QUEUE(9S) Data Structures for Drivers QUEUE(9S)

queue - STREAMS queue structure

#include <sys/stream.h>

Architecture independent level 1 (DDI/DKI)

A STREAMS driver or module consists of two queue structures: read for upstream processing and write for downstream processing. The queue structure is the major building block of a stream.

queue Structure Members

The queue structure is defined as type queue_t. The structure can be accessed at any time from inside a STREAMS entry point associated with that queue.

struct    qinit   *q_qinfo;     /* queue processing procedure */
struct    msgb    *q_first;     /* first message in queue */
struct    msgb    *q_last;      /* last message in queue */
struct    queue   *q_next;      /* next queue in stream */
void              *q_ptr;       /* module-specific data */
size_t             q_count;     /* number of bytes on queue */
uint_t             q_flag;      /* queue state */
ssize_t            q_minpsz;    /* smallest packet OK on queue */
ssize_t            q_maxpsz;    /* largest packet OK on queue */
size_t             q_hiwat;     /* queue high water mark */
size_t             q_lowat;     /* queue low water mark */

Contstraints and restrictions on the use of q_flag and queue_t fields and the q_next values are detailed in the following sections.

q_flag Field

The q_flag field must be used only to check the following flag values.


Queue is full.


Queue is used for upstream (read-side) processing.


Queue has been allocated.


Queue has been enabled for service by qenable(9F).


Queue will not be scheduled for service by putq(9F).


Upstream processing element wants to read from queue.


Downstream processing element wants to write to queue.

queue_t Fields

Aside from q_ptr and q_qinfo, a module or driver must never assume that a queue_t field value will remain unchanged across calls to STREAMS entry points. In addition, many fields can change values inside a STREAMS entry point, especially if the STREAMS module or driver has perimeters that allow parallelism. See mt-streams(9F). Fields that are not documented below are private to the STREAMS framework and must not be accessed.

The values of the q_hiwat, q_lowat, q_minpsz, and q_maxpsz fields can be changed at the discretion of the module or driver. As such, the stability of their values depends on the perimeter configuration associated with any routines that modify them.
The values of the q_first, q_last, and q_count fields can change whenever putq(9F), putbq(9F), getq(9F), insq(9F), or rmvq(9F) is used on the queue. As such, the stability of their values depends on the perimeter configuration associated with any routines that call those STREAMS functions.
The q_flag field can change at any time.
The q_next field will not change while inside a given STREAMS entry point. Additional restrictions on the use of the q_next value are described in the next section.

A STREAMS module or driver can assign any value to q_ptr. Typically q_ptr is used to point to module-specific per-queue state, allocated in open(9E) and freed in close(9E). The value or contents of q_ptr is never inspected by the STREAMS framework.

The initial values for q_minpsz, q_maxpsz, q_hiwat, and q_lowat are set using the module_info(9S) structure when mod_install(9F) is called. A STREAMS module or driver can subsequently change the values of those fields as necessary. The remaining visible fields, q_qinfo, q_first, q_last, q_next, q_count, and q_flag, must never be modified by a module or driver.

The illumos DDI requires that STREAMS modules and drivers obey the rules described on this page. Those that do not follow the rules can cause data corruption or system instability, and might change in behavior across updates or upgrades.

q_next Restrictions

There are additional restrictions associated with the use of the q_next value. In particular, a STREAMS module or driver:

Must not access the data structure pointed to by q_next.
Must not rely on the value of q_next before calling qprocson(9F) or after calling qprocsoff(9F).
Must not pass the value into any STREAMS framework function other than put(9F), canput(9F), bcanput(9F), putctl(9F), putctl1(9F). However, in all cases the "next" version of these functions, such as putnext(9F), should be preferred.
Must not use the value to compare against queue pointers from other streams. However, checking q_next for NULL can be used to distinguish a module from a driver in code shared by both.

close(9E), open(9E), bcanput(9F), canput(9F), getq(9F), insq(9F), mod_install(9F), put(9F), putbq(9F), putctl(9F), putctl1(9F), putnext(9F), putq(9F), qprocsoff(9F), qprocson(9F), rmvq(9F), strqget(9F), strqset(9F), module_info(9S), msgb(9S), qinit(9S), streamtab(9S)

Writing Device Drivers

STREAMS Programming Guide

May 13, 2017 OmniOS