STREAMS Programming Guide

Module Service Interface Example

The following code is part of a module that illustrates the concept of a service interface. The module implements a simple service interface and mirrors the service interface library example. The following rules pertain to service interfaces.


The service interface primitives are defined in the declarations:

#include <sys/types.h>
#include <sys/param.h>
#include <sys/stream.h>
#include <sys/errno.h>

/* Primitives initiated by the service user */

#define BIND_REQ                      1    /* bind request */
#define UNITDATA_REQ                  2    /* unitdata request */

 /* Primitives initiated by the service provider */

#define OK_ACK                        3    /* bind acknowledgment */
#define ERROR_ACK                     4    /* error acknowledgment */
#define UNITDATA_IND                  5    /* unitdata indication */
 * The following structures define the format of the
 * stream message block of the above primitives.

struct bind_req {                       /* bind request */
   t_scalar_t    PRIM_type;             /* always BIND_REQ */
   t_uscalar_t   BIND_addr;             /* addr to bind	*/
struct unitdata_req {                   /* unitdata request */
   t_scalar_t    PRIM_type;             /* always UNITDATA_REQ */
   t_scalar_t    DEST_addr;             /* dest addr */
struct ok_ack {                         /* ok acknowledgment */
   t_scalar_t    PRIM_type;             /* always OK_ACK */
struct error_ack {                      /* error acknowledgment */
   t_scalar_t    PRIM_type;             /* always ERROR_ACK */
   t_scalar_t    UNIX_error;            /* UNIX system error code*/
struct unitdata_ind {                   /* unitdata indication */
   t_scalar_t    PRIM_type;             /* always UNITDATA_IND */
   t_scalar_t    SRC_addr;              /* source addr */

union primitives {								/* union of all primitives */
   long                      type;
   struct bind_req           bind_req;
   struct unitdata_req       unitdata_req;
   struct ok_ack             ok_ack;
   struct error_ack          error_ack;
   struct unitdata_ind       unitdata_ind;
struct dgproto {                        /* structure minor device */
   short state;                         /* current provider state */
   long addr;                           /* net address */

/* Provider states */
#define IDLE 0
#define BOUND 1

In general, the M_PROTO or M_PCPROTO block is described by a data structure containing the service interface information. In this example, union primitives is that structure.

The module recognizes two commands:


Give this stream a protocol address (for example, give it a name on the network). After a BIND_REQ is completed, data from other senders will find their way through the network to this particular stream.


Send data to the specified address.

The module generates three messages:


A positive acknowledgment (ack) of BIND_REQ.


A negative acknowledgment (nak) of BIND_REQ.


Data from the network has been received.

The acknowledgment of a BIND_REQ informs the user that the request was syntactically correct (or incorrect if ERROR_ACK). The receipt of a BIND_REQ is acknowledged with an M_PCPROTO to ensure that the acknowledgment reaches the user before any other message. For example, a UNITDATA_IND comes through before the bind is completed, the application is confused.

The driver uses a per-minor device data structure, dgproto, which contains the following:


Current state of the service provider IDLE or BOUND


Network address that has been bound to this stream

It is assumed (though not shown) that the module open procedure sets the write queue q_ptr to point at the appropriate private data structure.

Service Interface Procedure

The write put procedure is:

static int protowput(queue_t *q, mblk_t *mp)
 	union primitives *proto;
 	struct dgproto *dgproto;
 	int err;
 	dgproto = (struct dgproto *) q->q_ptr;  /* priv data struct */
 	switch (mp->b_datap->db_type) {
 			/* don't understand it */
 			mp->b_datap->db_type = M_ERROR;
 			mp->b_rptr = mp->b_wptr = mp->b_datap->db_base;
 			*mp->b_wptr++ = EPROTO;
 			qreply(q, mp);
 	case M_FLUSH: /* standard flush handling goes here ... */
 	case M_PROTO:
 			/* Protocol message -> user request */
 			proto = (union primitives *) mp->b_rptr;
 			switch (proto->type) {
 				mp->b_datap->db_type = M_ERROR;
 				mp->b_rptr = mp->b_wptr = mp->b_datap->db_base;
 				*mp->b_wptr++ = EPROTO;
 				qreply(q, mp);
 			case BIND_REQ:
 				if (dgproto->state != IDLE) {
 						err = EINVAL;
 						goto error_ack;
 				if (mp->b_wptr - mp->b_rptr !=
 				 sizeof(struct bind_req)) {
 						err = EINVAL;
 						goto error_ack;
 				if (err = chkaddr(proto->bind_req.BIND_addr))
 						goto error_ack;
 				dgproto->state = BOUND;
 				dgproto->addr = proto->bind_req.BIND_addr;
 				mp->b_datap->db_type = M_PCPROTO;
 				proto->type = OK_ACK;
 				qreply(q, mp);
 				mp->b_datap->db_type = M_PCPROTO;
 				proto->type = ERROR_ACK;
 				proto->error_ack.UNIX_error = err;
 				mp->b_wptr = mp->b_rptr+sizeof(structerror_ack);
 				qreply(q, mp);
 			case UNITDATA_REQ:
 				if (dgproto->state != BOUND)
 						goto bad;
 				if (mp->b_wptr - mp->b_rptr !=
 					 sizeof(struct unitdata_req))
 						goto bad;
 						goto bad;
 				putq(q, mp);
 				/* start device or mux output ... */

The write put procedure switches on the message type. The only types accepted are M_FLUSH and M_PROTO. For M_FLUSH messages, the driver performs the canonical flush handling (not shown). For M_PROTO messages, the driver assumes the message block contains a union primitive and switches on the type field. Two types are understood: BIND_REQ and UNITDATA_REQ.

For a BIND_REQ, the current state is checked; it must be IDLE. Next, the message size is checked. If it is the correct size, the passed-in address is verified for legality by calling chkaddr. If everything checks, the incoming message is converted into an OK_ACK and sent upstream. If there was any error, the incoming message is converted into an ERROR_ACK and sent upstream.

For UNITDATA_REQ, the state is also checked; it must be BOUND. As above, the message size and destination address are checked. If there is any error, the message is discarded. If all is well, the message is put in the queue, and the lower half of the driver is started.

If the write put procedure receives a message type that it does not understand, either a bad b_datap->db_type or bad proto->type, the message is converted into an M_ERROR message and is then sent upstream.

The generation of UNITDATA_IND messages (not shown in the example) would normally occur in the device interrupt if this is a hardware driver or in the lower read put procedure if this is a multiplexer. The algorithm is simple: the data part of the message is prefixed by an M_PROTO message block that contains a unitdata_ind structure and sent upstream.