canput(D3DK) —

.IX \f4canput\fP(D3DK)
.IX STREAMS message queues
.IX message (STREAMS)

NAME

canput − test for room in a message queue

SYNOPSIS

#include <sys/stream.h>
int canput(queue_t ∗q);

ARGUMENTS

qPointer to the message queue.

DESCRIPTION

canput tests if there is room for a message in the queue pointed to by q. The queue must have a service procedure.

It is possible because of race conditions to test for room using canput and get an indication that there is room for a message, and then have the queue fill up before subsequently enqueuing the message, causing a violation of flow control. This is not a problem, since the violation of flow control in this case is bounded.

RETURN VALUE

canput returns 1 if a message can be placed on the queue. 0 is returned if a message cannot be enqueued because of flow control.

LEVEL

Base or Interrupt.

NOTES

Does not sleep.

The driver is responsible for both testing a queue with canput and refraining from placing a message on the queue if canput fails. Driver-defined basic locks, read/write locks, and sleep locks may be held across calls to this function.

The q argument may not reference q_next (for example, an argument of q->q_next is erroneous on a multiprocessor and is disallowed by the DDI/DKI). canputnext(q) is provided as a multiprocessor-safe equivalent to the common call canput(q->q_next), which is no longer allowed [see canputnext(D3DK)].

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