SVR5 and SCO OpenServer 5

canput(D3str)

canput -- test for room in a message queue

Synopsis

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

Description

canput searches through the stream (starting at q) 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 a message can be enqueued. If the queue is full, canput 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 the message queue.

Return values

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.

Usage

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

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.

In multithreaded drivers, 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). canputnext(D3str) is provided as a multiprocessor-safe equivalent to the common call canput(q->q_next), which is no longer allowed

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.

Examples

See bufcall(D3str) for an example of canput.

Hardware applicability

All

Version applicability

ddi: 1, 2, 3, 4, 5, 5mp, 6, 6mp, 7, 7mp, 7.1, 7.1mp, 8, 8mp
oddi: 1, 2, 2mp, 3, 3mp, 4, 4mp, 5, 5mp, 6, 6mp

References

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