Tribblix: manual page: testb.9f

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

NAME

testb - check for an available buffer

SYNOPSIS

#include <sys/stream.h>

int testb(size_t size, uint_t pri);

INTERFACE LEVEL

Architecture independent level 1 (DDI/DKI).

PARAMETERS

size
Size of the requested buffer.

pri
Priority of the allocb request.

DESCRIPTION

The testb() function checks to see if an allocb(9F) call is likely to
succeed if a buffer of size bytes at priority pri is requested. Even
if testb() returns successfully, the call to allocb(9F) can fail. The
pri argument is no longer used, but is retained for compatibility.

RETURN VALUES

Returns 1 if a buffer of the requested size is available, and 0 if
one is not.

CONTEXT

The testb() function can be called user, interrupt, or kernel
context.

EXAMPLES

Example 1 testb() example

In a service routine, if copymsg(9F) fails (line 6), the message is
put back on the queue (line 7) and a routine, tryagain, is scheduled
to be run in one tenth of a second. Then the service routine returns.

When the timeout(9F) function runs, if there is no message on the
front of the queue, it just returns. Otherwise, for each message
block in the first message, check to see if an allocation would
succeed. If the number of message blocks equals the number we can
allocate, then enable the service procedure. Otherwise, reschedule
tryagain to run again in another tenth of a second. Note that
tryagain is merely an approximation. Its accounting may be faulty.
Consider the case of a message comprised of two 1024-byte message
blocks. If there is only one free 1024-byte message block and no free
2048-byte message blocks, then testb() will still succeed twice. If
no message blocks are freed of these sizes before the service
procedure runs again, then the copymsg(9F) will still fail. The
reason testb() is used here is because it is significantly faster
than calling copymsg. We must minimize the amount of time spent in a
timeout() routine.

1 xxxsrv(q)
2 queue_t *q;
3 {
4 mblk_t *mp;
5 mblk_t *nmp;
...
6 if ((nmp = copymsg(mp)) == NULL) {
7 putbq(q, mp);
8 timeout(tryagain, (intptr_t)q, drv_usectohz(100000));
9 return;
10 }
...
11 }
12
13 tryagain(q)
14 queue_t *q;
15 {
16 register int can_alloc = 0;
17 register int num_blks = 0;
18 register mblk_t *mp;
19
20 if (!q->q_first)
21 return;
22 for (mp = q->q_first; mp; mp = mp->b_cont) {
23 num_blks++;
24 can_alloc += testb((mp->b_datap->db_lim -
25 mp->b_datap->db_base), BPRI_MED);
26 }
27 if (num_blks == can_alloc)
28 qenable(q);
29 else
30 timeout(tryagain, (intptr_t)q, drv_usectohz(100000));
31 }

NOTES

The pri argument is provided for compatibility only. Its value is
ignored.

January 16, 2006 TESTB(9F)