TESTB(9F)TESTB(9F)NAMEtestb - 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 approxima‐
tion. Its accounting may be faulty. Consider the case of a message com‐
prised 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 copy‐
msg(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 }
SEE ALSOallocb(9F), bufcall(9F), copymsg(9F), timeout(9F)
Writing Device Drivers
STREAMS Programming Guide
NOTES
The pri argument is provided for compatibility only. Its value is
ignored.
Jan 16, 2006 TESTB(9F)
