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  }

See Also

allocb(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.