SVR5

canputnext(D3str)

canputnext -- test for flow control in a stream

Synopsis (Not in ODDI)

   #include <sys/stream.h>
   #include <sys/ddi.h>
   
   int canputnext(queue_t *q);

Description

canputnext searches through the stream (starting at q->q_next) until it finds a queue containing a service routine, or until it reaches the end of the stream. If found, the queue containing the service routine is tested to see if there is room for a message in the queue. If the queue is full, canputnext marks the queue to automatically back-enable the caller's service routine when the amount of data in messages on the queue has reached its low water mark.

Arguments

q: Pointer to a message queue.

Return values

canputnext returns 1 if a message can be sent in the stream, or 0 if the stream is flow-controlled. If canputnext reaches the end of the stream without finding a queue with a service routine, then it returns 1.

Usage

The driver is responsible for both testing a queue with canputnext and refraining from placing a message on the queue if canputnext fails.

It is possible because of race conditions to test for room using canputnext 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.

The q argument may not reference q_next (for example, an argument of q->q_next is erroneous in a multithreaded driver and is disallowed by the DDI).

Context

Base or Interrupt.

Synchronization constraints

Does not block.

Driver-defined basic locks, read/write locks, and sleep locks may be held across calls to this function.

The caller cannot have the stream frozen (see freezestr(D3str)) when calling this function.

Hardware applicability

All

Version applicability

ddi: 3, 5, 5mp, 6, 6mp, 7, 7mp, 7.1, 7.1mp, 8, 8mp

References

bcanput(D3str), bcanputnext(D3str), canput(D3str), putbq(D3str), putnext(D3str)