QUEUE(9S)QUEUE(9S)NAMEqueue - STREAMS queue structure
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
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.
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.
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
o 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.
o 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 associ‐
ated with any routines that call those STREAMS functions.
o The q_flag field can change at any time.
o 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
The Solaris 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 behav‐
ior across patches or upgrades.
There are additional restrictions associated with the use of the q_next
value. In particular, a STREAMS module or driver:
o Must not access the data structure pointed to by q_next.
o Must not rely on the value of q_next before calling qproc‐
son(9F) or after calling qprocsoff(9F).
o 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.
o 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
SEE ALSOclose(9E), open(9E), bcanput(9F), canput(9F), getq(9F), insq(9F),
mod_install(9F), put(9F), putbq(9F), putctl(9F), putctl1(9F), put‐
next(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
Jan 10, 2006 QUEUE(9S)