success. Once a qband(9S) structure is allocated, it remains associated with the queue until the queue is freed. strqset(9F) and strqget(9F) cause qband(9S) allocation. Sending a message to a band causes all bands up to and including that one to be created.

Using qband(9S) Information

The STREAMS utility routines should be used when manipulating the fields in the queue and qband structures. strqget(9F) and strqset(9F) are used to access band information.

Drivers and modules can change the qb_hiwat and qb_lowat fields of the qband structure. Drivers and modules can only read the qb_count, qb_first, qb_last, and qb_flag fields of the qband structure. Only the fields listed previously can be referenced. There are fields in the structure that are reserved and are not documented.

Figure 7-11 shows a queue with two extra bands of flow.

Figure 7-11 Data Structure Linkage


Several routines are provided to aid you in controlling each priority band of data flow. These routines are

flushband(9F) is discussed in "Flush Handling". bcanputnext(9F) is discussed in "Flow Control in Service Procedures", and the other two routines are described in the following section. Appendix B, STREAMS Utilities also has a description of these routines.

Message Processing

Typically, put procedures are required in pushable modules, but service procedures are optional. If the put routine queues messages, there must exist a corresponding service routine that handles the queued messages. If the put routine does not queue messages, the service routine need not exist.

Example 7-3 shows typical processing flow for a put procedure which works as follows:

Figure 7-10 shows typical processing flow for a service procedure that works as follows:

Caution - Caution -

A service or put procedure must never block since it has no user context. It must always return to its caller.

If no processing is required in the put procedure, the procedure does not have to be explicitly declared. Rather, putq(9F) can be placed in the qinit(9S) structure declaration for the appropriate queue side to queue the message for the service procedure. For example:

static struct qinit winit = { putq, modwsrv, ...... };

More typically, put procedures process high-priority messages to avoid queueing them.

Device drivers associated with hardware are examples of STREAMS devices that might not have a put procedure. Since there are no queues below the hardware level, another module does not call the module's put procedure. Data comes into the Stream from an interrupt routine, and is either processed or queued it for the service procedure.

A STREAMS filter is an example of a module without a service procedure — messages passed to it are either passed or filtered. Flow control is described in "Flow Control in Service Procedures".

The key attribute of a service procedure in the STREAMS architecture is delayed processing. When a service procedure is used in a module, the module developer is implying that there are other, more time-sensitive activities to be performed elsewhere in this Stream, in other Streams, or in the system in general.

Note -

The presence of a service procedure is mandatory if the flow control mechanism is to be utilized by the queue. If you don't implement flow control, it is possible to overflow queues and hang the system.