illumos: manual page: qwait

QWAIT(9F) Kernel Functions for Drivers QWAIT(9F)

NAME

qwait, qwait_sig - STREAMS wait routines

SYNOPSIS

#include <sys/stream.h>
#include <sys/ddi.h>

void qwait(queue_t *q);

int qwait_sig(queue_t *q);

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

PARAMETERS

qp
Pointer to the queue that is being opened or closed.

DESCRIPTION

qwait() and qwait_sig() are used to wait for a message to arrive to the
put(9E) or srv(9E) procedures. qwait() and qwait_sig() can also be used
to wait for qbufcall(9F) or qtimeout(9F) callback procedures to execute.
These routines can be used in the open(9E) and close(9E) procedures in a
STREAMS driver or module.

The thread that calls close() does not necessarily have the ability to
receive signals, particularly when called by exit(2). In this case,
qwait_sig() behaves exactly as qwait(). Driver writers may use
ddi_can_receive_sig(9F) to determine when this is the case, and, if so,
arrange some means to avoid blocking indefinitely (for example, by using
qtimeout(9F).

qwait() and qwait_sig() atomically exit the inner and outer perimeters
associated with the queue, and wait for a thread to leave the module's
put(9E), srv(9E), or qbufcall(9F) / qtimeout(9F) callback procedures.
Upon return they re-enter the inner and outer perimeters.

This can be viewed as there being an implicit wakeup when a thread leaves
a put(9E) or srv(9E) procedure or after a qtimeout(9F) or qbufcall(9F)
callback procedure has been run in the same perimeter.

qprocson(9F) must be called before calling qwait() or qwait_sig().

qwait() is not interrupted by a signal, whereas qwait_sig() is
interrupted by a signal. qwait_sig() normally returns non-zero, and
returns zero when the waiting was interrupted by a signal.

qwait() and qwait_sig() are similar to cv_wait() and cv_wait_sig() except
that the mutex is replaced by the inner and outer perimeters and the
signalling is implicit when a thread leaves the inner perimeter. See
condvar(9F).

RETURN VALUES

0
For qwait_sig(), indicates that the condition was not necessarily
signaled, and the function returned because a signal was pending.

CONTEXT

These functions can only be called from an open(9E) or close(9E) routine.

EXAMPLES

Example 1: Using qwait()

The open routine sends down a T_INFO_REQ message and waits for the
T_INFO_ACK. The arrival of the T_INFO_ACK is recorded by resetting a flag
in the unit structure (WAIT_INFO_ACK). The example assumes that the
module is D_MTQPAIR or D_MTPERMOD.

xxopen(qp, ...)
queue_t *qp;
{
struct xxdata *xx;
/* Allocate xxdata structure */
qprocson(qp);
/* Format T_INFO_ACK in mp */
putnext(qp, mp);
xx->xx_flags |= WAIT_INFO_ACK;
while (xx->xx_flags & WAIT_INFO_ACK)
qwait(qp);
return (0);
}
xxrput(qp, mp)
queue_t *qp;
mblk_t *mp;
{
struct xxdata *xx = (struct xxdata *)q->q_ptr;

...

case T_INFO_ACK:
if (xx->xx_flags & WAIT_INFO_ACK) {
/* Record information from info ack */
xx->xx_flags &= ~WAIT_INFO_ACK;
freemsg(mp);
return;
}

...
}