illumos: 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)