Using Queue Information

The q_first, q_last, q_count, and q_flags components must not be modified by the module, and should be accessed using strqget(9F). The values of q_minpsz, q_maxpsz, q_hiwat, and q_lowat are accessed through strqget(9F), and are modified by strqset(9F). q_ptr can be accessed and modified by the module and contains data private to the module.

All other accesses to fields in the queue(9S) structure should be made through STREAMS utility routines (see Appendix B, "STREAMS Utilities"). Modules and drivers should not change any fields not explicitly listed previously.

strqget(9F) lets modules and drivers get information about a queue or particular band of the queue. This insulates the STREAMS data structures from the modules and drivers. The syntax of the routine is:

int
strqget(queue_t *q, qfields_t what, unsigned char pri, void *valp)

q specifies from which queue the information is to be retrieved; what defines the queue_t field value to obtain (see the following code example). pri identifies a specific priority band. The value of the field is returned in valp. The fields that can be obtained are defined in <sys/stream.h> and shown here as:

QHIWAT              /* high-water mark */
QLOWAT              /* low-water mark */
QMAXPSZ             /* largest packet accepted */
QMINPSZ             /* smallest packet accepted */
QCOUNT              /* approx. size (in bytes) of data */
QFIRST              /* first message */
QLAST               /* last message */
QFLAG               /* status */

strqset(9F) lets modules and drivers change information about a queue or a band of the queue. This also insulates the STREAMS data structures from the modules and drivers. Its format is:

int 
strqset(queue_t *q. qfields_t what, unsigned char pri, intptr_t val)

The q, what, and pri fields are the same as in strqget(9F), but the information to be updated is provided in val instead of through a pointer. If the field is read-only, EPERM is returned and the field is left unchanged. The following fields are read-only: QCOUNT, QFIRST, QLAST, and QFLAG.